In mammals, aldolase B is preferentially expressed in the liver, while aldolase A is expressed in muscle and erythrocytes and aldolase C is expressed in the brain. Slight differences in isozyme structure result in different activities for the two molecules, FBP and fructose 1-phosphate. Aldolase B exhibits no preference and thus catalyzes both reactions, while aldolases A and C prefer FBP, in humans, aldolase B is encoded by the ALDOB gene located on chromosome 9. The gene is 14,500 base pairs long and contains 9 exons, defects in this gene have been identified as the cause of hereditary fructose intolerance. The generic fructose bisphosphate aldolase enzyme cleaves a 6-carbon fructose sugar into two 3-carbon products in an aldol reaction. After Schiff base formation, the hydroxyl group on the fructose backbone is then deprotonated by an aspartate residue. Schiff base hydrolysis yields two 3-carbon products, depending on the reactant, F1P or FBP, the products are DHAP and glyceraldehyde or glyceraldehyde 3-phosphate, respectively. The ΔG°’ of this reaction is +23.9 kJ/mol, though the reaction may seem too uphill to occur, it is of note that under physiological conditions, the ΔG of the reaction falls to close to or below zero. For example, the ΔG of this reaction under physiological conditions in erythrocytes is -0.23 kJ/mol, click on genes, proteins and metabolites below to link to respective articles. Aldolase B is an enzyme, composed of four subunits with molecular weights of 36 kDa with local 222 symmetry. Each subunit has a weight of 36 kDa and contains an eight-stranded α/β barrel. Such regions have been denoted isozyme-specific regions and these regions are thought to give isozymes their specificities and structural differences. ISRs 1-3 are all found in exon 3 of the ALDOB gene, ISR4 is the most variable of the four and is found at the c-terminal end of the protein. ISRs 1-3 are found predominantly in patches on the surface of the enzyme and these patches do not overlap with the active site, indicating that ISRs may change specific isozyme substrate specificity from a distance or cause the C-terminus interactions with the active site. A recent theory suggests that ISRs may allow for different conformational dynamics in the enzyme that account for its specificity. Aldolase B plays a key role in metabolism as it catalyzes one of the major steps of the glycolytic-gluconeogenic pathway. Though it does catalyze the breakdown of glucose, it plays an important role in fructose metabolism, which occurs mostly in the liver, renal cortex. When fructose is absorbed, it is phosphorylated by fructokinase to form fructose 1-phosphate, aldolase B then catalyzes F1P breakdown into glyceraldehyde and DHAP
The aldol cleavage of fructose 1-phosphate by aldolase b yields dihydroxyacetone phosphate and glyceraldehyde. NOTE: THE STRUCTURES HERE ARE INCORRECT. EG. GLYCERALDEHYDE AND FRUCTOSE ARE MISSING THE TERMINAL HYDROXYL GROUPS
The aldol cleavage of fructose 1,6-bisphosphate by aldolase b demonstrates the different reaction products, dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Abbreviations: DHAP - dihydroxyacetone phosphate; Fru1,6bP - Fructose-1,6-bisphosphate; GAD - glyceraldehyde 3-phosphate".