1upm

From Proteopedia

Revision as of 15:56, 18 December 2007

ACTIVATED SPINACH RUBISCO COMPLEXED WITH 2-CARBOXYARABINITOL 2 BISPHOSPHAT AND CA2+.

Overview

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses CO(2), assimilation in biology. A prerequisite for catalysis is an activation, process, whereby an active site lysine is selectively carbamylated. The, carbamyl group is then stablised by a metal ion, which in vivo is Mg(2+)., Other divalent metal ions can replace Mg(2+) as activators in vitro, but, the nature of the metal ion strongly influences the catalytic activity of, the enzyme and has a differential effect on the ratio of the carboxylation, reaction and the competing oxygenation reaction. Biochemical studies show, that calcium promotes carbamylation but not catalysis. To investigate the, role of the metal in catalysis, we have determined two structures of the, enzyme complexed with Ca(2+) and the transition state analogue, 2-carboxy-D-arbinitol-1,5-bisphosphate (2CABP). One of the complexes was, prepared by soaking 2CABP into crystals of the enzyme-Ca(2+)-product, complex, while the other was obtained by cocrystallising the enzyme with, calcium and 2CABP under activating conditions. The two crystals belong to, different space groups, and one was merohedrally twinned. Both complexes, show very similar three-dimensional features. The enzyme is carbamylated, at Lys201, and requisite loops close over the bound ligands in the active, site, shielding them from the solvent in a manner similar to the, corresponding complex with Mg(2+). However, there are subtle differences, that could explain the particular role of Ca(2+) in these processes. The, larger radius of the calcium ion and its reduced Lewis-acid character, causes a significant increase in the required proton hop distance between, the C3 proton and the carbamate on Lys201 in the calcium complex. This, alone could explain the inability of calcium to sustain catalysis in, Rubisco. Similar effects are also expected on subsequent proton transfer, steps in the catalytic cycle. Here we also discuss the effect of metal, substitution on the dynamics of the ligands around the metal ion.

About this Structure

1UPM is a Protein complex structure of sequences from Spinacia oleracea with CA and CAP as ligands. Active as Ribulose-bisphosphate carboxylase, with EC number 4.1.1.39 Known structural/functional Site: . Full crystallographic information is available from OCA.

Reference

Calcium supports loop closure but not catalysis in Rubisco., Karkehabadi S, Taylor TC, Andersson I, J Mol Biol. 2003 Nov 14;334(1):65-73. PMID:14596800

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