GABA production by glutamic acid decarboxylase is regulated by a dynamic catalytic loop.
Gamma-aminobutyric acid (GABA) is synthesized by two isoforms of the pyridoxal 5'-phosphate-dependent enzyme glutamic acid decarboxylase (GAD65 and GAD67). GAD67 is constitutively active and is responsible for basal GABA production. In contrast, GAD65, an autoantigen in type I diabetes, is transiently activated in response to the demand for extra GABA in neurotransmission, and cycles between an active holo form and an inactive apo form. We have determined the crystal structures of N-terminal truncations of both GAD isoforms. The structure of GAD67 shows a tethered loop covering the active site, providing a catalytic environment that sustains GABA production. In contrast, the same catalytic loop is inherently mobile in GAD65. Kinetic studies suggest that mobility in the catalytic loop promotes a side reaction that results in cofactor release and GAD65 autoinactivation. These data reveal the molecular basis for regulation of GABA homeostasis.
Fenalti
Gustavo
G
Department of Biochemistry and Molecular Biology, Monash University, Clayton, Melbourne, VIC 3800, Australia.
Law
Ruby H P
RH
Buckle
Ashley M
AM
Langendorf
Christopher
C
Tuck
Kellie
K
Rosado
Carlos J
CJ
Faux
Noel G
NG
Mahmood
Khalid
K
Hampe
Christiane S
CS
Banga
J Paul
JP
Wilce
Matthew
M
Schmidberger
Jason
J
Rossjohn
Jamie
J
El-Kabbani
Ossama
O
Pike
Robert N
RN
Smith
A Ian
AI
Mackay
Ian R
IR
Rowley
Merrill J
MJ
Whisstock
James C
JC
D000595:Amino Acid Sequence; D001324:Autoantigens / Q000276:immunology; D001665:Binding Sites / Q000187:drug effects; D002384:Catalysis / Q000187:drug effects; D018360:Crystallography, X-Ray; D019281:Dimerization; D004789:Enzyme Activation / Q000187:drug effects; D005968:Glutamate Decarboxylase / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D018698:Glutamic Acid / Q000494:pharmacology; D006801:Humans; D007527:Isoenzymes / Q000737:chemistry / Q000276:immunology / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary / Q000187:drug effects; D005680:gamma-Aminobutyric Acid / Q000096:biosynthesis*
D001324:Autoantigens; D007527:Isoenzymes; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D005968:Glutamate Decarboxylase; C402565:glutamate decarboxylase 1; C401141:glutamate decarboxylase 2