<P> The role of glutathione (GSH) is to remove accumulated reactive oxygen species which may damage cells . During this process, its thiol side chain is oxidised and two glutathione molecules are connected by a disulphide bond to form a dimer (GSSG). In order to regenerate glutathione the disulphide bond has to be broken, In human cells, this is done by glutathione reductase (GR). </P> <P> Glutathione reductase is a dimer that contains two identical subunits . It requires one NADP and one FAD as the cofactors . The active site is located in the linkage between two subunits . The NADPH is involved in the generation of FADH -. In the active site, there are two cysteine residues besides the FAD cofactor and are used to break the disulphide bond during the catalytic reaction . NADPH is bound by three positively charged residues: Arg - 218, His - 219 and Arg - 224 . </P> <P> The catalytic process starts when the FAD is reduced by NADPH to accept one electron and from FADH.It then attacks the disulphide bond formed between 2 cysteine residues, forming one SH bond and a single S group . This S group will act as a nucleophile to attack the disulphide bond in the oxidised glutathione (GSSG), breaking it and forming a cysteine - SG complex. The first SG anion is released and then receives one proton from adjacent SH group and from the first glutathione monomer . Next the adjacent S group attack disulphide bond in cysteine - SG complex and release the second SG anion . It receives one proton in solution and forms the second glutathione monomer . </P> <P> Chymotrypsin is a serine endopeptidase that always presents in pancreatic juice and helps the hydrolysis of proteins and peptide . It catalyzes the hydrolysis of peptide bonds in L - isomers of tyrosine, phenylalanine, and tryptophan.In the active site of this enzyme three amino acid residues work together to form a catalytic triad which forms the catalytic site.In chymotrypsin, these enzymes are ser - 195, His - 157 and Asp - 102 . </P>

What is the relationship between enzymes substrates and active sites