Aldehyde lyases

Fructose-bisphosphate aldolase (EC 4.1.2.13), often just aldolase, is an enzyme catalyzing a reversible reaction that splits the aldol, fructose 1,6-bisphosphate, into the triose phosphates dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P). Aldolase can also produce DHAP from other (3S,4R)-ketose 1-phosphates such as fructose 1-phosphate and sedoheptulose 1,7-bisphosphate. Gluconeogenesis and the Calvin cycle, which are anabolic pathways, use the reverse reaction. Glycolysis, a catabolic pathway, uses the forward reaction. Aldolase is divided into two classes by mechanism. Fructose-bisphosphate aldolase (EC 4.1.2.13), often just aldolase, is an enzyme catalyzing a reversible reaction that splits the aldol, fructose 1,6-bisphosphate, into the triose phosphates dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P). Aldolase can also produce DHAP from other (3S,4R)-ketose 1-phosphates such as fructose 1-phosphate and sedoheptulose 1,7-bisphosphate. Gluconeogenesis and the Calvin cycle, which are anabolic pathways, use the reverse reaction. Glycolysis, a catabolic pathway, uses the forward reaction. Aldolase is divided into two classes by mechanism. The word aldolase also refers, more generally, to an enzyme that performs an aldol reaction (creating an aldol) or its reverse (cleaving an aldol), such as Sialic acid aldolase, which forms sialic acid. Class I proteins form a protonated Schiff base intermediate linking a highly conserved active site lysine with the DHAP carbonyl carbon. Additionally, tyrosine residues are crucial to this mechanism in acting as stabilizing hydrogen acceptors. Class II proteins use a different mechanism which polarizes the carbonyl group with a divalent cation like Zn2+. The Escherichia coli galactitol operon protein, gatY, and N-acetyl galactosamine operon protein, agaY, which are tagatose-bisphosphate aldolase, are homologs of class II fructose-bisphosphate aldolase. Two histidine residues in the first half of the sequence of these homologs have been shown to be involved in binding zinc. The protein subunits of both classes each have an α/β domain folded into a TIM barrel containing the active site. Several subunits are assembled into the complete protein. The two classes share little sequence identity. With few exceptions only class I proteins have been found in animals, plants, and green algae. With few exceptions only class II proteins have been found in fungi. Both classes have been found widely in other eukaryotes and in bacteria. The two classes are often present together in the same organism. Plants and algae have plastidal aldolase, sometimes a relic of endosymbiosis, in addition to the usual cytosolic aldolase. A bifunctional fructose-bisphosphate aldolase/phosphatase, with class I mechanism, has been found widely in archaea and in some bacteria. The active site of this archaeal aldolase is also in a TIM barrel.