Abstract
The transaldolase gene family includes representatives from all kingdoms
of life and the encoded enzymes are important for carbohydrate
metabolism. The family can be divided into two subfamilies; Classical
transaldolases and the MipB/TalC proteins. The reaction catalysed by the
classical transaldolases is a reversible transfer of a dihydroxyacetone
moiety from a ketose to an aldose sugar while members from the MipB/TaIC
subfamily catalyse either this same reaction or a reversible cleavage of
fructose 6-phosphate. The first part of these reactions is the formation
of a covalent Schiff base intermediate of the substrate with an active
site lysine, a feature common for all class 1 aldolases.
3D structures of members from both subfamilies have been determined by
protein crystallography and they all show a single domain alpha/beta
barrel fold common to all class I aldolases. Classical transaldolases
from Escherichia coli and Homo sapiens are both dimers and show high
overall similarity. Fructose 6-phosphate aldolase (FSA) from Escherichia
coli, a member of the MipB/TalC subfamily, folds into a more compact
barrel and is arranged as a decamer. The decamer is created through helix
swapping of the C-terminal helix of FSA, the equivalent helix in the
classical transaldolases covering the active site via a loop and being
involved in the dimer interface.
Site-directed mutagenesis in combination with structural analysis was
used to elucidate the mechanistic role of several active site residues in
the classical Escherichia coli transaldolase. Fructose 6-phosphate
aldolase catalyses a reversible cleavage instead of a transfer reaction
and analysis of its active site compared to the classical transaldolase
suggested explanations to this mechanistic difference.