Kinetic measurements of
enzyme activity indicate that type I
pantothenate kinase from Mycobacterium tuberculosis has dual substrate specificity for
ATP and
GTP, unlike the
enzyme from Escherichia coli, which shows a higher specificity for
ATP. A molecular explanation for the difference in the specificities of the two homologous
enzymes is provided by the crystal structures of the complexes of the M.
tuberculosis enzyme with (1) GMPPCP and pantothenate, (2)
GDP and
phosphopantothenate, (3)
GDP, (4)
GDP and pantothenate, (5)
AMPPCP, and (6) GMPPCP, reported here, and the structures of the complexes of the two
enzymes involving
coenzyme A and different adenyl
nucleotides reported earlier. The explanation is substantially based on two critical substitutions in the amino acid sequence and the local conformational change resulting from them. The structures also provide a rationale for the movement of
ligands during the action of the mycobacterial
enzyme. Dual specificity of the type exhibited by this
enzyme is rare. The change in locations of
ligands during action, observed in the case of the M.
tuberculosis enzyme, is unusual, so is the striking difference between two homologous
enzymes in the geometry of the binding site, locations of
ligands, and specificity. Furthermore, the dual specificity of the mycobacterial
enzyme appears to have been caused by a
biological necessity.