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Cysteine is considered a "newcomer" amino acid, being the 17th amino acid incorporated into the genetic code. [35] [36] Similar to other later-added amino acids such as methionine, tyrosine, and tryptophan, cysteine exhibits strong nucleophilic and redox-active properties.
The next step is nucleophilic attack by the deprotonated cysteine's anionic sulfur on the substrate carbonyl carbon. In this step, a fragment of the substrate is released with an amine terminus, the histidine residue in the protease is restored to its deprotonated form, and a thioester intermediate linking the new carboxy-terminus of the ...
It has been suggested that CHAP domain containing proteins utilise a catalytic cysteine residue in a nucleophilic-attack mechanism. [1] [2] The CHAP domain contains two invariant residues, a cysteine and a histidine. These residues form part of the putative active site of CHAP domain containing proteins.
An acid-base-nucleophile triad is a common motif for generating a nucleophilic residue for covalent catalysis. The residues form a charge-relay network to polarise and activate the nucleophile, which attacks the substrate , forming a covalent intermediate which is then hydrolysed to release the product and regenerate free enzyme.
Serine, threonine, and cysteine proteases use a nucleophilic residue (usually in a catalytic triad). That residue performs a nucleophilic attack to covalently link the protease to the substrate protein, releasing the first half of the product. This covalent acyl-enzyme intermediate is then hydrolyzed by activated water to complete catalysis by ...
Cysteine aminopeptidases, on the other hand, rely on a cysteine amino acid to perform catalysis. These enzymes are part of a broader group of cysteine proteases , all of which carve up proteins by using a nucleophilic cysteine thiol along with one or two other catalytic amino acids in a diad or triad.
Each of the tetrahedral reaction intermediates that occur during transfer of an acetyl group to and from the nucleophilic cysteine, respectively, have been observed in X-ray crystal structures of biosynthetic thiolase from A. fumigatus. [11] Thiolase Mechanism. The two-step, ping-pong mechanism for the thiolase reaction.
The mechanism by which papain breaks peptide bonds involves the use of a catalytic dyad with a deprotonated cysteine. [8] A nearby Asn-175 helps to orient the imidazole ring of His-159 to allow it to deprotonate the catalytic Cys-25. This cysteine then performs a nucleophilic attack on the carbonyl carbon of a peptide backbone. This forms a ...