The crystal structure of the H‐ras oncogene protein p21 complexed to the slowly hydrolysing GTP analogue GppNp has been determined at 1.35 A resolution. 211 water molecules have been built into the electron density. The structure has been refined to a final R‐factor of 19.8% for all data between 6 A and 1.35 A. The binding sites of the nucleotide and the magnesium ion are revealed in high detail. For the stretch of amino acid residues 61‐65, the temperature factors of backbone atoms are four times the average value of 16.1 A2 due to the multiple conformations. In one of these conformations, the side chain of Gln61 makes contact with a water molecule, which is perfectly placed to be the nucleophile attacking the gamma‐phosphate of GTP. Based on this observation, we propose a mechanism for GTP hydrolysis involving mainly Gln61 and Glu63 as activating species for in‐line attack of water. Nucleophilic displacement is facilitated by hydrogen bonds from residues Thr35, Gly60 and Lys16. A mechanism for rate enhancement by GAP is also proposed.

Mammalian xanthine oxidoreductases, which catalyze the last two steps in the formation of urate, are synthesized as the dehydrogenase form xanthine dehydrogenase (XDH) but can be readily converted to the oxidase form xanthine oxidase (XO) by oxidation of sulfhydryl residues or by proteolysis. Here, we present the crystal structure of the dimeric (Mr, 290,000) bovine milk XDH at 2.1-Å resolution and XO at 2.5-Å resolution and describe the major changes that occur on the proteolytic transformation of XDH to the XO form. Each molecule is composed of an N-terminal 20-kDa domain containing two iron sulfur centers, a central 40-kDa flavin adenine dinucleotide domain, and a C-terminal 85-kDa molybdopterin-binding domain with the four redox centers aligned in an almost linear fashion. Cleavage of surface-exposed loops of XDH causes major structural rearrangement of another loop close to the flavin ring (Gln 423OLys 433). This movement partially blocks access of the NAD substrate to the flavin adenine dinucleotide cofactor and changes the electrostatic environment of the active site, reflecting the switch of substrate specificity observed for the two forms of this enzyme. Milk xanthine oxidase is an archetypal enzyme, which was originally described as aldehyde oxidase in 1902 (1) and has since served as a benchmark for the whole class of complex metalloflavoproteins (2). Xanthine oxidoreductase enzymes have been isolated from a wide range of organisms, from bacteria to man, and catalyze the hydroxylation of a wide variety of purine, pyrimidine, pterin, and aldehyde substrates. All of these proteins have similar molecular weights and composition of redox centers (3, 4). The mammalian enzymes, which catalyze the hydroxylation of hypoxanthine and xanthine, the last two steps in the formation of urate, are synthesized as the dehydrogenase form xanthine dehydrogenase (XDH) and exist mostly as such in the cell but can be readily converted to the oxidase form xanthine oxidase (XO) by oxidation of sulfhydryl residues or by proteolysis. XDH shows a preference for NAD ϩ reduction at the flavin adenine dinucleotide (FAD) reaction site, whereas XO fails to react with NAD ϩ and exclusively uses dioxygen as its substrate, leading to the formation of superoxide anion and hydrogen peroxide (3). The enzyme is a target of drugs against gout and hyperuricemia (5), and the conversion of XDH to XO is of major interest as it has been implicated in diseases characterized by oxygen-radical-induced tissue damage, such as postischemic reperfusion injury (6). Recent work suggests that XO also might be associated with blood pressure regulation (7).The active form of the enzyme is that of a homodimer of molecular mass 290 kDa, with each of the monomers acting independently in catalysis. Each subunit contains one molybdopterin cofactor, two spectroscopically distinct [2Fe-2S] centers, and one FAD cofactor. The oxidation of xanthine takes place at the molybdopterin center (Mo-pt) and the electrons thus introduced are rapidly distributed to the other c...

The crystal structure of the guanine-nucleotide-binding domain of p21 (amino acids 1-166) complexed to the guanosine triphosphate analogue guanosine-5'-(beta, gamma-imido)triphosphate (GppNp) has been determined at a resolution of 2.6 A. The topological order of secondary structure elements is the same as that of the guanine-nucleotide-binding domain of bacterial elongation factor EF-Tu. Many interactions between nucleotide and protein have been identified. The effects of point mutations and the conservation of amino-acid sequence in the guanine-nucleotide-binding proteins are discussed.

We have determined the structure of the beta-carbonic anhydrase from the dicotyledonous plant Pisum sativum at 1.93 A resolution, using a combination of multiple anomalous scattering off the active site zinc ion and non-crystallographic symmetry averaging. The mol- ecule assembles as an octamer with a novel dimer of dimers of dimers arrangement. Two distinct patterns of conservation of active site residues are observed, implying two potentially mechanistically distinct classes of beta-carbonic anhydrases. The active site is located at the interface between two monomers, with Cys160, His220 and Cys223 binding the catalytic zinc ion and residues Asp162 (oriented by Arg164), Gly224, Gln151, Val184, Phe179 and Tyr205 interacting with the substrate analogue, acetic acid. The substrate binding groups have a one to one correspondence with the functional groups in the alpha-carbonic anhydrase active site, with the corresponding residues being closely superimposable by a mirror plane. Therefore, despite differing folds, alpha- and beta-carbonic anhydrase have converged upon a very similar active site design and are likely to share a common mechanism.

No abstract