Catalysis is the step(s) in an enzyme catalyzed reaction in which the substrate is converted to product.
In many enzyme catalyzed reactions, catalysis involves making and breaking covalent bonds. One of the best studied catalytic reactions involves the cleaving of peptide bonds by the enzyme chymotrypsin.
Chymotrypsin is classified as a serine protease because there is a serine in the active site of the enzyme. The 57th amino acid of the primary sequence of chymotrypsin is Histidine (His57), the 102nd amino acid is aspartic acid (asp102) and the 195th amino acid is serine (ser 195). The side chains of these three amino acids are part of the active site of chymotrypsin.
Since chymotrypsin is a protease, part of a protein will bind to the active site of the enzyme. Specifice amino acid side chains of the protein substrate bind to the active site. During catalysis a peptide bond is broken. A water molecule is also involved in catalysis.
During catalysis, a series of reactions occur in which covalent bonds are broken and formed. In an animation of catalysis carried out by chymotrypsin, the red lines and arrows indicate the series of covalent bonds that are borken or formed in the process of breaking one peptide bond
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