Structural enzymology

The microsymposium focused on enzymes that utilize unusual cofactors. J. Fontecilla-Camps discussed hydrogenases containing Fe in their active site or a combination of Fe and Ni coordinating CO, CN or cysteine thiolate. The function of the metal ions was discussed with emphasis on the spin state of Fe. The diffusion of H2 through long channels in the proteins was beautifully illustrated by loading them with Xe.

C. Wilmot showed snapshots of the reaction pathway of an amino oxidase which utilizes Cu and 2,4,5-trihydroxyphenylalanine quinone as cofactors. Individual steps of the reaction were flash-frozen and analyzed at high resolution. The quinone 'jumps' from one step to the next in the reaction cycle and, surprisingly, O₂ was seen to bind to Cu prior to oxidizing amine to the corresponding aldehyde.

U. Ermler described methyl-coenzyme M reductase, a 300 kDa heterohexamer catalyzing the last step of methane metabolism. This step involves the oxidation of coenzymes M and B to a heterodisulfide with the cooperation of coenzyme F₄₃₀ containing a Ni-porphinoid and transient formation of radicals and a Ni-Methyl complex.

Another multistep reaction, presented by K. Brown, concerned a Zn and NAD⁺ containing 3-dehydroquinate synthase in the shikimate pathway. The enzyme catalyzes alcohol oxidation, carbonyl reduction, ring opening, phosphate b-elimination, and intramolecular aldol condensation. Knowledge of the architecture of the active site will be of interest for drug design since several parasites, but no mammals, use this pathway.

A movie produced by A. Anderson showed clearly the reasons for anticooperativity in the homodimeric thymidylate synthetase enzyme. In complex with substrate dUMP and cofactor analog CB3717 the homodimer is asymmetric, binding two molecules of the former but only one of the latter. The reason for anticooperativity is seen in covalent bound formation between the catalytic cysteine and dUMP in one subunit that induces a conformational change in the other and prevents such bond formation.

A. Becker presenting the acetyl-CoA synthesizing enzyme pyruvate formate-lysase, a 340 kDa homodimer. In the reaction, a pyruvate bond is cleaved utilizing a free and stable radical located at Gly734. There is no metal ion involved in radical formation, contrasting with other known enzymes that engage Fe (III).