ABSTRACT
Histidine biosynthesis is arguably the most interesting of the amino acid biosynthetic pathways. Through a series of complex intennediates. beginning with ATP as a substrate. the molecule is formed at great energetic expense to the cell. The pathway shares a direct link with nucleotide metabolism. not only for starting materials. but also because a large purine intermediate is discarded as a side producL Additionally. histidine is thought to be an ancient molecule. perhaps one of the earliest amino acids. because of its synthesis directly from an RNA building block, and because of its frequent role in a catalytic position in a wide variety of enzymes.
