THE LIPOAMIDE ARM IN THE GLYCINE DECARBOXYLASE COMPLEX IS NOT FREELY SWINGING

Glycine decarboxylase consists of four protein components, Its structu ral and mechanistic heart is provided by the lipoic acid-containing H- protein which undergoes a cycle of reductive methylamination, methylam ine transfer and electron transfer, Lipoic acid attached to a specific lysine side chain is assumed to act as a 'swinging arm' conveying the reactive dithiolane ring from one catalytic centre to another, The X- ray crystal structures of two forms of the H-protein have been determi ned. The lipoate cofactor is located in the loop of a hairpin configur ation but following methylamine transfer it is pivoted to bind into a cleft at the surface of the H-protein. The lipoamide-methylamine arm i s, therefore, not free to move in aqueous solvent.