No abstract

The crystal structure of human aldose reductase in complex with citrate has been determined to a resolution of 0.82 A. The difference electron density for H atoms unequivocally shows that the cofactor is in the oxidized state corresponding to the situation after the catalytic event has occurred. A citrate molecule bound to the active site has been modelled in two different conformations. These two conformations correlate with a fully closed and a partially open conformation of the so-called safety-belt loop (Gly213-Ser226). The open conformation is observed for the first time with the cofactor bound to the protein and may be related to the initial phase of the opening of the safety belt. The structure suggests that after the catalytic event, a rearrangement of a bound ligand can trigger the opening of the safety-belt loop, thus initiating the release of the oxidized cofactor.

Rabphilin-3A is a neuronal protein containing a C2-domain tandem. To date, only the structure of the C2B domain has been solved. The crystal structure of the Ca2+-free C2A domain has been solved by molecular replacement and refined to 1.92 A resolution. It adopts the classical C2-domain fold consisting of an eight-stranded antiparallel beta-sandwich with type I topology. In agreement with its Ca2+-dependent negatively charged membrane-binding properties, this C2 domain contains all the conserved acidic residues responsible for calcium binding. However, the replacement of a conserved aspartic acid residue by glutamic acid allows formation of an additional strong hydrogen bond, resulting in increased rigidity of calcium-binding loop 1. The electrostatic surface of the C2A domain consists of a large positively charged belt surrounded by two negatively charged patches located at both tips of the domain. In comparison, the structurally very similar C2A domain of synaptotagmin I has a highly acidic electrostatic surface, suggesting completely unrelated functions for these two C2A domains.

There is considerable interest in organoaluminum compounds containing AlÀO bonds. This is primarily due to the discovery of methylalumoxane (MAO) as an extremely potent cocata-

There is considerable interest in organoaluminum compounds containing AlÀO bonds. This is primarily due to the discovery of methylalumoxane (MAO) as an extremely potent cocata-