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Date Submitted: 04/02/2012 08:40 PM
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 -...