AMINO ACID AND PROTEIN CHEMISTERY

AMINO ACID AND PROTEIN CHEMISTERY Proteins are the most abundant and functionally diverse molecules in living systems where they constitute 50% or more of their dry mass.The word protein is derived from the Greek Protos, which means the first or supreme. Proteins are nitrogenous macromolecules,composed of aminoacids linked by peptide bond.

AMINO ACIDSAmino acids are organic solvents.Have two functional groups NH and -COOH group.The amino group is basic while carboxylic group is acidic in nature.Soluble in water but insoluble in organic solvents e.g chloroform,acetone,ether,etc.All amino acids which make up proteins are L--aminoacids.All amino acids have chiral carbon, exept Glycine.

CLASSIFICATION OF AMINO ACIDSAlthough more than 300 naturally occurring amino acids are known but only 20 amino acids take part in the formation of all types of proteins,plants as well as animal in origion.These 20 amino acids are known as Primary,Standard or normal amino acids.Each of these amino acids has one or more genetic codon(s) which are present within the molecules of specific mRNA which themselves are produced under direction of genes occuring in DNA molecules.

Semi-essential aminoacids. These include Arginine and Histidine.These are growth promoting factors since they are not synthesized in sufficient quantity during growth.

6.CitrulineOrnithineArgininosuccinic acidThese three amino acids occur in the liver,where they takepart in the formation of urea.7.Pantathenic acid. It is a widely distributed vitamin.It forms a part of the molecule of Co.enzymeA.8.Homocysteine

9.Others includeN-methyllysine,Found in myosinCarboxyglutamate,Found in ProthrombinDesmosine,Found in elastinPEPTIDE BOND

Proteins are made by controlled polymerization of amino acids

37Peptide bond formation is not spontaneous (favorable in free energy) under normal conditions, and requires conversion of the carboxyl or the amino group to a form which reacts more readily. In general, the carboxylic acid moiety is activated by converting it to an ester, acid chloride, anhydride, azide or some other derivative that makes the carbonyl group more nucleophilic.Note that once you make a peptide bond, the peptide backbone is fairly chemically inert: the nitrogen is no longer as strong a nucleophile, and the linkage is pretty stable and slow to hydrolyze. Proteins are typically digested exhaustively by heating to near boiling temperatures in reasonably strong solutions of hydrochloric acid. As we will learn later, there are of course enzymes that hydrolyze proteins quite efficiently. Classification of peptidesDIPEPTIDESOLIGOPEPTIDES Composed of 3-10 amino acids linked together through peptide bond.e.g Tripeptide(3 amino acids linked together by 2 peptide bonds),Tetrapeptide(4 amino acids linked together by 3 peptide bonds).

UNUSUAL PEPTIDE BONDIn some cases the peptide bond in a peptide does not involve -COOH group.e.g Glutathione which has the sequence glutamic acid,cysteine and glycine.However,the COOH group of glutamic acid forming peptide bond with cysteine is not but .For this reason glutathione is chemically -glutamyl-cysteinyl-glycine.