736 Recent progress in computational modeling of the catalytic activation of cobalt–carbon bond cleavage shows that quantum chemical calculations could be an important part of coenzyme B 12 research. Particular emphasis has been placed on density functional theory, which is now emerging as a powerful tool to elucidate the electronic structure and spectroscopic properties of the active sites of metalloenzymes. Addresses Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, USA; e-mail address: pawel@louisville.edu Current Opinion in Chemical Biology 2001, 5:736–743 1367-5931/01/$ — see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations Ado-B 12 adenosylcobalamin (coenzyme B 12 ) BDE bond dissociation energy Cbl cobalamin DBI 5,6-dimethylbenzimidazole DD diol dehydratase DFT density functional theory HOMO highest occupied molecular orbital LUMO lowest unoccupied molecular orbital Me-B 12 methylcobalamin MM-CoA methylmalonyl Co-A PRDDO partial retention of diatomic differential overlap QM/MM quantum mechanics/molecular mechanics Introduction One of the central mechanistic questions in the bioinorganic chemistry of coenzyme B 12 (or adenosylcobalamin [Ado- B 12 ], Figure 1) is how enzymes activate the cobalt–carbon bond cleavage [1–6]. During enzymatic catalysis, the Co–C R bond (see Figure 1) of coenzyme B 12 is cleaved homolytical- ly, leading to the formation (at least formally) of a 5′-deoxyadenosyl radical and Co(II)Cbl (where Cbl is cobal- amin). Enzymes that bind and utilize the coenzyme, to a surprising extent, must act to mitigate the inherent kinetic inertness of the Co–C R bond. The homolysis of the free coenzyme occurs with a half-life of one year at 37°C, much slower than enzymatic substrate turnover rates of ~10–100 per second. The degree of Co–C R bond activation is spec- tacular, because the Co–C R bond is ordinarily quite stable. The Co–C R dissociation enthalpy is 31.4 ± 1.5 kcal mol –1 for Ado-B 12 (in solution) and the thermal homolysis rate at 25°C is only 10 –9±1 s –1 [7,8]. In Ado-B 12 -dependent enzymes, this rate is increased by a factor of 10 12±1 , implying a ~15 kcal mol –1 destabilization of the Co–C R bond [7–10]. Precisely how such a tremendous acceleration is achieved and controlled is a topic of intense interest. Despite the great effort that has been devoted to this problem, the mechanism of the catalytic activation is poorly understood. To the extent that has been addressed experimentally, evidence from model systems indicates that steric hindrance around coordi- nated alkyl ligands leads to a higher homolysis rate. Different models have been suggested, but none can be considered as fully satisfactory in light of a large body of experimental results [1,5]. Mechanisms that have been considered to account for the enzyme-accelerated Co–C R homolysis most frequently involve steric interaction of the cofactor with the protein matrix, such as distortion of the corrin ring (the ‘butterfly bending’), a direct lengthening of the Co–C R bond, separation of the Co and Ado moieties or angular distortion of the Co–C R bond [1–5]. Others, such as an alternation in the position of the trans axial base [11,12], or electron transfer have also been suggested [2]. This short review focuses on recent advances in theoretical modeling of Co–C R bond activation in B 12 -dependent Quantum chemical modeling of Co–C bond activation in B 12 -dependent enzymes Pawel M Kozlowski Figure 1 Molecular structure of coenzyme B 12 (5′-deoxy-5′adenosylcobalamin). The interligand N DBI –Co–C Ado bond lengths are denoted as Co–N B and Co–C R , respectively (see also Figure 2). N N N N Co N N Me Me Me Me Me Me Me Me Me Me Ribose R′′ R R R R′ R′ R′ H 2 C O OH HO A A = Adenine R = CH 2 CONH 2 R′ = CH 2 CH 2 CONH 2 R′′ = (CH 2 ) 2 CONHCH 2 CH(Me)OPO 3 Current Opinion in Chemical Biology