(1907-2000) American biochemist who, among other things, discovered the biologically active forms of vitamin B12. Horace Albert Barker, informally called "Nook," was educated at Stanford University and earned a PhD in chemistry in 1933. His research interest then turned to soil microbiology and microbial biochemistry. He set out on a two-year postdoctoral fellowship to study first with C. B. van Niel at the Hopkins Marine Station, Pacific Grove, California, and then a year in the Netherlands to study with van Niel's mentor, A. J. Kluyver in Delft. There he initiated an investigation that would later lead him to discover vitamin B12 coenzyme. Coenzymes are non-protein molecule that help the catalytic function of enzymes.