GLUTAMATE CYSTEINE LIGASE
Homology model of human catalytic subunit of glutamate cysteine ligase. In the ribbon diagram, cysteine residues are shown in ball and stick representation and bound ATP in space filling representation.
GCL catalyzes the committed, rate limiting step in glutathione biosynthesis, conjugating glutamate via its gamma-carboxyl group to cysteine at the expense of an ATP. In bacterial systems, GCL is a single polypeptide and glutathione production is largely controlled at the transcriptional level. However, in higher eukaryotes, GCL is a heterodimer comprised of a catalytic subunit (GCL-Cat) and a regulatory subunit (GCL-Mod) and this added complexity allows glutathione biosynthesis to be regulated post-translationally. Glutathione synthesis is often limited by free cysteine availability and is feedback regulated by the end product, glutathione.
Further regulation is provided by the interplay of the GCL-Cat and GCL-Mod subunits. Although GCL-Cat is capable of catalysis alone, addition of GCL-Mod enhances activity at physiological concentrations of glutamate and reduces feedback inhibition by glutathione. The stimulatory effects of GCL-Mod on catalysis may depend on the presence of an intersubunit disulfide bond with GCL-Cat, thus linking the redox state of the cell with glutathione biosynthesis. However, we do not know underlying molecular details since structural information is not available for either the catalytic or the regulatory subunit. Thus, although considerable data exist for bacterial GCL, including a crystal structure of the E.coli enzyme, it is unlikely that much of this information will be applicable to higher organisms since the modifier subunit is absent in lower organisms. Therefore, we are studying how eukaryotic GCL activity is regulated using biochemical and structural techniques.