27
Co
Cobalt
58.93
Essential:
Bacteria, Archaea, animals
Cobalt
Major functions in cells: (1)
-B12 cofactor for isomerases
-Cofactor for one carbon transfers
-Cofactor for dehalogenases
Environmental and health impacts:
-Some marine systems are B12 limited
-Cobalamin deficiency is a problem in human nutrition
Learn More!
(1) Cobalt: B12 Cofactor
Co, most commonly in the form of the corrinoid-based cofactor known as vitamin B12, is required by most prokaryotes (Bacteria and Archaea) and many eukaryotes. The major exceptions appear to be plants and fungi (Rodionov et al., 2003). Co is most often used as a cofactor for a variety of enzymes, including isomerases and dehalogenases.
​
Many organisms (including humans) that do not synthesize B12 nevertheless have a dietary requirement for it. In humans, B12 is needed for methionine synthesis and as a cofactor for propionate metabolism. Organisms that require B12 but lack the potential for synthesis have B12-transport systems for acquiring this key nutrient.
(2) Substituting Ribonucleotide Reductase
A common adaptation to B12 limitation is the substitution of B12- dependent enzymes with B12-independent isozymes. One example is in Streptomyces coelicolor, which uses two RNR enzymes. The class 1a enzyme is oxygen dependent while the class II enzyme requires B12. When B12 is available, the class 1a enzyme is repressed (Borovok et al., 2006).
(3) Substituting Methionine Synthase
A common adaptation to B12 limitation is the substitution of B12- dependent enzymes with B12-independent isozymes. Another example is that E. coli encodes two forms of methionine synthase (Drummond and Matthews, 1993). In the absence of exogenous B12 (which E. coli is unable to synthesize de novo), the Zn-dependent MetE protein is required for methionine synthesis (Hondorp and Matthews, 2009). Conversely, in the presence of B12, synthesis of MetE is reduced (Wu et al., 1992).
Reciprocal regulation of the two methionine synthase isozymes is also noted in Mycobacterium tuberculosis. In this organism, a B12-sensing riboswitch selectively represses the transcription of the B12-independent isozyme (Warner et al., 2007).
​
Functional redundancy of methionine synthases is also widespread in the algae. Among the algae, B12 is obtained from bacterial symbionts. When present, B12 represses the synthesis of the B12-independent (METE) isozyme and functionally activates the B12-dependent enzyme (METH).
​