Chemistry and Biochemistry of B12 Coenzymes and their Enzymic Partners

• Research unit: EU RFP
• Project number: 95.0606

Key words (English)

Vitamin B12; enzyme; coenzyme; rearrangements; radicals; models

Alternative project number (English)

EU project number: FMRXCT960018

Research programs (English)

EU-programme: 4. Frame Research Programme - 10.1 Stimulation of training and mobility

Short description (English)

See abstract

Partners and International Organizations (English)

Marburg (D), Karlsruhe (D), Ulm (D), Innsbruck (A), Graz (A), Newcastle (UK), associated: Budapest (HU)

Abstract (English)

The research activities of the partner group at Bern during the 08/01/2000 - 12/31/2000
The research activities fo the Bern group during this final period were mainly concerned with the aim to elaborate and to complete some of the ongoing projects. In more detail these activities have been centered around the following topics:
· Preparation of a new hexamethyl dicyano cob(III)yrinate monoacid
· Model for a mode of activation of the coenzyme B12 in the methylmalonyl-CoA mutase: statistical analysis of structural diffferences between protonated/complexed adenosyl moieties and their unprotonated/uncomplexed congeners
The space available to the coenzyme B12 in the methylmalonyl CoA mutase has been explored by L. Poppe, associated partner at Budapest, with respect to the adenosyl moiety on the b-side. In contrast there is no systematic investigation of the space requirements for the nucleotide loop on the a side of the coenzyme. Since this nucleotide loop is deeply buried in a pocket of the holoenzyme with the dimethyl-benzimidazole moiety being replaced by the imidazole ring of a protein derived histidine (His 610), it is of interest to explore the 'softness' of this pocket. As reported earlier we intended to prepare homocobyric acid by a novel route. As a step towards this goal we investigated the methanolysis of vitamin B12 under acidic conditions in greater detail. We isolated a hexamethyl cob(III)yrinate monoacid. The detailed NMR analysis by advanced methodology has led to the conclusion that the carboxylic group is located in ring A on the b-side rather than in the propionic acid on the a side of ring D. These results will be published in due course.
In the persuit of understanding the enigmatic activation of the coenzyme B12 in the methylmalonyl CoA mutase induced by association with the substrate we surmised that a protonation of the adenosyl moiety might lead to a structural change sufficiently large for inducing the homolysis of the Co-C bond. In order to gain insight into this perspective we explored the structural features of X-ray structures found in the Cambridge Structural Data Base for protonated/complexed adenosines and their uncomplexed/unprotonated congeners. On the basis of this statistical analysis it may be concluded that the impact of protonation or complexation at one (or two) N atoms of the adenine on structural changes in the exocyclic hydroxmethyl group of the ribose is insignificant. These results are taken as an indication that protonation of the adenine moiety in the coenzyme B12 upon formation of the active enzyme-substrate complex is insufficient for a ready cleavage of the Co-C bond.
Since these results are of general importance for all nucleosides and nucleotides, this study is extended to the corresponding compounds containing a pyrimidine rather than a purine ring system.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 95.0606