Tom Bobik Research Interests

Associate Professor
Genetics and biochemistry of vitamin B12

Dr. Bobik's laboratory is conducting research on the genetics and biochemistry of vitamin B12 in bacteria and in humans. Salmonella enterica is the primary organism under investigation because of the relative ease of genetic manipulation. Fundamental studies with this bacterium are paving the way for investigations in humans and other microbes.

Vitamin B12 which is also known as cyanocobalamin (CNCbl) is a precursor of two coenzymes, adenosylcobalamin (AdoCbl) and methylcobalamin (CH3Cbl). These coenzymes are required cofactors for a variety of enzymes found in bacteria and higher animals. CH3Cbl-dependent enzymes catalyze a variety of methyl transfer reactions and AdoCbl-dependent enzymes mediate difficult rearrangements and reductions via free radical mechanisms. Two human enzymes that require B12 are known and these enzymes are vital for good health. Among the Bacteria and Archaea, about 15 different B12-dependent enzymes have been reported and several have important biotechnology applications.

Fig. 1. Model for the conversion of vitamin B12 to the B12 coenzymes. Abbreviations are: CNCbl, cyanocobalamin, vitamin B12; GSCbl, glutahionylcobalamin; AdoCbl, adenosylcobalamin, coenzyme B12; CH3Cbl, methylcobalamin; Fre, Salmonella flavoprotein reductase; MSR, human methionine synthase reductase; MmaB, human adenosyltransferase; PduO, Salmonella adenosyltransferase; FldA, Esherichia coli flavodoxin A.

Currently, a major aim of Dr. Bobik's research is the isolation and characterization of the bacterial and human genes and enzymes used for the conversion of vitamin B12 to the B12 coenzymes. The proposed pathway for this process begins with the conversion of vitamin B12 (CNCbl) to glutathionylcobalamin (GSCbl) (Fig 1). Two successive one-electron reductions convert GSCbl to cob(II)alamin and then to cob(I)alamin which is alkylated to form AdoCbl. For CH3Cbl formation, cob(II)alamin associates with methionine synthase (or another methyl transferase enzyme) and is reductively methylated to form CH3Cbl.

A number of fundamental questions about vitamin B12 metabolism are answered. Several key genes and enzymes remain to be isolated and characterized from bacteria and humans. Dr. Bobik's laboratory is combining a variety of approaches including genetic, bioinformatic, genomic, and biochemical to fill these gaps in our knowledge about vitamin B12.

Dr. Bobik's research has importance in medicine and biotechnology. Defects in B12 metabolism result in elevated homocysteine levels (a major risk factor for heart disease) and altered folate pools (a key risk factor in cancer and neural tube defects). Therefore, fundamental knowledge about vitamin B12 is important for the prevention, diagnosis and treatment of these disorders as well as certain very serious rare pediatric diseases. New knowledge about vitamin B12 is also important to enhancing biotechnology processes that use coenzyme B12-dependent enzymes for chemical production since the efficient conversion of vitamin B12 (and related compounds) to the B12 coenzymes is essential for maintaining the activity B12-dependent enzymes. Such processes include the production of 1,3-propanediol (a substrate for synthetic fiber synthesis) and methylmalonyl-CoA (a precursor of macrolide antibiotics).