Proteins are the most abundant macromolecules in living cells. The name protein is derived from the Greek word ‘proteios’ meaning ‘of prime importance’. These are high molecular mass complex amino acids.
You will study about amino acids in the next section. Proteins are most essential class of biomolecules because they play the most important role in all biological processes. A living system contains thousands of different proteins for its various functions. In our every day food pulses, eggs, meat and milk are rich sources of proteins and are must for a balanced diet. Proteins are molecular tools that perform an astonishing variety of functions. In addition to serving as structural materials in all living organisms (e.g., actin and myosin in animal muscle cells), proteins are involved in such diverse functions as catalysis, metabolic regulation, transport, and defense. Proteins are composed of one or more polypeptides, unbranched polymers of 20 different amino acids. The genomes of most organisms specify the amino acid sequences of thousands or tens of thousands of proteins. Proteins are a diverse group of macromolecules. This diversity is directly related to the combinatorial possibilities of the 20 amino acid monomers. Amino acids can be theoretically linked to form protein molecules in any imaginable size or sequence.
An important reason for this remarkable discrepancy is demonstrated by the complex set of structural and functional properties of naturally occurring proteins that have evolved over billions of years in response to selection pressure. Among these are (1) structural features that make protein folding a relatively rapid and successful process, (2) the presence of binding sites that are specific for one or a small group of molecules, (3) an appropriate balance of structural flexibility and rigidity so that function is maintained, (4) surface structure that is appropriate for a protein’s immediate environment (i.e., hydrophobic in membranesand hydrophilic in cytoplasm), and (5) vulnerability of proteins to degradation reactions when they become damaged or no longer useful. Proteins can be distinguished based on their number of amino acids (called amino acid residues), their overall amino acyl composition, and their amino acid sequence. Molecules with molecular weights ranging from several thousand to several million daltons are called polypeptides. Those with low molecular weights, typically consisting of fewer than 50 amino acids, are called peptides. The term protein describes molecules with more than 50 amino acids. Each protein consists of one or more polypeptide chains.
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