Amino acids (2-aminocarboxylic acids)are the basic structural units of proteins. An amino acid consists of a central α carbon (Cα)bonded to four different chemical groups: one bond to an amino group (—NH2),one to a carboxyl group (—COOH),one to a hydrogen atom (H),and one to a variable side chain (R).The side chain is the major determinant of the individual functional property of each amino acid in a protein. The α carbon is asymmetric except in glycine ; therefore, amino acids exist in two mirror-image forms, D (dextro)and L (levo) isomers. Only the L forms occur in proteins,with rare exceptions. Amino acids are ionized in neutral solutions, the amino group taking on a proton (—NH3 + ) and the carboxyl group dissociating (—COO –).
Amino acids are classified according to their side chains and chemical reactivity. Each amino acid has its own three-letter and one-letter abbreviations. Essential amino acids for humans are valine (Val), leucine (Leu), isoleucine (Ile), phenylalanine (Phe), tryptophan (Trp),methionine (Met),threonine (Thr),and lysine (Lys).These have to be supplied by food intake.
A. Neutral amino acids
Simple amino acids have an aliphatic side chain, e.g.,glycine has a hydrogen atom (—H)and alanine a methyl group (—CH3),or a larger, hydrophobic (water-repellent)side chain as in valine, leucine, and isoleucine. Proline has an aliphatic side chain that is bound to both the central carbon and to the amino group in a ring
structure. Hydrophobic aromatic side chains occur in phenylalanine (a phenyl group bound via a methylene [—CH2 —]group)and tryptophan (an indol ring bound via a methylene group). Two hydrophobic amino acids contain sulfur (S)atoms: cysteine with a sulfhydryl group (—SH)and methionine with a thioether (—S —CH3).The sulfhydryl group in cysteine is very reactive and forms stabilizing disulfide bonds (—S —S —). These play an important role in stabilizing the three-dimensional forms of proteins. Selenocycsteine is a cysteine analogue occurring in a few proteins such as the enzyme glutathione peroxidase.
B. Hydrophilic amino acids
Serine, threonine, and tyrosine contain hydroxyl groups (—OH).Thus, they are hydrolyzed forms of glycine, alanine, and phenylalanine. The hydroxyl groups make them hydrophilic and more reactive than the nonhydrolyzed forms.Asparagine and glutamine both contain an amino and an amide group. At physiological pH their side chains are negatively charged.
C. Charged amino acids
These amino acids have either two ionized amino groups (basic)or two carboxyl groups (acidic).Basic amino acids (positively charged) are arginine,lysine,and histidine.Histidine has an imidazole ring and can be uncharged or positively charged, depending on its surroundings.
It is frequently found in the reactive centers of proteins, where it takes part in alternating bonds (e.g.,in the oxygen-binding region of hemoglobin).Aspartic acid and glutamic acid each have two carboxyl groups (—COOH)and thus are usually acidic.Seven of the 20 amino acids have slightly ionizable side chains, making them highly reactive (Asn,Glu,His,Cys, Tyr,Lys,Arg).