Since amino-acids contain both amino (—NH3) and acidic or carboxyl group (—COOH) in their molecules, therefore, they may act as acids or alkali bases at a time.
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The molecules of such acidic and basic properties are known as amphoteric molecules. Due to amphoteric properties the amino acids unite with one another forming large and complex protein molecules.
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The amino-acids are linked together in the protein molecule in chains. So, when the amino-acids link together one after another during protein synthesis, then the terminal OH of carboxyl group (—COOH) of one amino-acid combines with one H of the amino group (—NH3) of the next amino-acid to form water which is removed from there and the amino-acid link is joined.
The —OC.NH link so formed at the place where the water molecule has been removed is known as peptide bond.
When two amino-acids are united by peptide bond, the resultant product thus formed, is known as dipeptide.
This sort of process of linking can go on repeatedly to form tripeptide (with three amino-acid links) or polypeptides with larger number of amino-acid links.
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When a polypeptide contains fifty-or more amino-acid links it begins to show the characteristics of protein. Usually large peptides are sometimes also called peptones.
A polypeptide chain of amino-acids is an unbranched structure and an amino-acid unit in a polypeptide chain is called a residue.
A polypeptide chain has direction because its building blocks have different ends-namely, the a-amino and the a-carboxyl groups.
By contention, the amino end is taken to be the beginning of a polypeptide chain. The sequence of amino-acids in a polypeptide chain is written starting with the amino-terminal residue.
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Thus, in the tripeptide alanine-glycine-tryptophan, alanine is the amino-terminal residue and tryptophan is the carboxyl-terminal residue.
A polypeptide chain consists of a regularly repeating part, called the main chain, and a variable part, the distinctive side chain. The main chain is sometimes termed the backbone.
In some proteins, a few side chains are crossed-linked by disulphide bonds. These cross-bonds are formed by the oxidation of cysteine residues.
The resulting disulphide is called cystine. There are no other covalent cross-bonds in proteins.
Many proteins, such as myoglobin, consist of a single polypeptide chain. Others may contain two or more chains which may be same or different.
For example, haemoglobin is made up of feur chains out of which two are of one kind and two of another kind.
These four chains are held together by non-covalent forces. Alternatively, the polypeptide chains of some proteins are linked by disulphide bonds as in case of insulin.
Proteins have unique amino-acid sequences that are specified by genes, i.e., the amino-acid sequences of proteins are genetically determined.
The sequence of nucleotides in deoxyribonucleic acid specifies a complementary sequence of nucleotides in a ribonucleic acid (RNA), which in turn specifies the amino-acid sequences of a protein.