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 -amino acids because they all have an amino (NH3
+)
group and a carboxyl group (COOH) attached to C-2 carbon (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 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
amino group orientation determines
NH3
+ on left = L
NH3
+ on right = D
Can also use RS system of nomenclature.
2
Structures of 20 common amino acids: