Amino Acid

Chapter 3 - Amino Acids and Primary Structure of Proteins

Functions of proteins:

1- catalysts - enzymes for metabolic pathways

2- storage and transport - e.g. myoglobin and hemoglobin

3- structural - e.g. actin, myosin

4- mechanical work - movement of flagella and cilia, microtubule movement during mitosis, muscle contraction

5- decoding information - translation and gene expression

6- hormones and hormone receptors

7- specialized functions - e.g. antibodies

Structure of amino acids

There are 20 common amino acids called α-amino acids because they all have an amino (NH3+) group and a carboxyl group (COOH) attached to C-2 carbon (α carbon).

At pH of 7, amino group is protonated (-NH3+) and carboxyl group is ionized (COO-). The amino acid is called a zwitterion.

pKa of a carboxyl group = 1.8 - 2.5

pKa of a amino group = 8.7 - 10.7

The α carbon is chiral or asymmetric ( 4 different groups are attached to the carbon; exception is glycine.)

Amino acids exist as stereoisomers (same molecular formula, but differ in arrangement of groups).

Designated D(right) or L(left).

Amino acids used in nature are of L configuration.

carboxylate group at top --> points away side chain at bottom

α amino group orientation determines

NH3+ on left = L

NH3+ on right = D

Can also use RS system of nomenclature.

Structures of 20 common amino acids:

Amino acids are grouped based upon the properties and structures of side chains.

1) aliphatic (R groups consist of carbons and hydrogens)

glycine - R=H smallest a.a. with no chiral center

alanine - R=CH3 methyl group

valine R = branched; hydrophobic; important in protein folding

leucine R= 4 carbon branched side chain

isoleucine R = 2 chiral centers

proline R = ring; puts bends or kinks in proteins; contains a secondary amino group

2) aromatic (R groups have phenyl ring)

phenylalanine - very hydrophobic

tyrosine -...