A vibrational structure of 7,8-dihydrobiopterin bound to dihydroneopterin aldolase.
Dihydroneopterin aldolase (DHNA) catalyzes the conversion of 7, 8-dihydroneopterin to 6-hydroxymethyl-7,8-dihydropterin and glycolaldehyde. An inhibitor of the enzyme, 7,8-dihydrobiopterin, free in solution and bound in its complex with the enzyme has been studied by Raman difference spectroscopy. By using isotopically labeled 7,8-dihydrobiopterin and normal mode analyses based on ab initio quantum mechanic methods, we have positively identified some of the Raman bands in the enzyme-bound inhibitor, particularly the important N5=C6 stretch mode. The spectrum of the enzyme-bound inhibitor shows that the pK(a) of N5 is not significantly increased in the complex. This result suggests that N5 of 7,8-dihydroneopterin is not protonated before the bond cleavage of 7,8-dihydroneopterin during the DHNA-catalyzed reaction as has been suggested. Our results also show that the N5=C6 stretch mode of 7, 8-dihydrobiopterin shifts 19 cm(-)(1) upon binding to DHNA. Various possibilities on how the enzyme can bring about such large frequency change of the N5=C6 stretch mode are discussed.
Deng
H
H
Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
Callender
R
R
Dale
G E
GE
D000446:Aldehyde-Lyases / Q000037:antagonists & inhibitors*; D001665:Binding Sites; D001708:Biopterins / Q000031:analogs & derivatives* / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman
D001708:Biopterins; C017226:7,8-dihydrobiopterin; D000446:Aldehyde-Lyases; C020044:dihydroneopterin aldolase