Fructose metabolism takes place in the cytosol. D-Fructose can be synthesized from either isomerization of alpha-D-glucose via xylose isomerase or from sorbitol via sorbitol dehydrogenase. D-Fructose can then be directed to either Amino Sugar and Nucleotide Sugar Metabolism or used to synthesize beta-D-fructose 6-phosphate, either by hexokinase or fructokinase. Beta-D-fructose 6-phosphate can also be synthesized from D-fructose 2,6-bisphosphate via the enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Next, 6-phosphofructokinase uses ATP to catalyze the phosphorylation of D-fructose 6-phosphate to fructose 1,6-bisphosphate. It requires a magnesium ion as a cofactor. Alternatively, fructose 1,6-bisophosphate can be synthesized from beta-D-fructose 6-phosphate from pyrophosphate--fructose 6-phosphate 1-phosphotransferase. This enzyme has a regulatory alpha subunit and a catalytic beta subunit and requires magnesium ion as a cofactor. Fructose 1,6-bisophosphate can be re-synthesized into beta-D-fructose 6-phosphate by fructose-1,6-bisphosphatase. Next, fructose-bisphosphate aldolase catalyzes the conversion of beta-D-fructose 1,6-bisphosphate into D-glyceraldehyde 3-phosphate and glycerone phosphate. Triosephosphate isomerase can also catalyze the interconversion of D-glyceraldehyde 3-phosphate and glycerone phosphate. Fructose-bisphosphate aldolase also catalyzes the reversible conversion of D-fructose 1-phosphate into glycerone phosphate and D-glyceraldehyde. D-glyceraldehyde is theorized to be then be subsequently converted into D-glyceraldehyde 3-phosphate via the predicted protein triokinase (coloured in orange in the image).

Fructose Metabolism References