J. Biol. Chem., Vol. 264, Issue 1, 45-53, Jan, 1989
The structure of tyrosine aminotransferase. Evidence for domains involved in catalysis and enzyme turnover
JL Hargrove, HA Scoble, WR Mathews, BR Baumstark and K Biemann
Department of Anatomy and Cell Biology, Emory University School of Medicine, Atlanta, Georgia 30322.
The primary structure of tyrosine aminotransferase, as deduced from the
nucleotide sequence of complementary DNA, was confirmed by fast atom
bombardment mass spectrometry of tryptic peptides derived from the purified
protein. Limited digestion of the native enzyme with trypsin released an
acetylated, amino-terminal peptide; the new amino terminus in the modified
enzyme was Val65. Endogenous proteases generated a chromatographically
separable form of tyrosine aminotransferase that began at Lys35. Neither
trypsin nor the other proteases altered the catalytic activity of tyrosine
aminotransferase. Reduction of the holoenzyme with sodium borohydride
yielded a major tryptic peptide containing phosphopyridoxamine bound to
lysine 280, which probably functions in transamination. The carboxyl
terminus of tyrosine aminotransferase contains features that typify
proteins with short half- lives; it includes two negatively charged,
hydrophilic segments that are enriched for glutamyl residues and are
similar to a PEST region in ornithine decarboxylase (Rogers, S., Wells, R.,
and Rechsteiner, M. (1986) Science 234, 364-368). Tyrosine aminotransferase
belongs to a superfamily of enzymes which includes aspartate
aminotransferase and can be aligned so that many invariant, functional
residues coincide. Like the isoenzymes of aspartate aminotransferase,
tyrosine aminotransferase may contain two domains, with a central,
catalytic core, and a small domain made up of both amino- and
carboxyl-terminal components. We speculate that the exposed small domain
may confer the unusually rapid degradative rate that characterizes this
enzyme.