Mutagenesis identifies a conserved tyrosine residue important for the activity of uroporphyrinogen III synthase from Anacystis nidulans

Uroporphyrinogen III synthase from the cyanobacterium Anacystis nidulans was overproduced in Escherichia coli and analyzed by site specific mutagenesis. Of the nine conserved amino acids altered, only a single tyrosine mutant (Y166F) showed any significant decrease in activity suggesting this residue is critical for proper substrate binding and/or catalysis.

Biosynthesis of cobalamin (vitamin B(12))

The biosynthesis of vitamin B(12) is summarized, emphasizing the differences observed between the aerobic and anaerobic pathways. The biosynthetic route to adenosylcobalamin from its five-carbon precursor, 5-aminolaevulinic acid, can be divided into three sections: (1) the biosynthesis of uroporphyrinogen III from 5-aminolaevulinic acid, which is common to both pathways; (2) the conversion of uroporphyrinogen III into the ring-contracted, deacylated intermediate precorrin 6 or cobalt-precorrin 6, which includes the primary differences between the two pathways; and (3) the transformation of this intermediate to form adenosylcobalamin.

Isolation and characterization of 14 additional genes specifying the anaerobic biosynthesis of cobalamin (vitamin B12) in Propionibacterium freudenreichii (P. shermanii)

A search for genes encoding enzymes involved in cobalamin (vitamin B12) production in the commercially important organism Propionibacterium freudenreichii (P. shermanii) has resulted in the isolation of an additional 14 genes encoding enzymes responsible for 17 steps of the anaerobic B12 pathway in this organism. All of the genes believed to be necessary for the biosynthesis of adenosylcobinamide from uroporphyrinogen III have now been isolated except two (cbiA and an as yet unidentified gene encoding cobalt reductase). Most of the genes are contained in two divergent operons, one of which, in turn, is closely linked to the operon encoding the B12-dependent enzyme methylmalonyl-CoA mutase. The close linkage of the three genes encoding the subunits of transcarboxylase to the hemYHBXRL gene cluster is reported. The functions of the P. freudenreichii B12 pathway genes are discussed, and a mechanism for the regulation of cobalamin and propionic acid production by oxygen in this organism is proposed.

Reflections on the discovery of nature's pathways to vitamin B12

The chronology of the discoveries along the pathway of vitamin B12 biosynthesis is reviewed from a personal perspective, including discussion of the most recent finding that two pathways to B12 exist--one aerobic and one anaerobic--which differ mainly in the ring contraction mechanisms which convert porphyrin to corrin.

cis-peptide bond mimetic tetrazole analogs of the insect kinins identify the active conformation

The insect kinin neuropeptides have been implicated in the regulation of water balance, digestive organ contraction, and energy mobilization in a number of insect species. A previous solution conformation study of an active, restricted-conformation cyclic analog, identified two possible turn conformations as the likely active conformation adopted by the insect kinins at the receptor site. These were a cisPro type VI beta-turn over C-terminal pentapeptide core residues 1-4 and a transPro type I-like beta-turn over core residues 2-5, present in a ratio of 60:40. Synthesis and evaluation of the diuretic activity of insect kinin analogs incorporating a tetrazole moiety, which mimics a cis peptide bond, identifies the active conformation as the former. The discovery of a receptor interaction model can lead to the development of potent agonist and antagonist analogs of the insect kinins. Indeed, in this study a tetrazole analog with D stereochemistry has been shown to demonstrate partial antagonism of the diuretic activity of natural insect kinins, providing a lead for more potent and effective antagonists of this critical neuropeptide family. The future development of mimetic agonists and antagonists of insect kinin neuropeptides will provide important tools to neuroendocrinologists studying the mechanisms by which they operate and to researchers developing new, environmentally friendly pest insect control strategies.

Structure of the Methanococcus jannaschii mevalonate kinase, a member of the GHMP kinase superfamily

The mevalonate-dependent pathway is used by many organisms to synthesize isopentenyl pyrophosphate, the building block for the biosynthesis of many biologically important compounds, including farnesyl pyrophosphate, dolichol, and many sterols. Mevalonate kinase (MVK) catalyzes a critical phosphoryl transfer step, producing mevalonate 5'-phosphate. The crystal structure of thermostable MVK from Methanococcus jannaschii has been determined at 2.4 A, revealing an overall fold similar to the homoserine kinase from M. jannaschii. In addition, the enzyme shows structural similarity with mevalonate 5-diphosphate decarboxylase and domain IV of elongation factor G. The active site of MVK is in the cleft between its N- and C-terminal domains. Several structural motifs conserved among species, including a phosphate-binding loop, have been found in this cavity. Asp(155), an invariant residue among MVK sequences, is located close to the putative phosphate-binding site and has been assumed to play the catalytic role. Analysis of the MVK model in the context of the other members of the GHMP kinase family offers the opportunity to understand both the mechanism of these enzymes and the structural details that may lead to the design of novel drugs.