Proteins Silk, Keratin, Etc

Proteins are polymers, but are quite different from those discussed above. In cellulose and starch, the constituting unit (monomer) is a single chemical entity, glucose, and the polymer consists of a chain of a large number of this same unit connected. Not a single but about 20 different monomers called amino acids constitute a protein (polymer). The formation of proteins in our body was talked about in the previous chapter. 

Amino acid has a general chemical formula (NHj)CH(R)COOH, and its chemical identity is defined by the group R. Some of amino acids are glycine wit R=H, alanine with R=CHj, serine with R=OH, cysteine with R=SH, histidine with R=imidazole (C N Hj), glutamic acid with R=CH2CHjCOOH, and tyrosine with 

R=CHj(qH )OH. Two amino acids, (NHj)CH(R )COOH and (NHj)CH(R2)COOH, condense to form a dipeptide connected dirough a peptide bond (- ONH-)  

The third amino acid (NHj)CH(R )COOH then attaches itself to the end, and so on. The resulting long chain of amino acids is a polypeptide, and a protein can be a polypeptide or an ensanble of polypeptides. 

The relative spatial arrangement of amino acids in a polypeptide can take, approximately speaking, three forms a-helix, P-strand, and random coil. They are sketched in Fig. 5.4. By the way, these arrangements of amino adds are said to be of secondary structure of a polypeptide. The primary structure is the ordering of amino acids in the polypeptide chain. The tertiary structure is the overall three-dimensional structure of a polypeptide. Some proteins like hemoglobin consist of more than two polypeptides. This is the quaternary structure. A protein of a single polypeptide can take helix strnctnre in some portions and f)-strand structure or random coil in other portions, as illnstrated in Fig. 5.4. 

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