Proteases are divided into two major groups, i.e., exopeptidases and endopeptidases. Exopeptidases cleave the peptide bond very close to the amino or carboxy terminal of the substrate, while endopeptidases cleave peptide bonds distant from the amino and carboxy terminal of the substrate. As well as based on the functional group present at the active site, proteases are further classified into four prominent groups, i.e., serine proteases, aspartic proteases, cysteine proteases and metallo-proteases.

Based on their site of action at the amino and carboxy terminus, they are classified as Aminopeptidase and Carboxypeptidase, respectively. Aminopeptidases act at a free N terminus of the polypeptide chain and liberate a single amino acid residue, a dipeptide, or a tripeptide Aminopeptidases occur in a wide variety of microbial species including bacteria and fungi. A leucine aminopeptidase was purified about 670 fold from germinated grains of barley (Hordeum vulgare). The Bacillus licheniformis aminopeptidase has a molecular weight of 34.00 kDa and Bacillus stearothermophilus produces aminopeptidase. The Carboxypeptidase act at C terminals of the polypeptide chain and liberate a single amino acid or a dipeptide. Carboxypeptidases are divided into three major groups, serine carboxypeptidases, metallo-carboxypeptidases, and cysteine carboxypeptidases. The serine carboxypeptidases are isolated from Penicillium sp., Saccharomyces sp., and Aspergillus sp. Metallocarboxypeptidases are isolated from Saccharomyces sp. and Pseudomonas sp. which are hydrolyze the peptides in which the peptidyl group is replaced by a pteroyl moiety or by acyl groups.

Metalloproteases are the most diverse of the catalytic types of proteases which are characterized by the requirement for a divalent metal ion for their activity. They include enzymes from a variety of biological and native origins such as collagenases from higher organisms, hemorrhagic toxins from snake venoms, and thermolysin from bacteria. Based on the specificity of their action, metalloproteases can be divided into various groups - neutral, acid and alkaline. Extracellular proteases are important for the hydrolysis of proteins in cell-free environments and enable the cell to absorb and utilize hydrolytic products. At the same time, these extracellular proteases have also been commercially exploited to assist protein degradation in various industrial processes.

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Dr. Kirti Rani Sharma,
Assistant Professor (II),
Amity Institute of Biotechnology,
Amity University Uttar Pradesh, Noida
Sec-125, Gautam Buddha Nagar, Noida-201303 (UP), India.
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