High fructose corn syrup (HFCS) is a sweetening agent used in the food industry.

Question

High fructose corn syrup (HFCS) is a sweetening agent used in the food industry. HFCS is made enzymatically from cornstarch. Starch is first converted to short oligosaccharides and then to glucose monomers. Then an isomerase is used to convert glucose to fructose, and the resulting product is an approximately 50/50 mixture of glucose and fructose.

(a) Why bother? Why is glucose converted to fructose, rather than simply being used directly as a sweetener?

(b) Why is HFCS a 50/50 mixture? Why is it not possible to convert more of the glucose to fructose?

(c) In liver, fructose is metabolized by the pathway shown below. If glyceraldehyde-3- phosphate is converted to pyruvate by the reactions of glycolysis, what is the net ATP yield per molecule of fructose? [For interest: mutations in the gene encoding aldolase B cause hereditary fructose intolerance, which requires dietary exclusion of fructose].

Explanation / Answer

a) Fructose is much sweeter than glucose. So, HFCS can be used in small quantities to give desired sweetness.

c) Fructose metabolism also produces the same amount of ATP as glycolysis (38 ATP). In one pathway fructose-1-phosphate is formed in liver, which is then broken down into glyceraldehyde phosphate and DHAP by aldolase. GP is then phosphorylated to glyceraldehyde 3-phosphate. This pathway skips the rate limiting and regulatory step of phosphofructokinase.

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