Sucrose Phosphorylase - Metabolic Function

Metabolic Function

As mentioned above, sucrose phosphorylase is a very important enzyme in metabolism. The reaction catalyzed by sucrose phosphorylase produces the valuable byproducts α-D-glucose-1-phosphate and fructose. α-D-glucose-1-phosphate can be reversibly converted by phosphoglucomutase to glucose-6-phosphate (Tedokon et al. 1992), which is an important intermediate used in glycolysis. In addition, fructose can be reversibly converted into fructose 6-phosphate (Reid and Abratt 2005), also found in the glycolytic pathway. In fact, fructose-6-phosphate and glucose-6-phosphate can be interconverted in the glycolytic pathway by phosphohexose isomerase (Nelson and Cox 2005). The final product of glycolysis, pyruvate, has multiple implications in metabolism. During anaerobic conditions, pyruvate con be converted into either lactate or ethanol, depending on the organism, providing a quick source of energy. In aerobic conditions, pyruvate can be converted into Acetyl-CoA, which has many possible fates including catabolism in the Citric Acid Cycle for energy use and anabolism in the formation of fatty acids for energy storage. Through these reactions, sucrose phosphorylase becomes important in the regulation of metabolic functions.

The regulation of sucrose phosphorylase can also be used to explain its function in terms of energy consumption and preservation. The cAMP-CRP complex that enhances transcription of the sucrose phosphorylase gene (Reid and Abratt 2003) is only present when glucose levels are low. The purpose of sucrose phosphorylase, therefore, can be linked to the need for higher glucose levels, created through its reaction. The fact that glucose acts as a feedback inhibitor to prevent the formation of sucrose phosphorylase (Reid and Abratt 2005) further supports its catalytic role in the creation of glucose for energy use or storage.

The glucose-6-phosphate molecule created from the original α-D-glucose-1-phosphate product is also involved in the pentose phosphate pathway. Through a series of reactions, glucose-6-phosphate can be converted into ribose-5-phosphate, which is used for a variety of molecules such as nucleotides, coenzymes, DNA, and RNA (Nelson and Cox 2005). These connections reveal that sucrose phosphorylase is also important for the regulation of other cellular molecules.