HsaF and HsaG are an
aldolase and
dehydrogenase from the
cholesterol degradation pathway of Mycobacterium tuberculosis. HsaF could be heterologously expressed and purified as a soluble dimer, but the
enzyme was inactive in the absence of HsaG. HsaF catalyzes the
aldol cleavage of 4-hydroxy-2-oxoacids to produce
pyruvate and an
aldehyde. The
enzyme requires divalent metals for activity, with a preference for Mn(2+). The Km values for 4-hydroxy-2-oxoacids were about 20-fold lower than observed for the
aldolase homologue, BphI from the
polychlorinated biphenyl degradation pathway.
Acetaldehyde and
propionaldehyde were channeled directly to the
dehydrogenase, HsaG, without export to the bulk
solvent where they were transformed to
acyl-CoA in an
NAD(+) and
coenzyme A dependent reaction. HsaG is able to utilize
aldehydes up to five carbons in length as substrates, with similar catalytic efficiencies. The HsaF-HsaG complex was crystallized and its structure was determined to a resolution of 1.93 Å. Substitution of
serine 41 in HsaG with
isoleucine or
aspartate resulted in about 35-fold increase in Km for
CoA but only 4-fold increase in Km
dephospho-CoA, suggesting that this residue interacts with the 3'-ribose
phosphate of
CoA. A second
protein annotated as a
4-hydroxy-2-oxopentanoic acid aldolase in M.
tuberculosis (
MhpE, Rv3469c) was expressed and purified, but was found to lack
aldolase activity. Instead this
enzyme was found to possess
oxaloacetate decarboxylase activity, consistent with the conservation (with the 4-hydroxy-2-oxoacid aldolases) of residues involved in
pyruvate enolate stabilization.