The crystal structure of tyrosine aminotransferase
(TAT) from the parasitic protozoan Trypanosoma cruzi,
which belongs to the aminotransferase subfamily Iγ,
has been determined at 2.5 Å resolution with the
R-value R = 15.1%. T. cruzi
TAT shares less than 15% sequence identity with aminotransferases
of subfamily Iα but shows only two larger topological
differences to the aspartate aminotransferases (AspATs).
First, TAT contains a loop protruding from the enzyme surface
in the larger cofactor-binding domain, where the AspATs
have a kinked α-helix. Second, in the smaller substrate-binding
domain, TAT has a four-stranded antiparallel β-sheet
instead of the two-stranded β-sheet in the AspATs.
The position of the aromatic ring of the pyridoxal-5′-phosphate
cofactor is very similar to the AspATs but the phosphate
group, in contrast, is closer to the substrate-binding
site with one of its oxygen atoms pointing toward the substrate.
Differences in substrate specificities of T. cruzi
TAT and subfamily Iα aminotransferases can be attributed
by modeling of substrate complexes mainly to this different
position of the cofactor-phosphate group. Absence of the
arginine, which in the AspATs fixes the substrate side-chain
carboxylate group by a salt bridge, contributes to the
inability of T. cruzi TAT to transaminate acidic
amino acids. The preference of TAT for tyrosine is probably
related to the ability of Asn17 in TAT to form a hydrogen
bond to the tyrosine side-chain hydroxyl group.