Aspartate essential cysteine residues

Aspartate racemase requires no cofactors and contains an essential cysteine residue in the same manner as glutamate racemase1801. When l- or D-aspartate was incubated with aspartate racemase in tritiated water, tritium was incorporated preferentially into the product enantiomer. This is consistent with the results of glutamate racemase as described above1911. 

Yamauchi et al.11121 concluded that aspartate racemase also uses two bases to remove and return the a-proton of the substrate. Aspartate racemase contains three cysteine residues Cys 84, Cys 190 and Cys 197, and only Cys 84 is essential for the enzyme activity. The alkylation of one cysteine residue/dimer with 2-nitro-5-thiocya-nobenzoic acid results in a complete loss of activity. Therefore, the enzyme shows a half-of-the-sites-reactivity11121. Yamauchi et al.11121 suggested that the enzyme has a composite active site formed at the interface of two identical subunits in the same manner as proposed for proline racemase1921. 

Caspases are a family of at least 14 aspartate-specific cysteine proteases that are essential in the initiation and execution of apoptosis (Creagh et al., 2003 Cohen, 1997). Caspases are normally expressed as inactive proenzymes (zymogens) that become activated during apoptosis (Zhivotovsky et al., 1999). All members of the caspase family share a number of amino acid residues crucial for substrate binding... 

As the names already imply, the amino acids serine, cysteine and aspartic acid play an essential role in the catalytic site of the respective enzymes. Modification or blocking of these amino acid residues leads to complete inactivation of these enzymes. The serine proteases have a pH optimum at alkaline pH, the cysteine proteases show maximum activity at more neutral pH, whereas the aspartic acid proteases exhibit optimal activity at acidic pH. The metalloproteases contain an essential metal atom in their active site, usually zinc. They have optimal activity at neutral pH. 

Other residues that have been mutated are cysteine-242, aspartate-84, lysine-55 and lysine-59. Changing cysteine-242, the site of attachment of the cofactor, to a serine residue removes essentially all of the catalytic activity by preventing cofactor assembly. When aspartate-84 was changed to alanine or asparagine, all activity was lost, though assembly of the cofactor did still occur [48]. Changing aspartate-84 to a glutamate (i.e. addition of just one CHj) causes loss of 99% of the enzymic activity. The crystal structure of this mutant shows that the carboxylate... 

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