The transaldolase family : structure, function and evolution

Sammanfattning: 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.