Vitamin B12 (cobalamin) is an essential micronutrient required by organisms from all domains of life [1]. Cobalamin catalyzes complex methyl transfer and radical-based chemistry in biological systems, thereby impacting cellular metabolism in fundamental ways [2,3]. In humans, the methyl group in methionine derives from methylcobalamin (MeCbl), and adenosylcobalamin (AdoCbl) converts a toxic byproduct methylmalonyl-CoA to succinyl-CoA through a radical-based isomerization reaction. Structurally, B12 is classified as a tetrapyrrole along with heme, chlorophyll, siroheme, coenzyme F430, and bilins [4, 5, 6]. The features of vitamin B12 that distinguish it from other tetrapyrroles are (i) a cobalt-containing contracted macrocycle called the corrin ring, and (ii) upper and lower ligands coordinated to the cobalt ion (Figure 1a). The lower ligand may be in a coordinated (base-on) or free (base-off) conformation, and the base-on/off equilibrium plays an important role in the binding and catalysis of cobalamin within proteins and riboswitches (Figure 1b). Alongside B12, there exists a range of its naturally occurring analogues called cobamides which differ from one another mainly based on the lower ligand. Cobamides are produced by a >30 step route, a metabolically expensive endeavor [4,7]. Thus, their production is limited to a subset of bacteria and archaea. Several organisms depend on their diet, environment, or one another to obtain cobamides [8].

The presence (or absence) of multiple methyl (Me) groups throughout the structure of vitamin B12 imparts varying chemical and physical properties (Figure 1a, b). The methyl groups of B12 can be broadly classified into three categories: the catalytic methyl upper ligand in MeCbl, the conserved methyl groups on the corrin framework, and the variable methyl groups of the nucleotide loop and the lower ligand (Figure 1a). In this minireview, we discuss the molecular origins and structural significance of these methyl group categories in light of novel insights in the field, particularly from the past two years.