Corrinoid specificity of cytosolic cobalamin-binding protein of Euglena gracilis z

To elucidate the corrinoid specificity of the cytosolic cobalamin-binding protein of Euglena gracilis, inhibition of the binding of radioactive cyanocobalamin to the cytosolic binding protein was studied with a variety of cobalamin analogues. The cytosolic cobalamin-binding protein showed an absolute requirement for the alpha-axial ligand (the cobalt-coordinated nucleotide) in cobalamin binding, but was not able to recognize certain differences in the base or ribose moiety. Regarding the contributions of the b-, d-, and e-propionamide side chains in the binding of cobalamin to the cytosolic protein, the order of the contributions was shown to be b > d > e; in particular the b-propionamide side chain was essential for the formation of the protein-cobalamin complex. No involvement of the beta-axial ligand or the alkanolamine group in the binding of cobalamin to the protein was found.