These units have been so named, on account of having a-basic amino group (—NH3) and carboxyl group (—COOH) in their constitution.
Thus, proteins are polymers of amino-acids but the amount, the number and the arrangement of the amino-acids may differ in different protein molecules.
Amino acids are the basic structural units of proteins; therefore, Emil Fischer called them as building stones of the proteins.
An amino acid consists of an amino group (—NH3), a carboxyl group (—COOH), a hydrogen atom, and a distinctive R group bonded to a carbon atom, which is called the ot-carbon An R group is referred to as a side chain.
Amino acids in solution at neutral pH are predominantly dipolar ions (or Zwitterkms) rather than un-ionized molecules.
In the dipolar form of an amino acid, the amino group is protonated (—NH3+) and the carboxyl group is dissociated (—COO-).
The ionization state of an amino acid varies with pH. In acid solution, the carboxyl group is un-ionized (—COOH) and the amino group is ionized (—NH3+).
In alkaline solution (e. g., pH 11), the carboxyl group is ionized (—COO–) and the amino group is un-ionized (—NHt).
More than twenty amino acids have been recorded so far which are used by the living organisms to form the proteins.
It is not necessary that all amino acids should found in even’ protein. Some of the amino acids are referred as essential amino acids because they cannot be synthesized by the organisms.
These, therefore, must be included in the diet of the organisms from any external source. The remaining amino acids are referred to as dispensible or non-essential amino acids because they can be synthesized very well by the organisms themselves provided the other nutrient elements along with nitrogen and carbon compounds continue to be available.
Threonine, phenylalanine, lysine, tryptophan, valine, methionine, leucine, isoleucine, arginine are the essential amino acids, while alanine, aspartic acid, cysteine, glutamic acid, glycine, proline, serine, tyrosine are the nonessential amino-acids of man.
All amino acids except glycine are optically active because of the asymmetry of the a-carbon atom, which is covalently bonded to four different groups.
It is, therefore, possible to have a D-form and L-form of each amino acid. Only L-amino acids are constituents of proteins.
Twenty kinds of side chains varying in size, shape, charge, dehydrogen-bonding capacity, and chemical activity are commonly found in proteins.
Indeed, all proteins in all species, from bacteria to man, are constructed from the same set of twenty amino acids.
The remarkable range of functions mediated by proteins results from the diversity and versatility of these twenty kinds of building blocks because they create the intricate three-dimensional structure for proteins that enable proteins to participate in so many biological processes.