With few exceptions only class I proteins have been found in animals, plants, and green algae.[2] With few exceptions only class II proteins have been found in fungi. Both classes have been found widely in other eukaryotes and in bacteria.[3] 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.[4] The active site of this archaeal aldolase is also in a TIM barrel.
In gluconeogenesis and glycolysis
Gluconeogenesis and glycolysis share a series of six reversible reactions. In gluconeogenesis glyceraldehyde-3-phosphate is reduced to fructose 1,6-bisphosphate with aldolase. In glycolysis fructose 1,6-bisphosphate is made into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate through the use of aldolase. The aldolase used in gluconeogenesis and glycolysis is a cytoplasmic protein.
Three forms of class I protein are found in vertebrates.
Aldolase A is preferentially expressed in muscle and brain; aldolase B in liver, kidney, and in enterocytes; and aldolase C in brain. Aldolases A and C are mainly involved in glycolysis, while aldolase B is involved in both glycolysis and gluconeogenesis.[5] Some defects in aldolase B cause hereditary fructose intolerance. The metabolism of free fructose in liver exploits the ability of aldolase B to use fructose 1-phosphate as a substrate.[6] Archaeal fructose-bisphosphate aldolase/phosphatase is presumably involved in gluconeogenesis because its product is fructose 6-phosphate.[7]
In the Calvin cycle
The Calvin cycle is a carbon fixation pathway; it is part of photosynthesis, which convert carbon dioxide and other compounds into glucose. It and gluconeogenesis share a series of four reversible reactions. In both pathways 3-phosphoglycerate (3-PGA or 3-PG) is reduced to fructose 1,6-bisphosphate with aldolase catalyzing the last reaction. A fifth reaction, catalyzed in both pathways by fructose 1,6-bisphosphatase, hydrolyzes the fructose 1-6-bisphosphate to fructose 6-phosphate and inorganic phosphate. The large decrease in free energy makes this reaction irreversible. In the Calvin cycle aldolase also catalyzes the production of sedoheptulose 1,7-bisphosphate from DHAP and erythrose 4-phosphate. The chief products of the Calvin cycle are triose phosphate (TP), which is a mixture of DHAP and G3P, and fructose 6-phosphate. Both are also needed to regenerate RuBP. The aldolase used by plants and algae in the Calvin cycle is usually a plastid-targeted protein encoded by a nuclear gene.
Aldolase is also used in the part of the Calvin cycle shared with gluconeogenesis, with the irreversible phosphate hydrolysis at the end catalyzed by fructose 1,6-bisphosphatase
Triose-phosphate isomerase maintains DHAP and G3P in near equilibrium, producing the mixture called triose phosphate (TP)
G3P ⇌ DHAP
Thus both DHAP and G3P are available to aldolase.
Moonlighting properties
Aldolase has also been implicated in many "moonlighting" or non-catalytic functions, based upon its binding affinity for many other proteins including F-actin, α-tubulin, light chain dynein, WASP, Band 3 anion exchanger, phospholipase D (PLD2), glucose transporter GLUT4, inositol trisphosphate, V-ATPase and ARNO (a guanine nucleotide exchange factor of ARF6). These associations are thought to be predominantly involved in cellular structure, however, involvement in endocytosis, parasite invasion, cytoskeleton rearrangement, cell motility, membrane protein trafficking and recycling, signal transduction and tissue compartmentalization have been explored.[8][9][10]
^Ahn AH, Dziennis S, Hawkes R, Herrup K (1994). "The cloning of zebrin II reveals its identity with aldolase C". Development. 120 (8): 2081–90. doi:10.1242/dev.120.8.2081. PMID7925012.
