Pyridoxal-5'-phosphate (P5P) is a cofactor in more than a hundred enzyme reactions. One of its functions is to perform transamination, a key step in both breaking down and producing amino acids in the body. P5P is also necessary to produce the neurotransmitters dopamine, epinephrine, norepinephrine, GABA and serotonin, as well as the hemoglobin in red blood cells. And P5P plays a central role in fat metabolism.