The most common carbon–nitrogen functional groups in chemical biology are amines, imines, and amides, resulting from the fixation of ammonia into organic cosubstrates 2-ketoglutarate or glutamate scaffolds on the way to the two nitrogens of glutamine. The pyridoxal-P coenzyme form of vitamin B6 provides its aldehyde group for aldimine formation with dozens of amino acids as a prelude to cleavage of any one of the four bonds to C₂ of amino acid-PLP aldimine adducts. Three additional C–N groups that join amides in nucleic acid structures are amidines, ureas and guanidines, the H-bond donor and acceptors in Watson–Crick bonding of G–C and A–T pairs.