This is a rare family of glutamate--cysteine ligases, EC:6.3.2.2, demonstrated first in Thiobacillus ferrooxidans and present in a few other Proteobacteria [1]. It is the first of two enzymes for glutathione biosynthesis. It is also called gamma-glutamylcysteine synthetase. The structure of this family has been solved, and is similar to that of human glutathione synthetase and very different to gamma-glutamylcysteine synthetase from Escherichia coli.

1: The gene for gamma-glutamylcysteine synthetase from Thiobacillus ferrooxidans has low homology to its Escherichia coli equivalent and is linked to the gene for citrate synthase. Powles R, Deane S, Rawlings D; Microbiology. 1996;142:2543-2548. PMID:8828222