enzymes lectures

Enzymes Dr/ Faten

Biochemistry Diploma 3OBAD

LECTURE 1

DEFINITION, PROPERTIES AND CLASSIFICATION

Enzymes

* Enzymes are proteins with highly specialized catalytic functions, produced by all living organisms.

* Enzymes are responsible for many essential biochemical reactions in microorganisms, plants, animals, and

human beings.

* Enzymes are natural protein molecules that act as highly efficient catalysts in biochemical reactions, that

is, they help a chemical reaction take place quickly and efficiently.

Enzymes are highly specific. *

Enzymes not only work efficiently and rapidly, they are also biodegradable. *

* Enzymes are highly efficient in increasing the reaction rate of biochemical processes that otherwise

proceed very slowly, or in some cases, not at all.

* All enzymes are protein in nature enzymes are protein in nature, so have the same properties of proteins as

denaturation, precipitation, electrophoresis ………….etc.

* They are sensitive to changes in pH and temperature.

* They function in minute amounts, remain unchanged chemically during the reaction.

Chemical nature of enzymes

All enzymes are protein in nature and it may be:

: only protein chain(s) e.g. pepsin.Simple protein enzyme .1

Conjugated protein enzyme: (Holoenzyme) 2.

* Holoenzyme: it is an enzyme composed from protein part and non-protein part.

Protein part (apoenzyme) a)

Non-protein part : may be b)

Organic (coenzymes): loosely attached to apoenzyme

Inorganic (activator): loosely attached to apoenzyme

* Prosthetic group is a non-protein part firmly attached to the apoprotein

Enzymes Dr/ Faten


Inorganic catalyst Enzymes

Thermo- stable Thermo- labile

Inorganic, non biologically substances Organic, biologically substances

Non protein in nature, Non specific Protein in nature, Specific

Low catalytic effieciency High catalytic efficiency

Differentiate between holoenzymes, apoenzymes, co-factor, metal activated enzyme, prosthetic group,

coenzyme, metalloenzyme and isoenzyme.

A. Holoenzyme: it is an enzyme composed from proteinic part and non-proteinic part.

B. Apoenzyme: it is the proteinic part of the holoenzyme.

C. Co-factor: it is the non-proteinic part of the holoenzyme

D. Metal activated enzyme: holoenzymes which have a loosely bound metals on its prosthetic inorganic

group.

E. Co-enzyme: specific thermo stable low mol.wt non-protein organic substance bound tightly in usual.

F. Metalloenzyme: enzyme which has tightly bound metals as its prosthetic group.

G. Isoenzymes: enzymes which have different structures and same function.

- Cardiolipin is the prosthetic group of the enzyme cytochrome oxidase.

- Cu++, Fe++ ions are the metalloenzyme of cytochrome oxidase.

Turnover number:

The “turnover number” is the number of substrate molecules converted into product by an enzyme molecule

in a unit time when the enzyme is fully saturated.

Enzymes Dr/ Faten


Localization of the enzyme

Intracellular enzymes: they act inside the cells that make them.

Extracellular enzymes: they act outside the cells that secret them such as:

- digestive enzymes in the GIT

- coagulation enzymes in plasma.

Zymogens

* Digestive and coagulation enzymes are secreted in inactive forms, zymogens or proenzyme.

* Zymogens are activated by trimming of a short peptide blocking the active site, or by covalent

modification of the zymogen.

Mechanism of activation:

* The mechanism of activation may involve unmasking of a polypeptide chain that may be blocking or

masking the active centers of apoenzyme.

* Proteolytic enzymes are secreted inactive to prevent digestion of protein of the cell that synthesized them.

Activation takes place by:

HCL

1- pH – changes: Pepsinogen Pepsin

trypsin

2- Auto-activation: Trypsinogen Trypsin

Entrokinase

3- By other enzymes: Trypsinogen Trypsin

Terminology of enzymes

1-Some enzymes still retain their old names as digestion enzymes still use –in pepsin, trypsin.

2-End in –ase:

Identifies a reacting substance

sucrase – reacts sucrose

lipase - reacts lipid

3-Describes function of enzyme:

oxidase – catalyzes oxidation

hydrolase – catalyzes hydrolysis

4-Enzyme named by the name of both the substrate acted upon and the type of reaction

catalyzed. eg.: Succinic Dehydrogenase that remove hydrogen from succinic acid.

Enzymes Dr/ Faten


Nomenclature of enzymes

accepted for enzymes: Nomenclature systems are

1) The trivial name: which give no indication of the function of the enzyme, are commonly used.

In some cases, the trivial name is composed of the name of the substrate involved, the type of the reaction

catalyzed, and the ending – ase.

Lactate + dehydrogenation + ase = lactate dehydrogenase

2) The systematic name:

It is made up of the names of substrates of the chemical reaction catalyzed by the enzyme, the name of the type

of the catalyzed chemical reaction, and the ending –ase.

L – Lactate NAD+ Oxidoreductase

Substrate I Substrate II type of chemical reaction

The substrate

The substrate of an enzyme are the reactants that are activated by the enzyme

Enzymes are specific to their substrats

The specificity is determined by the active site

Classification of Enzymes

All the enzymes are classified into six groups. The name of the group indicates the type of the chemical reaction

catalyzed by the enzymes:

1) Oxidoreductases: are involved in oxidation and reduction.

2) Transferases: transfer functional groups (e.g., amino or phosphate groups) between donors and acceptors.

3) Hydrolases: transfer water; that is, they catalyze the hydrolysis of a substrate.

4) Lyases: add (or remove) the elements of water, ammonia, or carbon dioxide (CO2) to (or from) double

bonds.

mutases, as well as catalyze changes within one molecule; they include racemases and 5) Isomerases:

epimerases, cis-trans isomerases, intramolecular oxidoreductases, intramolecular transferases,

and intramolecular lyases.

6) Ligases (Synthetases): join two molecules together at the expense of a high-energy phosphate bond of

adenosine triphosphate (ATP).

Several subclasses of enzymes fall under each of these major classifications:

* Each group is assigned a definite number. The groups are dividing into subgroups; the latter are further

subdivided into subsubgroups.

The number of subgroups in a group varies, as well as the number of subsubgroups in a subgroup. *

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* According to the numerical classification system, each enzyme receives a four- part number whose

numerals are separated by a dot.

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B. Transeferase

Def: enzymes which catalyze the transfer of C- containing, N- containing and sulfur containing groups.

Ex:

ALT (GPT)

Glutamic acid + pyruvic acid α- ketoglutaric acid + Alanine

ALT (GPT) enzyme can be found in liver normally, but if it's found in the serum of the blood in high quantity

This indicates the presence of a disease in the liver.

Transferred group EC 2 Enzyme

Acetyl, Succinyl, Aminoacyl Acyl- transeferase (e.g: synthesis of TG and

phospholipids)

(phosphoryl) -H2PO3 Phospho-transeferase (e.g: Hexokinase, Glucokinase)

-NH2 (amino) Amino- transeferase(e.g: ALT, AST)

-SO3H (sulfuryl) Sulfo- transeferase (e.g: GAG synthesis)

Bond Substrate E.C. 3 Enzyme

ester Neutral fats, phospholipids,

acetylcholine

Estrases (e.g: lipase,

phospholipase, acetylcholine

esterase)

phosphodiester Nucleotides (cAMP – 5'Amp) Phosphodiesterases (e.g: cAMP

phosphodiesterase)

phosphomonoester Phosphatester (G-6-P) Phosphotases (e.g Glucose-6-

phosphotases)

glycoside Polysaccharides, disacharides Glycosidases (e.g: amylase,

lipase)

Proteines, peptides Proteases, peptidases (e.g:

Trypsin)

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Removed group E.C. 4 Enzyme

- CO2 Decarboxylase (e.g: amino acid decarboxylase)

- aldehyde Aldolase (e.g: aldolase A/B)

CO2 + NH3 GLY Synthase (e.g: GLY synthase)

- H2O Dehydratse (e.g: cysthationin synthase)

-NH3 Desaminase (e.g: Gln desaminase)

Changing position of the group Isomerated group E.C 5 Enzyme

C1 C2 carbonyl Glucose – 6- phosphate isomerase

C2 C3 phosphoryl Phosphoglycerate phosphomutase

D L Hydroxyl Epimerase (e.g: UDP-GLU, UDP-

GAL)

Enzymes Dr/ Faten


URE 2LECT

ENZYME STRUCTURE, SPECIFICITY AND ACTION

Structure of the Enzyme

* Enzymes are proteins, and their function is determined by their complex structure. The reaction takes

place in a small part of the enzyme called the active site, while the rest of the protein acts as

"scaffolding".

* The enzyme is tertiary or quaternary structure that has spatial configuration.

This makes the enzyme specific for one reaction only, as other molecules won't fit into the active site –

their shape is wrong.

* It has pockets on its surface. Each pocket has its own function.

The catalytic or active site.

The allosteric site.

1) The catalytic or active site:

- It is the site at which the substrate binds to the enzyme.

- It should be fit to the substrate (fitness is made by the tertiary structure of the enzyme molecule).

- Any factor affecting this structure will alter the fitness and formation of enzyme-substrate complex.

2) The Allosteric site:

- It is a site for fitting of a small molecule whose binding alters the affinity of the catalytic site to the

substrate.

- This small molecule is called allosteric modifier for binding of the substrate:

Stimulatory (making it more fit)

Inhibitory (making the catalytic site unfit)

Enzymes Dr/ Faten


Enzyme Action

- An enzyme binds a substrate in a region called the active site

Only certain substrates can fit the active site forming enzyme substrate complex. -

- The active site contain specific groups of Amino acid help substrate bind.

- Enzyme-substrate complex decomposes, giving rise to free enzyme and products of the reaction.

E + S E S E+ P

Mode of enzyme action

• The reactants should raise their energy levels to reach a transition state.

• The transition state represents the energy barrier between the reactants and products.

• This energy is known as energy of activation.

• The enzyme makes the reaction needs lower activation energy.

Enzymes Dr/ Faten


How do enzymes work?

- There are three parts to our thinking about enzyme catalysis.

- They each describe different aspects of the same process, and you should know about each of them.

1. Reaction Mechanism

In any chemical reaction, a substrate (S) is converted into a product (P). -

- In an enzyme-catalysed reaction, the substrate first binds to the active site of the enzyme to form an

enzyme-substrate (ES) complex, then the substrate is converted into product whilst attached to the

enzyme, and finally the product is released, thus allowing the enzyme to start all over again.

2. Molecular Geometry:

- The substrate molecule is complementary in shape to that of the active site. It was thought that the

substrate exactly fitted into the active site of the enzyme molecule like a key fitting into a lock (the

now discredited ‘lock and key’ theory).

- It is now known that the substrate and the active site both change shape when the enzyme-substrate

complex is formed, bending (and thus weakening) the target bonds.

3. Energy Changes:

- The way enzymes work can also be shown by looking at the energy changes during a chemical reaction.

- In a reaction where the product has a lower energy than the substrate, the substrate naturally turns into

product (i.e. the equilibrium lies in the direction of the product).

- Before it can change into product, the substrate must overcome an "energy barrier" called the activation

energy.

- The larger the activation energy is, the slower the reaction will be. This is because only a few substrate

molecules will have sufficient energy to overcome the activation energy barrier.

Enzymes Dr/ Faten


Enzymes reduce the activation energy of a reaction so that the kinetic energy of most molecules exceeds

the activation energy required and so they can react.

Enzyme specificity

The specificity of an enzyme is determined by:

- The functional groups of the substrate (or product).

- The functional groups of the enzyme and its cofactors.

- The physical proximity of these various functional groups.

;Two theories have been proposed to explain the specificity of enzyme action

The active site of the enzyme is complementary in The lock and key theory: A)

conformation to the substrate, so that enzyme and substrate "recognize" one another.

Enzymes Dr/ Faten


E S complex E + P E + S

Induced Fit ModelB)

- The active site of the enzyme is flexible, not rigid

- When the active site identifies the substrate it brings a change in the active site shape so it can

accommodate the substrate.

- It has a shape complementary to that of the substrate only after the substrate is bound to the enzyme.

- This model similar to the rubber gloves.

E + S ES complex E + P

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There are 5 types of specificity:

1) Absolute specificity :

e.g. urease enzyme acts on urea, uricase enzyme acts on uric acid and arginase enzyme acts on arginine.

cificity: 2) Relative Spe

In this type, the enzyme acts on a group of closely related substrates i.e. which are similar in structure and

posses the same type of bonds e.g.

Lipase catalyzes the process of hydrolysis of ester linkage present in triglycerides containing different types of

fatty acids.

specificity or optical specificity:-3) Stereo

The enzyme works on one of two isomers e.g.: L – Amino acid oxidase acts cn L – amino acids only. &

Enzymes of glycolysis act on D-sugars only.

Exception:

There is only one exception which is racemase enzyme catalyses reversible interconversion of D and L

isomers.

4) Dual specificity:

- The enzyme acts on 2 different substrates e.g.:

Xanthine oxidase can oxidize hypoxanthine and xanthine to uric acid.

Isocitric acid dehydrogenase acting on isocitric acid causing dehydrogenation and decarboxylation.

specificity or structural specificity: –5) Group

-The enzyme shows specificity not only to the bond and its position but also towards the chemical groups

or atoms surrounding this bond e.g.

carboxypeptidase acts on the free carboxyl end of polypeptide chain .

aminopeptidase acts on the free amino end of polypeptide chain .

Autolysis

- More commonly known as self-digestion, refers to the destruction of a cell through the action of its

own enzymes. It may also refer to the digestion of an enzyme by another molecule of the same enzyme.

- In the living body these enzymes are usually inactive since their optimum pH is slightly acidic, while

the pH of the body fluids is slightly alkaline and unsuitable for their activity.

- After death, lactic acid accumulate in the tissues and cathepsins become activated .

Enzymes Dr/ Faten


LECTURE 3

FACTORS AFFECTING ENZYMES

Factors Affecting Enzyme Action

- The activity of the enzyme is evaluated by measuring the rate or the velocity of the reaction.

- velocity of the reaction is measured by how many moles of the substrate are converted into products per

unit of time (minute).

The factors include the following:

1- Substrate 2- Temperature 3- PH

4- Time 5- Cofactor 6- Enzyme Inhibitors

7- Hormones 8- Radiation & Light 9- Product concentration

10- Enzyme activators 11- Antienzyme & antibodies 12- Concentrations of Co- Factors

1- Temperature

- Little activity at low temperature.

- Rate increases with temperature.

- Each enzyme has an optimum temperatures

at which the enzyme acts maximally.

- Most active at optimum temperatures

(usually 37°C in humans and 65°C for plant

enzymes).

- Activity lost with denaturation at high

temperatures.

2- pH

- Each enzyme has an optimum pH at which

the enzyme acts maximally.

- Maximum activity at optimum pH

- Most lose activity in low or high pH

3-Substrate Concentration

As substrate concentration increases,

- the rate of reaction increases (at constant

enzyme concentration).

- the enzyme eventually becomes saturated

giving maximum activity.

Enzymes Dr/ Faten


4-Enzyme Concentration

Increasing enzyme concentration increases the rate of reaction.-

- Further increase in the enzyme can not increase the velocity of

reaction because the amount of substrate may not be sufficient to

permit of maximum velocity.

Michaelis Constant (Km)

Km is equal to the substrate concentration [S] at

which the reaction is half of its maximum

(1/2Vmax).

It expresses the affinity of the enzyme to its

substrate.

Low Km means high affinity of the enzyme to the

substrate.

High Km means low affinity of the enzyme to the

substrate.

Burk Equation-Lineweaver –Enzymes

5-Hormones

e.g. insulin hormone stimulates glucokinase enzyme while glucocorticoids inhibit it.

Steroid hormones are known to increase the rate of synthesis of many enzyme.

Enzymes Dr/ Faten


6-Time

As time is passed the rate of the enzyme catalyzed reaction diminishes due to:

*Decline of substrate concentration.

*The accumulated product may cause feedback inhibition of the enzyme.

7-Product concentration

As you increase the product concentration you

decrease the rate of the reaction.

The excess amount of product accumulates and

occupies the active site of the enzyme.

8-Radiation and light

Light inhibit most enzyme activity although some enzymes e.g. amylase is activated by red or green light.

Ultraviolet rays and ionized radiations cause denaturation of most enzymes.

9-Enzyme activators

Certain inorganic ions e.g.:-

CL¯activate salivary and pancreatic amylase.

Ca++ activate thrombokinase.

Bile salts activate lipase enzyme.

- Some enzymes are secreted in inactive form and are activated by:

pH: pepsinogen is activated by HCL giving pepsin.

Autoactivation: pepsinogen pepsin

Kinase: which activate zymogen or proenzymes.

10- Enzyme Inhibitors

Definition: substances which inhibit (stop) the enzyme activity.

It causes a loss of catalytic activity

Change the protein structure of an enzyme.

It may be :

Non-specific or specific.

Enzymes Dr/ Faten


11-Antienzymes and antibodies

Antienzymes: ascaris living in the lumen of intestine secrete antipepsin and antitrypsin to prevent digestion

of the worm by these enzymes.

Antibodies: if enzyme is injected, the immune system of the body produces antibodies which will inactivate

these enzymes.

12-Concentration of cofactors

The rate of enzyme reaction is directly proportional to the concentration of the cofactors.

protein molecule that is needed by some enzymes to help the reaction-An additional nonDefinition:

Tightly bound cofactors are called prosthetic groups

Cofactors that are bound and released easily are called coenzymes

Many vitamins are coenzymes.

Coenzyme = thiamine pyrophosphate (TPP)

used in decarboxylation and transketolation

contains pyrimidine and thiazole.

disease due to deficiency: beri-beri, Wernicke’s disease.

peripheral nerves, muscle cramps, numbness

Coenzyme :flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD)

both act as prosthetic groups

-- use = redox reactions

-- its vitamin = riboflavin or B2

adenine dinucleotide phosphate(NADP)-adenine dinucleotide (NAD), nicotinamide-Nicotinamide

used in redox reactions with H transfer.

Its vitamin =niacin or B3 =nicotinamide& nicotinic acid

Disease due to deficiency pellagra, skin lesions,

swollen tongue, nervous/mental disorders

Enzymes Dr/ Faten


Coenzyme = pyrodoxal phosphate

Used in decarboxylations, transaminations and racemases.

Its vitamin = pyridoxine, or vitamin B6

(CoA) Coenzyme = Coenzyme A

use = activates carbonyl groups and in acyl transfer(acetyl- CoA, synthesis of fats and steroids)

its vitamin = pantothenic acid

disease due to deficiency GI problems, emotional instability, burning sensation in extemities

tetrahydrofolate (the reduced form)Coenzyme = folate or

Used in transfer of one carbon unit or formate

Its vitamin = folic acid

Disease due to deficiency megablastic anemia, birth defects

Coenzyme = biotin

a prosthetic group

-- use = carboxylations

-- its vitamin = biotin

Coenzyme = cyanocobalamin

Used in methyl group transfer; folate metabolism, myelin synthesis

Its vitamin is cyanocobalamin or vitamin B12 disease due to deficiency pernicious anemia

Coenzyme = lipoic acid

(reduced SH or oxidized form -S-S-)prosthetic group

Used in redox reactions

Its vitamin = lipoic acid

(humans probably produce enough so it is not always considered a vitamin)

Enzymes Dr/ Faten


LECTURE 4

ENZYME INHIBITORS

substances which inhibit (stop) the enzyme activity. Definition:

It causes a loss of catalytic activity

Change the protein structure of an enzyme.

It may be :

Non-specific or specific.

specific inhibitors:-Non

These are inhibitors which exert their effect on all enzymes or on

wide variety of enzymes; e.g.

Agents which precipitate or denaturate proteins.

Specific inhibitors:

These are inhibitors which exert their effect on one enzyme or

on a small group of related enzymes.

May be competitive or noncompetitive

*The molecule resembling the substrate.

*Can bind to the active site of the enzyme and so it can form enzyme inhibitor complex EI.

*Decreases the affinity of enzymes for substrate.

*Excessive concentrations of substrate will break the EI complex and then S can bind to the enzyme

*Reversible.

*It depends on Substrate and Inhibitor.

Enzymes Dr/ Faten


Examples of competitive inhibitors:

Allopurinol competes with hypoxanthine for xanthine oxidase inhibiting the formation of uric acid, so it is used

in treatment of hyperuricemia (gout).

Dicumarol or Warfarine compete with vitamin K, for epoxide

reductase, so they are used to reduce prothrombin synthesis.

Statins (e.g. atorvastatin) competes with HMGCoA for its reductase,so, it inhibits cholesterol synthesis.

Methotrexate competes with dihydrofolic acid for dihydrofolate

reductase, so, it inhibits DNA synthesis and used in treatment of cancers.

Succinate dehydrogenase

Succinate Fumarate + 2H++ 2e-

Competitive Inhibition

Product Substrate Competitive Inhibitor

Succinate Dehydrogenase

Does not have a structure like substrate

Binds to the enzyme but not active site

Changes the shape of enzyme and active site

Substrate cannot fit altered active site

No reaction occurs

Effect is not reversed by adding substrate

Enzymes Dr/ Faten


Enzyme Inhibition (Mechanism)

Non-Competitive Inhibition: Competitive Inhibition:

* A non-competitive inhibitor molecule is quite

different in structure from the substrate and

does not fit into the active site.

* It binds to another part of the enzyme

molecule, changing the shape of the whole

enzyme, including the active site, so that it can no

longer bind substrate molecules.

* Non-competitive inhibitors therefore simply

reduce the amount of active enzyme.

* A competitive inhibitor molecule has a

similar structure to the substrate molecule,

and so it can fit into the active site of the

enzyme.

* It therefore competes with the substrate for

the active site, so the reaction is slower.

* Increasing the concentration of substrate

restores the reaction rate and the inhibition is

usually temporary and reversible.

Allosteric inhibitors are low-molecular weight substances, they regulate the enzyme activity. -

- The inhibitor is not similar in structure to the substrate and it is bound to apoenzyme at sites far from the

active site this site is called allosteric site.

- It is not reversable by increasing the concentration of the substrate.

- This interaction causes conformational changes in the catalytic site that makes it unfavorable for binding to

Substrate.

Enzymes Dr/ Faten


- When Allosteric inhibitor is consumed the activity of the enzyme is regained ,so Allosteric inhibition is

reversible. e.g. glucose-6-phosphate is allosteric inhibitor for hexokinase enzyme.

- Some time conformational change of apoenzyme produce by binding to the allosteric effector, makw the

active center more fit to bind with the substrate, thus the enzyme activity is increased. In this case the

effector is called allosteric activator e.g. AMP and ADP are allosteric activator to phosphofructokinase.

End- product of a metabolic pathway

inhibits the initial enzyme in the pathway,

this called feed-back inhibition.

* Agents that block the coenzyme will stop enzyme action e.g. phenylhydrazine will block the aldehyde

group of pyridoxal phosphate, which is the coenzyme of transaminase.

* Agents that block the prosthetic group will stop enzyme action e.g.cyanide and carbon monoxide

block the iron of heme of cytochrome oxidase enzyme.

Enzymes Dr/ Faten


1. Covalent modification:(short term regulation)

The enzyme may be phosphorylated in the OH group of serine, threonine, or tyrosine. This process is

catalyzed by kinase. ATP is the source of phosphate.

The phosphoprotein produced is dephosphorylated by phosphatase.

phosphoprotein enzyme is the active form of the enzyme, e.g. glycogen phosphorylase.

the dephosphorylated form is the active form of the enzyme, e.g. glycogen synthase.

2. Induction and repression of enzyme synthesis

Enzymes synthesis (long term regulation ).

• Synthesis is stimulated by "induction" or inhibited by "repression".

• Control of enzyme synthesis is under hormonal control that affects gene

expression related to that particular enzyme.

• Example:

• Insulin induces synthesis of glucokinase and phosphofructokinase, but represses

glucose-6-phosphatase and fructose 1,6,bisphosphatase.

• Steroid and thyroid hormones effects on enzyme generation.

• Control of enzyme activity by induction or repression consumes hours or

days to be achieved.

3. Allosteric modification (short term regulation):

occurs within seconds or minutes

LECTURE 5

Enzymes Dr/ Faten


ISOENZYMES

These are different isomers of the same enzyme which differ by having a different electrophoretic mobility

and different tissue source.

Each of these is called isoenzyme and all of them have the same catalytic activity e.g. lactate dehydrogenase

(LDH) and creatin kinase (CK).

Isozymes are different molecular forms of enzymes that may be isolated from the same or different tissues.

Analysis of the distribution of isozymes of particular enzymes is sometimes a useful tool in clinical

diagnosis.

1) Creatine kinase (CK)

occurs as a dimer consisting of two subunits can be present as two distinct molecular forms

brain type (B) and muscle type (M).

Thus, three isozymes are possible CK-MM, CK-BB, and CK-MB.

These isozymes can be readily distinguished and quantitated by electrophoresis.

CK-MB is normally present in trace amounts only in the myocardium.

Elevation of CK-MB levels to greater than 6% of the total CK is diagnostic of a myocardial infarction.

CPK is made of three slightly different substances:

CPK-1 (also called CPK-BB) is found mostly in the brain and lungs.

CPK-2 (also called CPK-MB) is found mostly in the heart.

CPK-3 (also called CPK-MM) is found mostly in skeletal muscle.

2) Lactate dehydrogenase (LDH)

is a tetramer of four subunits which can be present as two distinct molecular forms.

Type H is found primarily in heart, and type M is found primarily in muscle or liver.

Five isozmes of LDH composed of different combinations of these subunits are possible:

M4, M3H, M2H2, MH3 and H4.

In normal serum the H3M isozyme is present. In an individual who has suffered a myocardial infarction,

the serum levels of the H-containing isozymes, particularly H4, are elevated.

The increase in H4 such that ratio of H4/H3M is greater than 1 confirms the diagnosis that the patient

suffered a myocardial infarction.

lightly in subunits:LDH exists in 5 isoenzymes , which differ s

LDH-1 HHHH is found primarily in heart muscle and red blood cells.

LDH-2 HHHM is concentrated in white blood cells.

LDH-3 HHMM is highest in the lung.

LDH-4 HMMM is highest in the kidney, placenta, and pancreas.

LDH-5 MMMM is highest in the liver and skeletal muscle.

Enzymes Dr/ Faten


The use of isozymes in the diagnosis of myocardial infarction

High levels of the MB isozyme of creatine (CK-MB) relative to other CK isozymes and the analysis of

lactate dehydrogenase (LDH) isozymes indicating higher serum levels of the H-containing isozymes,

particularly H4 with elevated ratio of H4/H3M greater than 1, are indicative with the clinical picture of

myocardial infarction.

Conditions other than myocardial infarction can cause similar changes in the isozyme patterns of either CK

or LDH, but it is unlikely that both patterns would be affected in this manner.

Therefore, data such as those described above can be reliably used to distinguish between the occurrence of

myocardial infarction and other conditions that may caused the reported symptoms.

Examples for medical important enzymes:

Lipase: it is elevated in acute pancreatitis and pancreatic carcinoma.

Amylase: it is elevated in parotitis, acute pancreatitis and pancreatic carcinoma.

Trypsin: also increased in pancreatic diseases.

Cholinesterase: is lowered in exposure to insecticides and is elevated in conditions of active heamopoiesis.

Serum alkaline phosphatase : it is elevated in :

1- Liver diseases as obstructive jaundice.

2- Bone diseases: rickets, osteogenic sarcoma and Paget's disease.

3- Hyperparathyroldism and malignancy.

Serum Acid Phosphatase is elevated in cancer prostate.

Lactic Acid Dehydrogenase (LDH) is elevated in cases of myocardial infarction.

Creatine Phosphokinase (CK) is elevated in cardiac and skeletal muscle diseases.

Transaminases :

1- Glutamic pyruvic transaminase GPT increased in any disease causing damage or destruction to liver

cells e.g. infective hepatitis.

2- Glutamic oxaloactic transaminase GOT is elevated in damage of heart e.g. coronary thrombosis and

liver disease.

Enzymes Dr/ Faten


Non-Functional plasma enzymes

These are enzymes which have no specific function in blood and having no substrate to act upon.

They are present in low concentration in blood and originally they are present in different organs inside their

cells.

If the cells of theses organs are destroyed by any disease, these enzymes are liberated and appear in higher

concentrations in blood.

So, the presence of these non-functional enzymes in higher concentrations than normal in blood is of great

clinical importance for diagnosis of such diseases.

Diagnostic Uses Assayed Enzyme

Prostate cancer Acid phosphatase

Viral hepatitis, liver Alanine aminotransferase (ALT)

Liver disease, bone disorders Alkaline phosphatase

Acute pancreatitis Amylase

Muscle disorders, heart attack Creatine kinase (CK)

Heart attack Lactate dehydrogenase (LDH)

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simple protein enzyme

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