Lecture on the topic: Аmino acids and proteins.For the 1st year students of specialty “General

medicine".Discipline: Chemistry.Lecturer: PhD, Associate Professor

Vlassova Lenina.Karaganda 2014

Karaganda State Medical University. Chemistry department.

The purpose of the lecture is the study of properties of amino acids, as well as their structure, chemical properties and the biological role of proteins and peptides. The main objectives 1. You need to know the biochemical classification of amino acids.  2. You need to understand the structure and chemical properties of amino acids.  3.You should be aware of the structural organization of the molecules of protein and to characterize the biological significance of proteins and peptides....

Amino acides - are bifunctional organic compounds which contain a carboxyl group-COOH and an amino group-NH2.Depending on the relative position of both functional groups it are distinguished on ά-, β -, γ-amino acids, and so on:

The 20 most important α-amino acids encoded by the genetic code are included in the polypeptide chains during protein synthesis.The general formula of α-amino acids:

Amino acids can be classified according to several criteria:

1) By ability of body to synthesize amino acids from predecessors: amino acids are divided into essential and nonessential.

Essential: tryptophan, phenylalanine, lysine, threonine, methionine, leucine, isoleucine, valine.Nonessential: tyrosine, cysteine , histidine, arginine, glycine, alanine, serine, glutamine, glutamic acid, aspartic acid, asparagine, proline.

2) Amino acids are divided into proteinogenic (20 α-amino acids) and non proteinogenic (4 amino acids).

3). Amino acids are classified according to the chemical nature of the radicals (R-groups)The most important proteinogenic amino acids.

Common name 3-letter code formula

1. Amino acids having non-polar radical

alanine ALA

valine VAL

leucine LEU

NH2 - CH - COOH

CH3

NH2 - CH - COOH

CH3 CH3

CH

NH2 - CH - COOH

CH3 CH3

CH

CH2

isoleucine ILE

tryptophan TRP

proline PRO

NH2 - CH - COOH

CH3

CH3CH

CH2

NH2 - CH - COOH

CH2

N

H

N

H

COOH

phenylalanine PHE

methionine МЕТ

NH2 - CH - COOH

CH2

CH3

NH2 - CH - COOH

(CH2)2

S

2. Amino acids with uncharged polar radical.

glycine GLY

serine SER

threonine THR

tyrosine TYR

H

NH2 - CH - COOH

NH2 - CH - COOH

CH2

OHNH2 - CH - COOH

CH3

CH OH

NH2 - CH - COOH

CH2

OH

asparagine АСN

glutamine GLN

cysteine 

CYS

NH2 - CH - COOH

CH2

C ONH2

NH2 - CH - COOH(CH2)2

C ONH2

NH2 - CH - COOH

CH2

SH

3. Amino acids having a negatively charged radical.

aspartic acid ASP

glutamic acid GLU

NH2 - CH - COOH

CH2

COOH

(CH2)2

NH2 - CH - COOH

COOH

4. Amino acids with positively charged radical.

lysine LYS

arginine ARG

histidine HIS

(CH2)4

NH2

NH2 - CH - COOH

(CH2)3

C

NH2

NH2 - CH - COOH

NH

NH

NH2 - CH - COOH

CH2

H

N

N

1. NH2 - CH2 - CH2 - COOH

(CH2)3

NH2

2. NH2 - CH - COOH

(CH2)3

C

NH2

3. NH2 - CH - COOH

NH

O

4. NH2 - CH2 - CH2 - CH2 - COOH

Essential nonproteinogenic amino acids.

β – Alanine

Ornithine

Citrulline

γ - Aminobutyric acid

Chemical properties.1.The Acid - base properties. Amino acids - are organic amphoteric compounds,

because they contain two functional groups - the amino group with basic properties and the carboxyl group having acidic properties. Therefore, the amino acids react with acids and bases :

2. Transamination - it is one of the reactions of amino acids metabolism, it is transportation of the amino group (NH2) of the amino acid to the keto acid; thereby forming an other keto acid and other amino acid.For example, glutamic acid is reacted with pyruvic acid to form the a-ketoglutaric acid and alanine.

3. Desamidization- is splitting off amino group (-NH2) from molecule of organic compounds. Desamidization can be of several types depending on the reaction conditions: reductive, hydrolytic, intramolecular and oxidative.

4. Decarboxylation - a process of elimination the carboxyl groups of amino acids in the form of CO2, resulting in the formation of reaction products - biogenic amines, having a strong pharmacological action for a plurality of physiological functions of human and animals.

5. Transformations of amino acids by heating.α-Amino acids and their esters more easily by heating form a cyclic peptides - diketopiperazines:

By heating, β-amino acids form α, β-unsaturated acids.This process is linked with elimination of amino group and hydrogen atom from adjacent carbon atoms;

γ-amino acids easily cleaved off water and it are cyclized to form the internal amides - the lactams:

6. Qualitative reaction into amino acids.A) Ninhydrin reaction - the general qualitative reaction of α-amino acids.All amino acids are oxidized with ninhydrin to form products stained blue-purple color.

Ninhydrin reaction

B) Xanthoprotein reaction - a reaction for the detection of aromatic and heterocyclic alpha-amino acids (from the Greek. Xanthos - yellow).C) Biuret reaction is used to detect the peptide bonds in peptides and proteins.

Biuret reaction.

Proteins - a natural polypeptides

with high molecular weight (from 10 000 to tens of millions) consisting of more than 100 amino acid residues.They are part of all living organisms and carry out various biological functions.

Proteins.

There are four levels in the structure of the polypeptide chain.

Primary structure of the

protein -is a specific

sequence of amino acids in the polypeptide

chain.

Peptide bond formation.

Secondary structure.

Secondary structure - is a polypeptide chain conformation, ie twisting way in the space behind the chain by hydrogen bonds between NH and CO groups.Two ways of stacking chains exist. This is the α - helix and β - sheet.

Tertiary structure of the proteins.

The tertiary structure of the proteins – is a three-dimensional configuration of a swirling spiral space. The tertiary structure is formed by disulfide bridges-S-S-between cysteine residues located in different places polypeptide chain , ionic interactions of oppositely charged groups of NH3 + and COO-, and hydrophobic interactions.

Quaternary structure.

The quaternary structure is formed by the interaction of several polypeptide chains.

The organization of the molecule of protein.

Thank you.