Peter Hof

Crystal structure of the xanthine oxidase-related aldehyde oxido-reductase from D. gigas

The crystal structure of the aldehyde oxido-reductase (Mop) from the sulfate reducing anaerobic Gram-negative bacterium Desulfovibrio gigas has been determined at 2.25 Å resolution by multiple isomorphous replacement and refined. The protein, a homodimer of 907 amino acid residues subunits, is a member of the xanthine oxidase family. The protein contains a molybdopterin cofactor (Mo-co) and two different [2Fe-2S] centers. It is folded into four domains of which the first two bind the iron sulfur centers and the last two are involved in Mo-co binding. Mo-co is a molybdenum molybdopterin cytosine dinucleotide. Molybdopterin forms a tricyclic system with the pterin bicycle annealed to a pyran ring. The molybdopterin dinucleotide is deeply buried in the protein. The cis-dithiolene group of the pyran ring binds the molybdenum, which is coordinated by three more (oxygen) ligands.

The 1.8 A crystal structure of human cathepsin G in complex with Suc-Val-Pro-PheP-(OPh)2: a Janus-faced proteinase with two opposite specificities.

The crystal structure of human neutrophil cathepsin G, complexed with the peptidyl phosphonate inhibitor Suc‐Val‐Pro‐PheP‐(OPh)2, has been determined to a resolution of 1.8 A using Patterson search techniques. The cathepsin G structure shows the polypeptide fold characteristic of trypsin‐like serine proteinases and is especially similar to rat mast cell proteinase II. Unique to cathepsin G, however, is the presence of Glu226 (chymotrypsinogen numbering), which is situated at the bottom of the S1 specificity pocket, dividing it into two compartments. For this reason, the benzyl side chain of the inhibitor PheP residue does not fully occupy the pocket but is, instead, located at its entrance. Its positively charged equatorial edge is involved in a favourable electrostatic interaction with the negatively charged carboxylate group of Glu226. Arrangement of this Glu226 carboxylate would also allow accommodation of a Lys side chain in this S1 pocket, in agreement with the recently observed cathepsin G preference for Lys and Phe at P1. The cathepsin G complex with the covalently bound phosphonate inhibitor mimics a tetrahedral substrate intermediate. A comparison of the Arg surface distributions of cathepsin G, leukocyte elastase and rat mast cell protease II shows no simple common recognition pattern for a mannose‐6‐phosphate receptor‐independent targeting mechanism for sorting of these granular proteinases.

The crystal structure of the Physarum polycephalum actin–fragmin kinase: an atypical protein kinase with a specialized substrate‐binding domain

Coordinated temporal and spatial regulation of the actin cytoskeleton is essential for diverse cellular processes such as cell division, cell motility and the formation and maintenance of specialized structures in differentiated cells. In plasmodia of Physarum polycephalum, the F‐actin capping activity of the actin–fragmin complex is regulated by the phosphorylation of actin. This is mediated by a novel type of protein kinase with no sequence homology to eukaryotic‐type protein kinases. Here we present the crystal structure of the catalytic domain of the first cloned actin kinase in complex with AMP at 2.9 Å resolution. The three‐dimensional fold reveals a catalytic module of ∼160 residues, in common with the eukaryotic protein kinase superfamily, which harbours the nucleotide binding site and the catalytic apparatus in an inter‐lobe cleft. Several kinases that share this catalytic module differ in the overall architecture of their substrate recognition domain. The actin–fragmin kinase has acquired a unique flat substrate recognition domain which is supposed to confer stringent substrate specificity.