Multi-enzyme Complexes [R. Sterner]

Glutamine amidotransferases are a family of enzymes, whose members contain two active sites that are often located on two different polypeptide chains. The glutaminase subunit hydrolyses glutamine to glutamate and ammonia, which is added to a specific acceptor substrate at the active site of the synthase subunit. There are two important questions regarding the mechanism of glutamine amidotransferases: How are the activities at the two active sites coordinated? How is nascent ammonia transferred from the glutaminase to the synthase subunit without getting in contact with bulk water?

We are investigating these questions using imidazole glycerol phosphate (ImGP) synthase from Thermotoga maritima, which is a key metabolic enzyme that links histidine and purine biosynthesis. ImGP synthase consists of a complex of the glutaminase subunit HisH and the synthase subunit HisF, which has the (ba)₈-barrel fold. A combination of biochemical investigations and X-ray crystallography revealed that glutamine hydrolysis at HisH is induced by ammonia consumption at the active site of HisF to which it migrates through a long bi-partite channel that comprises the interior of the b-barrel of HisF.