Re: What is a Protease?

PROTEASES (proteinases) are large group of ENZYMES - enzymes are the 
protein molecules which play the role of biocatalysts in the organism, 
they catalyse the reactions of all metabolic processes. Enzymes are 
divided into classes, one of which is the class of HYDROLASES - these 
enzymes catalyse the reaction of hydrolysis of various bonds (peptide 
bonds, ester bonds etc.) with the participation of a water molecule. 
PROTEOLYTIC ENZYMES (PROTEASES) belong to the class of HYDROLASES. 

Proteases are involved in splitting the peptide bonds which link the amino 
acid residues (elementar units of PROTEINS). Thus proteins are the 
SUBSTRATES for proteases. These enzymes "digest" long protein chain to 
shorter fragments. Some of them can detach the terminal amino acids from 
the protein chain (EXOPEPTIDASES - like aminopeptidases, carboxipeptidase 
A), the others "attack" internal peptide bonds of a protein (ENDOPEPTIDASES 
- like trypsin, chymotrypsin, pepsin, papain, elastase). 

Proteases are divided into four major groups according to the character of 
their active site (catalytic site) and conditions of action: SERINE 
PROTEINASES, CYSTEINE (THIOL) PROTEINASES, ASPARTIC PROTEINASES AND 
METALLOPROTEINASES. Attachment of a protease to a certain group depends on 
the structure of catalytic site and the amino acid (as one of the 
consituents) essential for its activity. 

Proteases are everywhere and they are involved in various metabolic 
processes. Acid proteases secreted into the stomach (such as PEPSIN) and 
serine proteases present in duodeum (TRYPSIN, CHYMOTRYPSIN), enable us to 
digest the protein in food, proteases present in blood serum (THROMBIN, 
PLASMIN, HAGEMAN FACTOR etc.) play important role in blood clotting, as well
as lysis of the clots, and the correct action of the immune system. Other 
proteases are present in leukocytes (ELASTASE, CATHEPSIN G) and play several 
different roles in metabolic control. Proteases determine the lifetime of 
other proteins playing important physiological role like hormones, 
antibodies, or other enzymes - this is one of the fastest "switching on" 
and "switching off" regulatory mechanisms in the physiology of an organism. 
By complex cooperative action the proteases may proceed as "cascade" 
reactions which result in amplification of the organism response to the 
physiological signal, and make this response very fast.

Generally we can say that proteases are involved (in different way) in all 
physiological processes.