3D architecture and structural flexibility revealed in the subfamily of large glutamate dehydrogenases by a mycobacterial enzyme

Abstract

Glutamate dehydrogenases (GDHs) are widespread metabolic enzymes that play key roles in nitrogen homeostasis. Large glutamate dehydrogenases composed of 180\u2009kDa subunits (L-GDHs 180 ) contain long N- and C-terminal segments flanking the catalytic core. Despite the relevance of L-GDHs 180 in bacterial physiology, the lack of structural data for these enzymes has limited the progress of functional studies. Here we show that the mycobacterial L-GDH 180 (mL-GDH 180 ) adopts a quaternary structure that is radically different from that of related low molecular weight enzymes. Intersubunit contacts in mL-GDH 180 involve a C-terminal domain that we propose as a new fold and a flexible N-terminal segment comprising ACT-like and PAS-type domains that could act as metabolic sensors for allosteric regulation. These findings uncover unique aspects of the structure-function relationship in the subfamily of L-GDHs. Lázaro et. al . report the first 3D structure of a large glutamate dehydrogenase (L-GDH), the one corresponding to the Mycobacterium smegmatis enzyme composed of 180\u2009kDa subunits (mL-GDH 180 ), obtained by X-ray crystallography and cryo-electron microscopy. This structure reveals that mL-GDH 180 assembles as tetramers with the N- and C-terminal domains being involved in inter-subunit contacts and unveils unique features of the subfamily of L-GDHs.