Amino acids, peptides and proteins Kharkiv National Medical University Department of Medical and Bioorganic chemistry «Biological and Bioorganic Chemistry» Lecture № 4 Amino acids, peptides and proteins Lecturer: As. Professor, Department of Medical and Bioorganic Chemistry,, Ph.D. Levashova O.L.

Plan of lecture Classification of amino acids. Stereoisomerism. Ionic state Quantitative reactions of amino acids. Specific reactions of α, β and γ-amino acids. Biologically important chemical reactions. Peptides and proteins. Primary, secondary and tertiary protein structure.

Amino acid α-aminobutyric acid β -aminobutyric acid γ -aminobutyric acid Nearly all the naturally occurring amino acids are α-amino acids.

Stereoisomerism

all naturally occurring amino acids belong to L-series

Aliphatic Amino acids found in proteins Alanine (Ala) Glycine (Gly) Leucine (Leu) Valine (Val) Isoleucine (Ile)

Amino acids containing OH-group Serine (Ser) Threonine (Thr)

Amino acids containing COOH-group Aspartic acid (Asp) Glutamic acid (Glu)

Amino acids containing NH2 CO-group Asparagine (Asn) Glutamine (Gln)

Amino acids containing NH2 -group Lysine (Lys) Arginine (Arg)

Sulfur containing Amino acids Cysteine (Cys) Methionine (Met)

Aromatic Amino acids Phenylalanine (Phe) Tyrosine(Tyr)

Heterocyclic Amino acids Tryptophan (Trp) Histidine (His) Proline (Pro)

Essential amino acids Valine Leucine Isoleucine Threonine Lysine Essential amino acids – are amino acids which can not be synthesized in the human body and must be supplied to the diet. Valine Leucine Isoleucine Threonine Lysine Methionine Phenylalanine Thryptophan

Ionic state zwitterion cation anion

Chemical properties 1. Amphoteric character H2O

Specific reactions α-amino acids Dipeptide + H2O a) b) Diketopiperazine

Specific reactions β-amino acids Unsaturated carboxylic acid

Specific reactions γ-amino acids Lactam (cyclic amide) - H2O

Biologically important chemical reactions 1. Transamination. Transamination – is the main way of biosynthesis of α-amino acids from α-oxo acids in the organism. The process takes place in the presence of enzymes – transaminases and coenzyme – pyridoxal phosphate (vit B6).

2. Deamination. a) reductive deamination b) oxydative deamination

3. Decarboxytation. Decarboxylation takes place in the organism under the influence of enzymes called decarboxylases and pyridoxal phosphate coenzyme (vitamin B6). Lys

His Histamine (participates in allergic reactions) Glu α β γ γ-aminobutyric acid (neuromediator)

Tryptophan 5-hydroxytryptophan Serotonine 5-hydroxy-β-indolyl-acetic acid

Peptides and proteins + H2O

Classification of proteins on the basis of molecular structure Proteins Globular albumins, enzymes, hormones etc. Fibrous keratin, in skin, hair, nails and wool, collagen in tendons, fibroin in silk, myosin in muscle, etc.

Proteins on the basis of hydrolysis products Conjugated Simple nucleoproteins, glycoproteins, lipoproteins, phosphoproteins, metallo-proteins (hemoglobin). Simple albumins, globulins, keratin etc.

Structure of proteins Primary structure Primary structure – is the sequence in which the amino acids are linked in polypeptide chain. Gly-Ala-Gly

Secondary structure Secondary structure The secondary structure - is the manner in which the polypeptide chains are arranged. Secondary structure α-Helix structure β-Pleated sheet structure

α-Helix structure is right-handed helix with 3.6 amino acid residues per turn. Examples: myosin (found in muscles), keratin (hair, wool, nails). С О N H

β-Pleated sheet structure In this structure, the long peptide chains lie side by side in a zigzag manner to form a flat sheet. Each chain is held to the two neighboring chains by hydrogen bonds. Silk fibroin has this type of structure.

Tertiary structure is three-dimensional stricture. This structure gives the overall shape of proteins. The tertiary structure of a protein can be obtained due to folding and superimposition of various secondary structures. The examples of the proteins having tertiary structures are globular proteins. Myoglobin

Quaternary structure Quaternary structure – is the overall structure of a protein having multiple subunits. Many proteins exist as stable and ordered noncovalent aggregates of more than one polypeptide chain. The quaternary structure of hemoglobin, for example, involves four subunits. Hemoglobin

Forces that stabilize protein structures NH3 + N H Disulphide bond Ionic Hydrogen Hydrophobic interaction

Thank you for your attention!