page_title: Active site
section_title: Glutathione reductase
hierarchical_section_title: Active site / Examples of enzyme catalysis mechanisms / Glutathione reductase
caption_attribution_description: English: GSR catalyic cycle
context_page_description: In biology, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate and residues that catalyse a reaction of that substrate. Although the active site occupies only ~10–20% of the volume of an enzyme, it is the most important part as it directly catalyzes the chemical reaction. It usually consists of three to four amino acids, while other amino acids within the protein are required to maintain the tertiary structure of the enzyme.
  Each active site is evolved to be optimised to bind a particular substrate and catalyse a particular reaction, resulting in high specificity. This specificity is determined by the arrangement of amino acids within the active site and the structure of the substrates. Sometimes enzymes also need to bind with some cofactors to fulfil their function. The active site is usually a groove or pocket of the enzyme which can be located in a deep tunnel within the enzyme, or between the interfaces of multimeric enzymes. An active site can catalyse a reaction repeatedly as residues are not altered at the end of the reaction.
context_section_description: 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).
  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.
  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.
caption: the mechanism of glutathione reductase
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