GTP Cyclohydrolase II Structure and Mechanism

Jingshan Ren,Masayo Kotaka,Michael Lockyer,Heather K Lamb,Alastair R Hawkins,David K Stammers

doi:10.1074/jbc.m507725200

Abstract

GTP cyclohydrolase II converts GTP to 2,5-diamino-6-beta-ribosyl-4(3H)-pyrimidinone 5'-phosphate, formate and pyrophosphate, the first step in riboflavin biosynthesis. The essential role of riboflavin in metabolism and the absence of GTP cyclohydrolase II in higher eukaryotes makes it a potential novel selective antimicrobial drug target. GTP cyclohydrolase II catalyzes a distinctive overall reaction from GTP cyclohydrolase I; the latter converts GTP to dihydroneopterin triphosphate, utilized in folate and tetrahydrobiopterin biosynthesis. The structure of GTP cyclohydrolase II determined at 1.54-A resolution reveals both a different protein fold to GTP cyclohydrolase I and distinctive molecular recognition determinants for GTP; although in both enzymes there is a bound catalytic zinc. The GTP cyclohydrolase II.GMPCPP complex structure shows Arg(128) interacting with the alpha-phosphonate, and thus in the case of GTP, Arg(128) is positioned to act as the nucleophile for pyrophosphate release and formation of the proposed covalent guanylyl-GTP cyclohydrolase II intermediate. Tyr(105) is identified as playing a key role in GTP ring opening; it is hydrogen-bonded to the zinc-activated water molecule, the latter being positioned for nucleophilic attack on the guanine C-8 atom. Although GTP cyclohydrolase I and GTP cyclohydrolase II both use a zinc ion for the GTP ring opening and formate release, different residues are utilized in each case to catalyze this reaction step.

Highlights

The GCHII reaction involves a guanine ring-opening step and formate release, in common with the named GTP cyclohydrolase I (EC 3.5.4.16) (GCHI), but other components of the two reactions catalyzed are rather different
GCHI and GCHII are of similar size, and both contain a zinc ion for guanine ring opening
GCHI and GCHII provide an example of convergent evolution in which a bound zinc is present for a common catalytic function

Summary

Introduction

The GCHII reaction involves a guanine ring-opening step and formate release, in common with the named GTP cyclohydrolase I (EC 3.5.4.16) (GCHI), but other components of the two reactions catalyzed are rather different. 18O from solvent is incorporated into the phosphate group of DARP but not into the initially released pyrophosphate [9] Such kinetic data, along with the lack of substrate activity for GMP [8], have been interpreted as being consistent with the formation of a covalent guanylated GCHII intermediate. A ring-opened guanine formamide derivative of GTP, (2-amino-5-formylamino-6-ribosylamino-4(3H)pyrimidinone triphosphate), has been shown to act as a substrate for GCHII, it does not fulfill the criteria for a kinetically competent intermediate [11]. Both GCHI and GCHII have much lower turnover numbers (between 1 and 5 minϪ1) than normally found for most enzymes [9].

Methods

Results

Conclusion