Question

A \(6\) month old infant presented with hypoglycemia, vomiting, diarrhea, protein-loss enteropathy, and hepatic fibrosis. Measurement of the glycosylation state of endogenous serum transferrin revealed Type Ib CDG (congenital disorders of glycosylation), which is a defect in phosphomannose isomerase activity. The role of phosphomannose isomerase is the interconversion of A. mannose 6 -phosphate and mannose 1 -phosphate. B. glucose 6 -phosphate and mannose 6 -phosphate. C. fructose 6 -phosphate and mannose 6 -phosphate. D. fructose 1 -phosphate and mannose 1 -phosphate. E. glucose 6 -phosphate and mannose 1 -phosphate.

Short answer

Answer: Phosphomannose isomerase interconverts glucose 6-phosphate and mannose 6-phosphate.

Step-by-step solution

Understanding Phosphomannose Isomerase

Phosphomannose isomerase (PMI) is an essential enzyme in the metabolism of carbohydrates. It participates in the hexose monophosphate pathway, which helps in the synthesis of glycoproteins and glycolipids. Its primary role is the interconversion of hexose phosphates to generate essential precursors for various metabolic reactions.

Analyzing the Options

Now let's analyze each of the given options and determine which one describes the role of phosphomannose isomerase accurately: A. Mannose 6-phosphate and mannose 1-phosphate B. Glucose 6-phosphate and mannose 6-phosphate C. Fructose 6-phosphate and mannose 6-phosphate D. Fructose 1-phosphate and mannose 1-phosphate E. Glucose 6-phosphate and mannose 1-phosphate

Option A: Mannose 6-phosphate and mannose 1-phosphate

This option involves interconversion between two phosphorylated forms of mannose. However, phosphomannose isomerase is not responsible for interconversion between different phosphorylated forms of the same sugar.

Option B: Glucose 6-phosphate and mannose 6-phosphate

This is the correct answer. Phosphomannose isomerase catalyzes the interconversion between glucose 6-phosphate and mannose 6-phosphate. This conversion is essential because mannose 6-phosphate is required for glycoprotein and glycolipid synthesis, while glucose 6-phosphate is an essential metabolic intermediate.

Option C: Fructose 6-phosphate and mannose 6-phosphate

This option involves interconverting fructose and mannose, two different sugars. While phosphomannose isomerase does catalyze interconversion between hexose phosphates, it does not involve fructose in this case.

Option D: Fructose 1-phosphate and mannose 1-phosphate

This option involves interconverting fructose and mannose, two different sugars. Furthermore, phosphomannose isomerase is involved in the conversion of hexose 6-phosphates, not 1-phosphates.

Option E: Glucose 6-phosphate and mannose 1-phosphate

This option does involve interconversion between glucose and mannose, but it considers the 1-phosphate form of mannose, which is not the correct substrate for phosphomannose isomerase.

Conclusion

Based on our analysis, the correct answer is (B) glucose 6-phosphate and mannose 6-phosphate. Phosphomannose isomerase catalyzes the interconversion of these two important metabolic intermediates, allowing the synthesis of glycoproteins and glycolipids.