Fig. 1.

Ascorbic acid synthesis pathway in the presence of GULO or GULOP. Glucose is the starting carbohydrate for ascorbic acid synthesis. Phosphorylation of glucose by hexokinase forms glucose-6-phosphate which can be shuttled to the ascorbic acid synthesis pathway or used in glycolysis or the pentose phosphate pathway. In animals that have a functional GULO gene, the L-gulono-gamma lactone precursor is converted to ascorbate (vitamin C), but in animals with a GULOP gene, the pathway is non-functional. Those animals require dietary vitamin C and are prone to scurvy in its absence