Which of the following enzymes is unique to gluconeogenesis and is NOT involved in glycolysis?

Explanation

## Gluconeogenesis-Specific Enzymes **Key Point:** Gluconeogenesis requires four enzymes that are either absent from glycolysis or catalyze irreversible reactions in the opposite direction. Fructose-1,6-bisphosphatase (FBPase-1) is the classic gluconeogenesis-specific enzyme. ### The Four Gluconeogenesis-Unique Steps | Step | Substrate | Product | Enzyme | Glycolysis Equivalent | |------|-----------|---------|--------|----------------------| | 1 | Pyruvate | Oxaloacetate | Pyruvate carboxylase | Irreversible (PK) | | 2 | Oxaloacetate | Phosphoenolpyruvate | PEPCK | Irreversible (PK) | | 3 | Fructose-1,6-bisphosphate | Fructose-6-phosphate | **FBPase-1** | Irreversible (PFK-1) | | 4 | Glucose-6-phosphate | Glucose | Glucose-6-phosphatase | Irreversible (HK) | **High-Yield:** FBPase-1 is the rate-limiting enzyme of gluconeogenesis and is inhibited by AMP and fructose-2,6-bisphosphate (F-2,6-BP) — the same signals that activate glycolysis. ### Why Other Options Are Wrong - **Phosphofructokinase-1 (PFK-1):** The rate-limiting enzyme of *glycolysis*, not gluconeogenesis. It catalyzes the reverse reaction (F-6-P → F-1,6-BP) in gluconeogenesis, but this is catalyzed by FBPase-1, not PFK-1. - **Pyruvate kinase:** A glycolytic enzyme; its reverse reaction in gluconeogenesis is catalyzed by PEPCK, not PK. - **Aldolase:** A shared enzyme present in both pathways; catalyzes aldol cleavage/condensation in both directions. **Clinical Pearl:** Deficiency of FBPase-1 causes fructose-1,6-bisphosphatase deficiency, a rare autosomal recessive disorder presenting with severe hypoglycemia, lactic acidosis, and hepatomegaly during fasting. **Mnemonic:** **PPPG** — The four gluconeogenesis-unique enzymes: **P**yruvate carboxylase, **P**EPCK, **P**hosphatase (FBPase-1), **G**lucose-6-phosphatase.