EFFECT OF THE COBALT-N COORDINATION ON THE COBAMIDE RECOGNITION BY THE HUMAN VITAMIN-B12 BINDING-PROTEINS INTRINSIC-FACTOR, TRANSCOBALAMIN AND HAPTOCORRIN

The binding of several corrinoids to the binding site of human intrinsic factor, transcobalamin or haptocorrin was investigated. p-Cresolyl cobamide and 2-amino-vitamin B-12 are complete corrinoids, whose nucleotide at the lower face of the corrin ring is not coordinated to the cobalt. These corrinoids were greater-than-or-equal-to 10(3) times less efficiently recognized by intrinsic factor or transcobalamin than vitamin B-12, which contains a Co-coordinated nucleotide. Pseudovitamin B-12, With a weak Co-N coordination bond, revealed only moderate affinity to intrinsic factor. From these findings it is concluded that the cobamide binding to intrinsic factor and transcobalamin is strongly affected by the Co-N coordination bonds of their lower cobalt nucleotide ligands. We suggest that the Co-N coordination bond positions the nucleotide at a critical distance to the corrin ring, which is recognized by the binding proteins. Human haptocorrin, however, disclosed to distinctive selectivity regarding the different corrinoid structures. The protein bound all corrinoids with similar efficiency, independent of the strength of their Co-N coordinations, or the structures of their lower Co-alpha-ligands. Hence, the corrin ring, rather than a structural feature induced by the Co-N coordination, has to be considered responsible for the corrinoid binding to haptocorrin.