amino&acids,&pep.des,&and& proteins& amino... · •...
TRANSCRIPT
Amino Acids, Pep.des, and Proteins
1
• Amino acids contain a basic amino group and an acidic carboxyl group
• Joined as amides between the –NH2 of one amino acid and the –CO2H to the next amino acid
• Chains with fewer than 50 units are called pep.des • Protein: large chains that have structural or cataly.c func.ons
in biology
Proteins – Amides from Amino Acids
2
• Amino acids are the fundamental building blocks of proteins
• To see how amino acids are incorporated into proteins and the structures of proteins
Why this Chapter?
3
• In neutral solu.on, the COOH is ionized and the NH2 is protonated
• The resul.ng structures have “+” and “-‐” charges (a dipolar ion, or zwi,erion)
• They are like ionic salts in solu.on
Structures of Amino Acids
4
• 20 amino acids form amides in proteins • All are α-‐amino acids -‐ the amino and carboxyl are connected to the same C • They differ by the other subs.tuent aWached to the α carbon, called the
side chain, with H as the fourth subs.tuent • Proline is a five-‐membered secondary amine, with N and the α C part of a
five-‐membered ring • See table 26.1 to examine names, abbrevia.ons, physical proper.es, and
structures of 20 commonly occurring amino acids
The Common Amino Acids
5
Alanine A, Ala
Arginine R, Arg Asparagine N, Asn
Aspar.c acid D, Asp Cysteine C, Cys
Glutamine Q, Gln
Glutamic Acid E, Glu Glycine G, Gly
His.dine H, His Isoleucine I, Ile
Leucine L, Leu Lysine K, Lys Methionine M, Met Phenylalanine F, Phe Proline P, Pro Serine S, Ser Threonine T, Thr Tryptophan W, Trp Tyrosine Y, Tyr Valine V, Val
Abbrevia.ons and Codes
6
• Glycine, 2-‐amino-‐ace.c acid, is achiral • In all the others, the α carbons of the amino acids are centers
of chirality • The stereochemical reference for amino acids is the Fischer
projec.on of L-‐serine • Proteins are derived exclusively from L-‐amino acids
Chirality of Amino Acids
7
• Neutral: Fibeen of the twenty have neutral side chains
• Asp and Glu have a second COOH and are acidic • Lys, Arg, His have addi.onal basic amino groups side chains (the N in tryptophan is a very weak base)
• Cys, Ser, Tyr (OH and SH) are weak acids that are good nucleophiles
Types of side chains
8
• Contains an imidazole ring that is par.ally protonated in neutral solu.on
• Only the pyridine-‐like, doubly bonded nitrogen in his.dine is basic. The pyrrole-‐like singly bonded nitrogen is nonbasic because its lone pair of electrons is part of the 6 π electron aroma.c imidazole ring.
His.dine
9
• All 20 of the amino acids are necessary for protein synthesis
• Humans can synthesize only 10 of the 20
• The other 10 must be obtained from food
Essen.al Amino Acids
10
• In acidic solu.on, the carboxylate and amine are in their conjugate acid forms, an overall ca.on
• In basic solu.on, the groups are in their base forms, an overall anion
• In neutral solu.on ca.on and anion forms are present • This pH where the overall charge is 0 is the isoelectric point, pI
Amino Acids, the Henderson Hasselbalch Equa.on, and Isoelectric Points
11
• If pKa values for an amino acid are known the frac.ons of each protona.on state can be calculated (Henderson-‐Hasselbach Equa.on)
• pH = pKa – log [A-‐]/[HA] • This permits a .tra.on curve to be calculated or pKa to be
determined from a .tra.on curve
Titra.on Curves of Amino Acids
12
• The 15 amino acids with thiol, hydroxyl groups or pure hydrocarbon side chains have pI = 5.0 to 6.5 (average of the pKa’s)
• D and E have acidic side chains and a lower pI • H, R, K have basic side chains and higher pI
pI Depends on Side Chain
13
• Proteins have an overall pI that depends on the net acidity/basicity of the side chains
• The differences in pI can be used for separa.ng proteins on a solid phase permeated with liquid
• Different amino acids migrate at different rates, depending on their isoelectric points and on the pH of the aqueous buffer
Electrophoresis
14
• Bromina.on of a carboxylic acid by treatment with Br2 and PBr3 then use NH3 or phthalimide to displace Br
Synthesis of Amino Acids
15
• Based on malonic ester synthesis. • Convert diethyl acetamidomalonate into enolate ion with base,
followed by alkyla.on with a primary alkyl halide • Hydrolysis of the amide protec.ng group and the esters and
decarboxyla.on yields an α-‐amino acid
The Amidomalonate Synthesis
16
• Reac.on of an α-‐keto acid with NH3 and a reducing agent produces an α-‐amino acid
Reduc.ve Amina.on of α-‐Keto Acids
17
• Amino acids (except glycine) are chiral and pure enan.omers are required for any protein or pep.de synthesis
• Resolu.on of racemic mixtures is inherently ineffecient since at least half the material is discarded
• An efficient alterna.ve is enan.oselec.ve synthesis
Enan.oselec.ve Synthesis of Amino Acids
18
• Chiral reac.on catalyst creates diastereomeric transi.on states that lead to an excess of one enan.omeric product
• Hydrogena.on of a Z enamido acid with a chiral hydrogena.on catalyst produces S enan.omer selec.vely
Enan.oselec.ve Synthesis of Amino Acids (cont’d)
19
• Proteins and pep.des are amino acid polymers in which the individual amino acid units, called residues, are linked together by amide bonds, or pep.de bonds
• An amino group from one residue forms an amide bond with the carboxyl of a second residue
Pep.des and Proteins
20
• Two dipep.des can result from reac.on between A and S, depending on which COOH reacts with which NH2 we get AS or SA
• The long, repe..ve sequence of ⎯N⎯CH⎯CO⎯ atoms that make up a con.nuous chain is called the protein’s backbone
• Pep.des are always wriWen with the N-‐terminal amino acid (the one with the free ⎯NH2 group) on the leb and the C-‐terminal amino acid (the one with the free ⎯CO2H group) on the right
• Alanylserine is abbreviated Ala-‐Ser (or A-‐S), and serylalanine is abbreviated Ser-‐Ala (or S-‐A)
Pep.de Linkages
21
• The sequence of amino acids in a pure protein is specified gene.cally
• If a protein is isolated it can be analyzed for its sequence • The composi.on of amino acids can be obtained by automated
chromatography and quan.ta.ve measurement of eluted materials using a reac.on with ninhydrin that produces an intense purple color
Amino Acid Analysis of Pep.des
22
• The Edman degrada.on cleaves amino acids one at a .me from the N-‐terminus and forms a detectable, separable deriva.ve for each amino acid
Pep.de Sequencing: The Edman Degrada.on
23
• Pep.de synthesis requires that different amide bonds must be formed in a desired sequence
• The growing chain is protected at the carboxyl terminal and added amino acids are N-‐protected
• Aber pep.de bond forma.on, N-‐protec.on is removed
Pep.de Synthesis
24
• Usually converted into methyl or benzyl esters • Removed by mild hydrolysis with aqueous NaOH • Benzyl esters are cleaved by cataly.c hydrogenolysis of the
weak benzylic C–O bond
Carboxyl Protec.ng Groups
25
• An amide that is less stable than the protein amide is formed and then removed
• The tert-‐butoxycarbonyl amide (BOC) protec.ng group is introduced with di-‐tert-‐butyl dicarbonate
• Removed by brief treatment with trifluoroace.c acid
Amino Group Protec.on
26
• Amides are formed by trea.ng a mixture of an acid and amine with dicyclohexylcarbodiimide (DCC)
Pep.de Coupling
27
Overall Steps in Pep.de Synthesis
28
• Pep.des are connected to beads of polystyrene, reacted, cycled and cleaved at the end
Automated Pep.de Synthesis: The Merrifield Solid-‐Phase Technique
29
• The primary structure of a protein is simply the amino acid sequence.
• The secondary structure of a protein describes how segments of the pep.de backbone orient into a regular paWern.
• The terHary structure describes how the en.re protein molecule coils into an overall three-‐dimensional shape.
• The quaternary structure describes how different protein molecules come together to yield large aggregate structures
Protein Structure
30
• α-‐Helix stabilized by H-‐bonds between amide N–H groups and C=O groups four residues away
α-‐Helix
31
• β-‐pleated sheet secondary structure is exhibited by polypep.de chains lined up in a parallel arrangement, and held together by hydrogen bonds between chains
β-‐Pleated Sheet
32
• The ter.ary structure of a globular protein is the result of many intramolecular aWrac.ons that can be disrupted by a change of the environment, causing the protein to become denatured
• Solubility is dras.cally decreased as in hea.ng egg white, where the albumins unfold and coagulate
• Enzymes also lose all cataly.c ac.vity when denatured
Denatura.on of Proteins
33
• An enzyme is a protein that acts as a catalyst for a biological reac.on.
• Most enzymes are specific for substrates while enzymes involved in diges.on, such as papain aWack many substrates
Enzymes and Coenzymes
34
• Enzymes are usually grouped according to the kind of reac.on they catalyze, not by their structures
Types of Enzymes by Func.on
35
• Citrate synthase catalyzes a mixed Claisen condensa.on of acetyl CoA and oxaloacetate to give citrate
• Normally Claisen condensa.ons require a strong base in an alcohol solvent but citrate synthetase operates in neutral solu.on
How Do Enzymes Work? Citrate Synthase
36
• Determined by X-‐ray crystallography
• Enzyme is very large compared to substrates, crea.ng a complete environment for the reac.on
The Structure of Citrate Synthase
37
• A cleb with func.onal groups binds oxaloacetate
• Another cleb opens for acetyl CoA with H 274 and D 375, which have carboxylate that abstract a proton from acetyl CoA
• The enolate (stabilized by a ca.on) adds to the carbonyl group of oxaloacetate
• The thiol ester in citryl CoA is hydrolyzed
Mechanism of Citrate Synthetase
38