This is some scientific jargon that I don't understand, I was always terrible at chemistry. BUT, according to a website I found, these are the proper answers. Base-on cobalamin fall into strongly coordinating ligands which stabilise cobalt against reduction. Base-off/his-on binding is important for the catalytic activity of the enzymes. Location: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2995210/