required reading & learning objectives amino acids structure and function

8/3/2019 REquired Reading & Learning Objectives Amino Acids Structure and Function

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Amino Acids: Structure and FunctionCase Based Learning #1

Required Reading:Some resources and a limited reference list for this case are provided below in

order to assist you during the process of problem solving. This reference list isnot exhaustive, and additional literature search will be productive and is stronglyencouraged. You are also encouraged to consult with the resource faculty forassistance in filling any gaps in your understanding of the material. We mustemphasize, however, that the responsibility for, and benefits from, learning andmastering the necessary knowledge and content relevant to this case are yours.

1. Workgroup Notes (see below)From: The Medical Biochemistry Page:

2. Lieberman and Marks Text: Chapter 6, pps 73-82, Chapter 7, pps. 92-1023. Emedicine article on alpha-1-antitrypsin deficiency, selected sections

(see below).

CaseIngrid Ernster, a 44-year-old white female, complains of shortness of breath,which she has experienced for the past year. She tires easily, even after modestexertion.

Ms. Ernster tells the doctor that she smoked a pack a day since she was 16.,however discontinuing smoking did not change the shortness of breath orfatigability. She denies any chronic productive cough or sputum productionPulmonary function tests show chronic obstructive lung disease (COPD) and the

doctor refers her tests.

Serum 1-antitrypsin level was 23 mg/dl (normal 93-224 mg/dl).

Electrophoretic testing of serum proteins reveals the presence of Z protein andno M protein.

Ingrid Ernster is diagnosed with emphysema due to an 1-antitrypsin deficiency,which is compounded by her long history of smoking. In order to determine thespecific mutation in the 1-antitrypsin gene, a DNA analysis was done:Polymerase Chain Reaction (PCR) and restriction mapping test reveal asubstitution of A for G in exon 5 of the gene encoding 1-AT. This mutationreplaces a glutamic acid at position 342 in the 1-antitrypsin protein with a lysine.This is consistent with the ZZ phenotype.
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Case Objectives

1) Understand the general chemical properties of amino acidsas building blocks of proteins in humans2) Discuss the different classes of amino acids and how the

different side chains give them distinct properties3) Define primary, secondary, tertiary and quaternary structureof proteins4) Explain the difference between a conservative and anonconservative amino acid substitution, and the consequences ofsuch mutations on the structure and function of proteins.5) Understand the pathophysiology of alpha-1-antitrypsindeficiency.

Core Concepts

1. Covalent bonds2. Hydrogen bonds3. Van der Waals bonds4. Water is a polar molecule5. Water solvates polar molecules6. 20 amino acids build proteins. Amino acids in solution at neutralpH are predominantly dipolar ions; the amino group is protonated and thecarboxyl group is dissociated.7. Amino acids vary in size, shape, charge, hydrogen-bondingcapacity, and chemical reactivity.8. Amino acid having aliphatic side chain: gly, ala, val, leu, ile, pro.

9. Amino acids having aromatic side chains: phe, tyr, trp.10. Amino acids with sulfur containing side chain: cys, met.11. Amino acids having aliphatic hydroxyl side chain: ser, thr.12. Amino acids having basic side chains: lys, arg, his.13. Amino acids having acidic side chains and their amide derivative:asp, glu, asn, gln.14. Amino acids are linked by peptide bond to form polypeptide chains.15. Proteins primary structure; proteins have unique amino acidsequences that are specified by genes.16. Secondary structure: polypeptide chains can fold into regularstructures such as -helix, pleated sheet, turn and loop.17. The alpha helix is a coiled structure stabilized by inter-chainhydrogen bonds.18. Beta sheets are stabilized by hydrogen bonding betweenpolypeptide strands.19. Polypeptide chains can change direction by making loops andturns.20. Tertiary structure: water-soluble proteins fold into compactstructures with polar core.


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21. Quaternary structure: polypeptide chains can assemble into multisubunits structures.22. The amino acid sequence of a protein determines its threedimensional structure.23. Amino acids side chains have different properties for forming

secondary, tertiary, and quaternary structures.24. Pathological conditions can result from inappropriate conformation:Prion diseases, Alzheimers etc.25. Proteins fold by stabilization of intermediates.26. Molecular chaperones are proteins that bind to unfolded andpartially folded polypeptides to prevent the improper associations ofexposed segments that might lead to non-native folding, aggregation andprecipitation in vivo.27. The proteome is the set of proteins, their isoforms, modifications,and interactions, in any given cell at any given moment.28. Proteomics is the systemic analysis of cellular proteins. It

encompasses the type, functions and interactions of proteins.29. Cellular functions are performed by structured ensembles ofproteins.

Workgroup Notes

Chemical Nature of the Amino Acids

All peptides and polypeptides are polymers of -amino acids. There are 20 -

amino acids that are relevant to the make-up of mammalian proteins (see below).Several other amino acids are found in the body free or in combined states (i.e.not associated with peptides or proteins). These non-protein associated aminoacids perform specialized functions. Several of the amino acids found in proteinsalso serve functions distinct from the formation of peptides and proteins, e.g.,tyrosine in the formation of thyroid hormones or glutamate acting as aneurotransmitter.

The -amino acids in peptides and proteins (excluding proline) consist of acarboxylic acid (COOH) and an amino (NH2) functional group attached to thesame tetrahedral carbon atom. This carbon is the -carbon. Distinct R-groups,

that distinguish one amino acid from another, also are attached to the alpha-carbon (except in the case of glycine where the R-group is hydrogen). The fourthsubstitution on the tetrahedral -carbon of amino acids is hydrogen.


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Table of -Amino Acids Found in Proteins

AminoAcid

Symbol

Structure*pK1

(COOH)

pK2(NH2

)

pK RGrou

p

Amino Acids with Aliphatic R-Groups

GlycineGly

G2.4 9.8

Alanine

Ala

A 2.4 9.9

ValineVal

V2.2 9.7

LeucineLeu

L2.3 9.7

Isoleucine Ile I 2.3 9.8

Non-Aromatic Amino Acids with Hydroxyl R-Groups

SerineSer

S

2.2 9.2 13

ThreonineThr

T2.1 9.1 13


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Amino Acids with Sulfur-Containing R-Groups

CysteineCys

C1.9 10.8 8.3

MethionineMet

M2.1 9.3

Acidic Amino Acids and their Amides

AsparticAcid

Asp D

2.0 9.9 3.9

AsparagineAsn

N2.1 8.8

GlutamicAcid

Glu E

2.1 9.5 4.1

Glutamine Gln Q

2.2 9.1

Basic Amino Acids

ArginineArg

R1.8 9.0 12.5

LysineLys

K2.2 9.2 10.8


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HistidineHis

H1.8 9.2 6.0

Amino Acids with Aromatic Rings

Phenylalanine

Phe F

2.2 9.2

TyrosineTyr

Y2.2 9.1 10.1

TryptophanTrp

W2.4 9.4

Imino Acids

ProlinePro

P2.0 10.6

*Backbone of the amino acids is red, R-groups are black

Amino Acid Classifications

Each of the 20 -amino acids found in proteins can be distinguished by the R-group substitution on the -carbon atom. There are two broad classes of aminoacids based upon whether the R-group is hydrophobic or hydrophilic.

The hydrophobic amino acids tend to repel the aqueous environment and,therefore, reside predominantly in the interior of proteins. This class of aminoacids does not ionize nor participate in the formation of H-bonds. The hydrophilicamino acids tend to interact with the aqueous environment, are often involved inthe formation of H-bonds and are predominantly found on the exterior surfacesproteins or in the reactive centers of enzymes.


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Acid-Base Properties of the Amino Acids

The -COOH and -NH2 groups in amino acids are capable of ionizing (as are

the acidic and basic R-groups of the amino acids). As a result of their ionizabilitythe following ionic equilibrium reactions may be written:

R-COOH R-COO + H+

R-NH3+ R-NH2+ H

+

The equilibrium reactions, as written, demonstrate that amino acids contain atleast two weakly acidic groups. However, the carboxyl group is a far strongeracid than the amino group. At physiological pH (around 7.4) the carboxyl groupwill be unprotonated and the amino group will be protonated. An amino acid with

no ionizable R-group would be electrically neutral at this pH. This species istermed a zwitterion.

Like typical organic acids, the acidic strength of the carboxyl, amino andionizable R-groups in amino acids can be defined by the association constant, Kaor more commonly the negative logrithm of Ka, the pKa. The net charge (thealgebraic sum of all the charged groups present) of any amino acid, peptide orprotein, will depend upon the pH of the surrounding aqueous environment. As thepH of a solution of an amino acid or protein changes so too does the net charge.This phenomenon can be observed during the titration of any amino acid orprotein. When the net charge of an amino acid or protein is zero the pH will beequivalent to the isoelectric point: pI.


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Functional Significance of Amino Acid R-Groups

In solution it is the nature of the amino acid R-groups that dictate structure-function relationships of peptides and proteins. The hydrophobic amino acids willgenerally be encountered in the interior of proteins shielded from direct contactwith water. Conversely, the hydrophilic amino acids are generally found on theexterior of proteins as well as in the active centers of enzymatically activeproteins. Indeed, it is the very nature of certain amino acid R-groups that allow

enzyme reactions to occur.

The imidazole ring of histidine allows it to act as either a proton donor or acceptorat physiological pH. Hence, it is frequently found in the reactive center ofenzymes. Equally important is the ability of histidines in hemoglobin to buffer theH+ ions from carbonic acid ionization in red blood cells. It is this property ofhemoglobin that allows it to exchange O2 and CO2 at the tissues or lungs,respectively.


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The primary alcohol of serine and threonine as well as the thiol (SH) of cysteineallow these amino acids to act as nucleophiles during enzymatic catalysis.Additionally, the thiol of cysteine is able to form a disulfide bond with othercysteines:

Cysteine-SH + HS-Cysteine Cysteine-S-S-Cysteine

This simple disulfide is identified as cystine. The formation of disulfide bonds between

cysteines present within proteins is important to the formation of active structuraldomains in a large number of proteins. Disulfide bonding between cysteines in different

polypeptide chains of oligomeric proteins plays a crucial role in ordering the structure of

complex proteins, e.g. the insulin receptor.

Optical Properties of the Amino Acids

A tetrahedral carbon atom with 4 distinct constituents is said to be chiral. The

one amino acid not exhibiting chirality is glycine since its '"R-group" is a hydrogenatom. Chirality describes the handedness of a molecule that is observable by theability of a molecule to rotate the plane of polarized light either to the right(dextrorotatory) or to the left (levorotatory). All of the amino acids in proteinsexhibit the same absolute steric configuration as L-glyceraldehyde. Therefore,they are all L--amino acids. D-amino acids are never found in proteins, althoughthey exist in nature. D-amino acids are often found in polypetide antibiotics.

The aromatic R-groups in amino acids absorb ultraviolet light with an absorbancemaximum in the range of 280nm. The ability of proteins to absorb ultraviolet lightis predominantly due to the presence of the tryptophan which strongly absorbs

ultraviolet light.

The Peptide Bond

Peptide bond formation is a condensation reaction leading to the polymerizationof amino acids into peptides and proteins. Peptides are small consisting of fewamino acids. A number of hormones and neurotransmitters are peptides.Additionally, several antibiotics and antitumor agents are peptides. Proteins arepolypeptides of greatly divergent length. The simplest peptide, a dipeptide,

contains a single peptide bond formed by the condensation of the carboxyl groupof one amino acid with the amino group of the second with the concomitantelimination of water. The presence of the carbonyl group in a peptide bond allowselectron resonance stabilization to occur such that the peptide bond exhibitsrigidity not unlike the typical C=C double bond. The peptide bond is, therefore,said to have partial double-bond character.


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Resonance stabilization forms of the peptide bond

Primary Structure of Proteins

The primary structure of peptides and proteins refers to the linear number andorder of the amino acids present. The convention for the designation of the orderof amino acids is that the N-terminal end (i.e. the end bearing the residue withthe free -amino group) is to the left (and the number 1 amino acid) and the C-terminal end (i.e. the end with the residue containing a free -carboxyl group) isto the right.

Secondary Structure in Proteins

The ordered array of amino acids in a protein confer regular conformationalforms upon that protein. These conformations constitute the secondary structures

of a protein. In general proteins fold into two broad classes of structure termed,globular proteins or fibrous proteins. Globular proteins are compactly folded andcoiled, whereas, fibrous proteins are more filamentous or elongated. It is thepartial double-bond character of the peptide bond that defines the conformationsa polypeptide chain may assume. Within a single protein different regions of thepolypeptide chain may assume different conformations determined by theprimary sequence of the amino acids.

The -Helix

The -helix is a common secondary structure encountered in proteins of the

globular class. The formation of the -helix is spontaneous and is stabilized by H-bonding between amide nitrogens and carbonyl carbons of peptide bondsspaced four residues apart. This orientation of H-bonding produces a helicalcoiling of the peptide backbone such that the R-groups lie on the exterior of thehelix and perpendicular to its axis.


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Typical -Helix

Not all amino acids favor the formation of the (-helix due to steric constraints ofthe R-groups. Amino acids such as A, D, E, I, L and M favor the formation of -helices, whereas, G and P favor disruption of the helix. This is particularly true forP since it is a pyrrolidine based imino acid (HN=) whose structure significantlyrestricts movement about the peptide bond in which it is present, thereby,interfering with extension of the helix. The disruption of the helix is important as itintroduces additional folding of the polypeptide backbone to allow the formationof globular proteins.

-Sheets

Whereas an -helix is composed of a single linear array of helically disposedamino acids, -sheets are composed of 2 or more different regions of stretchesof at least 5-10 amino acids. The folding and alignment of stretches of thepolypeptide backbone aside one another to form -sheets is stabilized by H-bonding between amide nitrogens and carbonyl carbons. However, the H-bonding residues are present in adjacently opposed stretches of the polypetidebackbone as opposed to a linearly contiguous region of the backbone in the -


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helix. -sheets are said to be pleated. This is due to positioning of the -carbonsof the peptide bond which alternates above and below the plane of the sheet. -sheets are either parallel or antiparallel. In parallel sheets adjacent peptidechains proceed in the same direction (i.e. the direction of N-terminal to C-terminalends is the same), whereas, in antiparallel sheets adjacent chains are aligned in

opposite directions. -sheets can be depicted in ball and stick format or asribbons in certain protein formats.

Ball and Stick Representation of a -Sheet Ribbon Depiction of -Sheet

Super-Secondary Structure

Some proteins contain an ordered organization of secondary structures that form

distinct functional domains or structural motifs. Examples include the helix-turn-helix domain of bacterial proteins that regulate transcription and the leucinezipper, helix-loop-helix and zinc finger domains of eukaryotic transcriptionalregulators. These domains are termed super-secondary structures.

Tertiary Structure of Proteins

Tertiary structure refers to the complete three-dimensional structure of the polypeptide

units of a given protein. Included in this description is the spatial relationship of different

secondary structures to one another within a polypeptide chain and how these secondarystructures themselves fold into the three-dimensional form of the protein. Secondary

structures of proteins often constitute distinct domains. Therefore, tertiary structure also

describes the relationship of different domains to one another within a protein. Theinteractions of different domains is governed by several forces: These include hydrogen

bonding, hydrophobic interactions, electrostatic interactions and van der Waals forces.
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Forces Controlling Protein Structure

Hydrogen Bonding:

Polypeptides contain numerous proton donors and acceptors both in their

backbone and in the R-groups of the amino acids. The environment in whichproteins are found also contains the ample H-bond donors and acceptors of thewater molecule. H-bonding, therefore, occurs not only within and betweenpolypeptide chains but with the surrounding aqueous medium.

Hydrophobic Forces:

Proteins are composed of amino acids that contain either hydrophilic orhydrophobic R-groups. It is the nature of the interaction of the different R-groupswith the aqueous environment that plays the major role in shaping proteinstructure. The spontaneous folded state of globular proteins is a reflection of a

balance between the opposing energetics of H-bonding between hydrophilic R-groups and the aqueous environment and the repulsion from the aqueousenvironment by the hydrophobic R-groups. The hydrophobicity of certain aminoacid R-groups tends to drive them away from the exterior of proteins and into theinterior. This driving force restricts the available conformations into which aprotein may fold.

Electrostatic Forces:

Electrostatic forces are mainly of three types; charge-charge, charge-dipole anddipole-dipole. Typical charge-charge interactions that favor protein folding are

those between oppositely charged R-groups such as K or R and D or E. Asubstantial component of the energy involved in protein folding is charge-dipoleinteractions. This refers to the interaction of ionized R-groups of amino acids withthe dipole of the water molecule. The slight dipole moment that exist in the polarR-groups of amino acid also influences their interaction with water. It is,therefore, understandable that the majority of the amino acids found on theexterior surfaces of globular proteins contain charged or polar R-groups.

van der Waals Forces:

There are both attractive and repulsive van der Waals forces that control protein

folding. Attractive van der Waals forces involve the interactions among induceddipoles that arise from fluctuations in the charge densities that occur betweenadjacent uncharged non-bonded atoms. Repulsive van der Waals forces involvethe interactions that occur when uncharged non-bonded atoms come very closetogether but do not induce dipoles. The repulsion is the result of the electron-electron repulsion that occurs as two clouds of electrons begin to overlap.


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Although van der Waals forces are extremely weak, relative to other forcesgoverning conformation, it is the huge number of such interactions that occur inlarge protein molecules that make them significant to the folding of proteins.

Quaternary Structure

Many proteins contain 2 or more different polypeptide chains that are held inassociation by the same non-covalent forces that stabilize the tertiary structuresof proteins. Proteins with multiple polypetide chains are oligomeric proteins. Thestructure formed by monomer-monomer interaction in an oligomeric protein isknown as quaternary structure.

Oligomeric proteins can be composed of multiple identical polypeptide chains ormultiple distinct polypeptide chains. Proteins with identical subunits are termedhomo-oligomers. Proteins containing several distinct polypeptide chains aretermed hetero-oligomers.

Hemoglobin, the oxygen carrying protein of the blood, contains two and two subunits arranged with a quaternary structure in the form, 22. Hemoglobin is,therefore, a hetero-oligomeric protein.

Structure of Hemoglobin
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Complex Protein Structures

Proteins also are found to be covalently conjugated with carbohydrates. Thesemodifications occur following the synthesis (translation) of proteins and are,therefore, termed post-translational modifications. These forms of modification

impart specialized functions upon the resultant proteins. Proteins covalentlyassociated with carbohydrates are termed glycoproteins. Glycoproteins are oftwo classes, N-linked and O-linked, referring to the site of covalent attachment ofthe sugar moieties. N-linked sugars are attached to the amide nitrogen of the R-group of asparagine; O-linked sugars are attached to the hydroxyl groups ofeither serine or threonine and occasionally to the hydroxyl group of the modifiedamino acid, hydroxylysine.

There are extremely important glycoproteins found on the surface oferythrocytes. It is the variability in the composition of the carbohydrate portions ofmany glycoproteins and glycolipids of erythrocytes that determines blood group

specificities. There are at least 100 blood group determinants, most of which aredue to carbohydrate differences. The most common blood groups, A, B, and O,are specified by the activity of specific gene products whose activities are toincorporate distinct sugar groups onto RBC membrane glycoshpingolipids as wellas secreted glycoproteins.

Structural complexes involving protein associated with lipid via noncovalentinteractions are termed lipoproteins. The distinct roles oflipoproteins aredescribed on the linked page. Their major function in the body is to aid in thestorage transport of lipid and cholesterol.

Clinical Significances

This discussion is not intended to be a complete review of all disorders that resultfrom defects in protein structure and function. Visit the Inborn Errors page for amore complete listing of diseases related to abnormal proteins and also click onthe links to the specific examples below for more information.

The substitution of a hydrophobic amino acid (V) for an acidic amino acid (E) inthe -chain of hemoglobin results in sickle cell anemia (HbS). This change of asingle amino acid alters the structure of hemoglobin molecules in such a way thatthe deoxygenated proteins polymerize and precipitate within the erythrocyte,

leading to their characteristic sickle shape.

Collagens are the most abundant proteins in the body. Alterations in collagenstructure arising from abnormal genes or abnormal processing of collagenproteins results in numerous diseases, including Larsen syndrome, scurvy,osteogenesis imperfecta and Ehlers-Danlos syndrome.
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Ehlers-Danlos syndrome is actually the name associated with at least ten distinctdisorders that are biochemically and clinically distinct yet all manifest structuralweakness in connective tissue as a result of defective collagen structure.Osteogenesis imperfecta also encompasses more than one disorder. At leastfour biochemically and clinically distinguishable maladies have been identified as

osteogenesis imperfecta, all of which are characterized by multiple fractures andresultant bone deformities. Marfan syndromemanifests itself as a disorder of theconnective tissue and was originally believed to be the result of abnormalcollagens. However, recent evidence has shown that Marfan syndrome resultsfrom mutations in the extracellular protein, fibrillin, which is an integral constituentof the non-collagenous microfibrils of the extracellular matrix.

Several forms offamilial hypercholesterolemia are the result of genetic defects inthe gene encoding the receptor for low-density lipoprotein (LDL). These defectsresult in the synthesis of abnormal LDL receptors that are incapable of binding toLDLs, or that bind LDLs but the receptor/LDL complexes are not properly

internalized and degraded. The outcome is an elevation in serum cholesterollevels and increased propensity toward the development of atherosclerosis.

A number of proteins can contribute to cellular transformation and carcinogenesiswhen their basic structure is disrupted by mutations in their genes. These genesare termed proto-oncogenes. For some of these proteins, all that is required toconvert them to the oncogenic form is a single amino acid substitution. Thecellular gene, RAS, is observed to sustain single amino acid substitutions atpositions 12 or 61 with high frequency in colon carcinomas. Mutations in RAS aremost frequently observed genetic alterations in colon cancer.
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Alpha1-Antitrypsin DeficiencyPaul Fairman, MD, Director, Pulmonary Hypertension Service, Professor,Department of Internal Medicine, Division of Pulmonary and Critical CareMedicine, Virginia Commonwealth University

Rajiv Malhotra, DO, Fellow, Department of Internal Medicine, Division ofPulmonary and Critical Care Medicine, Virginia Commonwealth UniversityHealth System

Updated: Apr 16, 2009

Introduction

Background

Alpha1-antitrypsin (AAT) deficiency, first described in 1963, is one of the most

common inherited disorders among white persons. Its primary manifestation is

early-onset panacinar emphysema. About 1-3% of patients with diagnosed

chronic obstructive pulmonary disease (COPD) are predicted to have alpha1-

antitrypsin deficiency. Slowly progressive dyspnea is the primary symptom,

though many patients initially have symptoms of cough, sputum production, or

wheezing.

Treatment involves smoking cessation, bronchodilation, and physical

rehabilitation in a program similar to that designed for patients with smoking-

related COPD. In addition, intravenous (IV) augmentation therapy with alpha1-antitrypsin benefits some patients. Alpha1-antitrypsin deficiency is also an

unusual cause of hepatic cirrhosis in children and adults.

Pathophysiology

The genetic defect in alpha1-antitrypsin (AAT) deficiency alters the configuration

of the alpha1-antitrypsin molecule and prevents its release from hepatocytes. As

a result, serum levels of alpha1-antitrypsin are decreased, leading to low alveolar

concentrations, where the alpha1-antitrypsin molecule normally would serve as

protection against antiproteases. The resulting protease excess in alveoli

destroys alveolar walls and causes emphysema. The accumulation of excess

alpha1-antitrypsin in hepatocytes can also lead to destruction of these cells and

ultimately, clinical liver disease.


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Frequency

United States

Alpha1-antitrypsin (AAT) deficiency is 1 of the 3 most common lethal genetic

diseases among adult white persons, affecting 1 per 3000-5000 individuals.Severe alpha1-antitrypsin deficiency affects an estimated 100,000 individuals,

and approximately 25 million people carry of at least 1 deficient gene. However,

less than 6% of severely deficient individuals are currently identified.

International

Alpha1-antitrypsin deficiency has been identified in all populations, but it is most

common in individuals of Northern European and Iberian descent. Similar rates

are found among white persons worldwide, with an estimated 117 million carriersand 3.4 million affected individuals.

Mortality/Morbidity

Specific morbidity and mortality rates are unknown. Not all patients with

homozygous deficiency develop symptomatic emphysema or cirrhosis; however,

among those who develop symptomatic disease, the mortality rate is high.

Race

Racial groups other than whites are affected less frequently.

Sex

Women and men are affected in equal numbers.

Age

The enzyme deficiency is congenital and has a bimodal distribution with respect

to symptoms. It can be seen in neonates as a cause of neonatal jaundice andhepatitis. It can present in infants as cholestatic jaundice and in children as

hepatic cirrhosis or liver failure. Alpha1-antitrypsin deficiency is also the leading

cause of liver transplantation in children.

In adults, alpha1-antitrypsin deficiency leads to chronic liver disease in the fifth


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decade. As a cause of emphysema, it is seen in nonsmokers in the fifth decade

of life and during the fourth decade of life in smokers.

Clinical

History

Symptoms of alpha1-antitrypsin (AAT) deficiency emphysema are limited to the

respiratory system.

The initial symptoms of alpha1-antitrypsin deficiency include cough,

sputum production, and wheezing. Symptoms are initially intermittent, and,

if wheezing is the predominant symptom, patients often are told they have

asthma. If recurrent episodes of cough are most prominent, patients may

be treated with multiple courses of antibiotics and evaluated for sinusitis,

postnasal drip, or gastroesophageal reflux.

o Dyspnea is the symptom that eventually dominates alpha1-

antitrypsin deficiency.

o Similar to other forms of emphysema, the dyspnea of

alpha1-antitrypsin deficiency is initially evident only with strenuous

exertion. Over several years, it eventually limits even mild activities.

o Patients with alpha1-antitrypsin deficiency frequently

develop dyspnea 20-30 years earlier (at age 30-45 y) than dosmokers with emphysema and normal alpha1-antitrypsin levels.

Cigarette smoking accelerates the progression of emphysema in

patients with alpha1-antitrypsin deficiency. Symptoms develop about 10

years earlier in alpha1-antitrypsindeficient individuals who smoke

regularly.

By the time dyspnea becomes the dominant manifestation and a

diagnosis is established, most patients will have seen several physicians

over several years. Efforts to improve the interval between the onset of

symptoms and the diagnosis of alpha1-antitrypsin deficiency have been

disappointing. Between 1968 and 2003 a significant improvement has not

been noted in the average interval (approximately 8 y), although

improvement has been shown in the alpha1-antitrypsin deficiency

detection in older individuals.[1 ]


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Physical

No single physical sign confirms a diagnosis of alpha1-antitrypsin deficiency

emphysema. Signs characteristic of increased respiratory work, airflowobstruction, and hyperinflation eventually develop but are dependent on the

severity of emphysema at the time of diagnosis.

Increased respiratory work is evident as tachypnea, scalene and

intercostal muscle retraction, and tripod position.

Airflow obstruction manifests as pursed-lip breathing, wheezing,

and pulsus paradox.

Hyperinflation results in barrel chest, increased percussion note,

decreased breath sound intensity, and distant heart sounds.

Patients with mild emphysema generally have no abnormal findings

on physical examination.

o Even moderate disease may be evident only when a

complicating acute infection occurs.

o Most of the signs generally considered a part of emphysema

(from any cause) are signs of moderate-to-severe disease.

o Mild-to-moderate disease is easily missed if the physician

relies solely on physical findings.

Causes

Alpha1-antitrypsin deficiency is an uncommon but not rare disease. It is under

diagnosed. The responsible genetic defect affects 1 in 3000-5000 individuals,

making it 1 of the 3 most common lethal genetic diseases among whites. (The

other 2 common fatal genetic defects are cystic fibrosis and Down syndrome.)

Fortunately, not every individual with alpha1-antitrypsin deficiency develops

clinically significant disease.

The major biochemical activity of the alpha1-antitrypsin molecule is

inhibition of several neutrophil-derived proteases (eg, trypsin, elastase,

proteinase 3, cathepsin G). Therefore, the protein is more accurately

termed alpha1-antiprotease. However, most physicians, and virtually all


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patients, refer to the disease as alpha1-antitrypsin deficiency, and doctors

and patients often refer to those who are affected as "alphas."

Hepatocytes synthesize alpha1-antiprotease.

o After its release from the liver, alpha1-antiprotease circulates

unbound and diffuses into interstitial and alveolar lining fluids. Its

principle function in the lung is to inactivate neutrophil elastase, an

enzyme that is released during normal phagocytosis of organisms

or particulates in the alveolus.

o Alpha1-antiprotease constitutes about 95% of all the

antiprotease activity in human alveoli, and neutrophil elastase is

considered the protease largely responsible for alveolar

destruction. In patients with the Z allele, the alpha1-antitrypsin

produced has a lysine substituted for glutamate. This results inspontaneous polymerization within the endoplasmic reticulum of the

hepatocyte, which leads to decreased serum levels of alpha1-

antitrypsin and thus a deficiency of peripheral alpha1-antitrypsin.

o Additionally, the accumulation of intrahepatic alpha1-

antitrypsin is thought to result in apoptosis of hepatocytes. This

initially can manifest as laboratory abnormalities, but also can

progress to hepatitis, followed by fibrosis and cirrhosis. [2 ]

In healthy persons, alpha1-antiprotease serves as a protective

screen that prevents alveolar wall destruction. The lungs have a large

surface area and are continuously exposed to a high burden of airborne

pathogens, which results in a cellular immune response. This is

characterized by local release of oxidants and proteases. The presence of

alpha1-antiprotease serves to keep these proteases in check and protect

the lungs from unregulated protease activity. Individuals with the alpha1-

antitrypsin genetic defect do not release alpha1-antiprotease from the

liver, and serum and alveolar levels of the protein are low. Consequently,

alveoli lack antiprotease protection. The imbalance of proteases-antiproteases in the alveolus leads to unimpeded neutrophil elastase

digestion of elastin and collagen in the alveolar walls and progressive

emphysema.

Alveolar cell apoptosis may also play an important role in

emphysema pathogenesis. Recent evidence suggests that alpha1-


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antiprotease may inhibit alveolar cell apoptosis and protect against

emphysema in the absence of neutrophilic inflammation.[3 ]

Cigarette smoking accelerates the onset of symptomatic disease by

approximately 10 years by increasing the number of neutrophils (and

neutrophil elastase) in the alveolus and inactivating the remaining small

amounts of antiprotease. Other factors that can accelerate the onset or

worsen symptoms of disease include infections and exposures to dust and

fumes, which can also cause the recruitment of neutrophils to the alveoli.

The production of alpha1-antiprotease is controlled by a pair of

genes at the protease inhibitor (Pi) locus. The SERPINA1 (formerly known

as Pi) gene responsible for encoding alpha1-antitrypsin is located on

chromosome 14 and is highly pleomorphic, with more than 100 allelic

variants. The variants are classified based on serum levels of alpha1-antitrypsin protein. M alleles are the most common and normal

variants. Most patients with clinical disease are homozygous SS or ZZ or

heterozygous MS, MZ, or SZ.

o Nearly 24 variants of the alpha1-antiprotease molecule have

been identified, and all are inherited as codominant alleles. The

most common (90%) allele is M (PiM), and homozygous individuals

(MM) produce normal amounts of alpha1-antiprotease (serum

levels of 20-53 mol/L or 150-350 mg/dL).

o The most common form of alpha1-antitrypsin deficiency is

associated with allele Z, or homozygous PiZ (ZZ). Serum levels of

alpha1-antitrypsin in these patients are about 3.4-7 mol/L, 10-15%

of normal serum levels. Serum levels greater than 11 mol/L

appear to be protective. Emphysema develops in most (but not all)

individuals with serum levels less than 9 mol/L.

Other genotypes associated with severe alpha1-antitrypsin

deficiency include PiSZ, PiZ/Null, and PiNull. The Sgene is more frequent

among individuals of Spanish or Portuguese descent, whereas thefrequency of the Zgene is highest in patients of Northern or Western

European descent.

o Patients with the PiSZ phenotype have a 20-50% increased

likelihood of developing emphysema compared with MM

homozygotes. Serum levels of patients with PiSZ alpha1-antitrypsin

deficiency are 75-120 mg/dL.


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o Patients with the null gene for alpha1-antitrypsin will not

produce any alpha1-antitrypsin and are high risk for emphysema

(100% by the age of 30 y). None with the null gene develop liver

disease because of a lack of production, and thus accumulation, of

alpha1-antitrypsin in the hepatocytes. The null gene is the least

common of the known alleles associated with alpha1-antitrypsin

deficiency.

o Carriers or heterozygotes (MZ, MS or M/Null) have levels

approximately 35% of normal levels, but they do not develop

disease.

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