Substrate specificity and structure of human aminoadipate aminotransferase/kynurenine aminotransferase II.
KAT (kynurenine aminotransferase) II is a primary enzyme in the brain for catalysing the transamination of kynurenine to KYNA (kynurenic acid). KYNA is the only known endogenous antagonist of the N-methyl-D-aspartate receptor. The enzyme also catalyses the transamination of aminoadipate to alpha-oxoadipate; therefore it was initially named AADAT (aminoadipate aminotransferase). As an endotoxin, aminoadipate influences various elements of glutamatergic neurotransmission and kills primary astrocytes in the brain. A number of studies dealing with the biochemical and functional characteristics of this enzyme exist in the literature, but a systematic assessment of KAT II addressing its substrate profile and kinetic properties has not been performed. The present study examines the biochemical and structural characterization of a human KAT II/AADAT. Substrate screening of human KAT II revealed that the enzyme has a very broad substrate specificity, is capable of catalysing the transamination of 16 out of 24 tested amino acids and could utilize all 16 tested alpha-oxo acids as amino-group acceptors. Kinetic analysis of human KAT II demonstrated its catalytic efficiency for individual amino-group donors and acceptors, providing information as to its preferred substrate affinity. Structural analysis of the human KAT II complex with alpha-oxoglutaric acid revealed a conformational change of an N-terminal fraction, residues 15-33, that is able to adapt to different substrate sizes, which provides a structural basis for its broad substrate specificity.
Han
Qian
Q
Department of Biochemistry, Virginia Tech, Blacksburg, VA 24061, U.S.A. qianhan@vt.edu
Cai
Tao
T
Tagle
Danilo A
DA
Robinson
Howard
H
Li
Jianyong
J
D000596:Amino Acids / Q000378:metabolism; D006801:Humans; D007656:Ketoglutaric Acids / Q000378:metabolism; D007700:Kinetics; D007736:Kynurenic Acid / Q000378:metabolism; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013379:Substrate Specificity; D013696:Temperature; D000637:Transaminases / Q000737:chemistry* / Q000378:metabolism*
D000596:Amino Acids; D007656:Ketoglutaric Acids; D011994:Recombinant Proteins; D000637:Transaminases; C021481:kynurenine-oxoglutarate transaminase; D007736:Kynurenic Acid