phosphoenolpyruvate carboxykinase

**Reaction:** Oxaloacetate (OAA) + GTP → phosphoenolpyruvate + GDP + CO2. It is a decarboxylation plus phosphorylation. Pyruvate carboxylase (biotin, ATP, CO2; mitochondrial; activated by acetyl-CoA) makes OAA from pyruvate first. PEPCK then converts OAA to PEP, bypassing glycolytic pyruvate kinase.

**Location:** Isoforms are cytosolic (PEPCK-C, PCK1) and mitochondrial (PEPCK-M, PCK2). In humans, both are present. Gluconeogenesis occurs mainly in the liver, with a smaller contribution from the renal cortex, and a minor one from the small intestine. OAA is made in mitochondria and must be shuttled out as malate (via malate dehydrogenase) or aspartate, because OAA cannot cross the inner mitochondrial membrane. The malate shuttle also carries NADH reducing equivalents to the cytosol, where they are needed for the glyceraldehyde-3-phosphate dehydrogenase step.

**Regulation (mainly transcriptional):** - Induced by glucagon (via cAMP/CREB), cortisol (glucocorticoid response element), and epinephrine. - Repressed by insulin. - The pathway is also controlled at other steps: pyruvate carboxylase (activated by acetyl-CoA), fructose-1,6-bisphosphatase (inhibited by fructose-2,6-bisphosphate and AMP), and glucose-6-phosphatase.

**Four key gluconeogenic enzymes (bypass irreversible glycolytic steps):** "Pathway Produces Fresh Glucose" 1. **P**yruvate carboxylase (mitochondria) 2. **P**EP carboxykinase (cytosol) 3. **F**ructose-1,6-bisphosphatase (cytosol) 4. **G**lucose-6-phosphatase (ER; liver, kidney, intestine only)

**Clinical significance:** - **Fasting/starvation:** PEPCK induction maintains blood glucose from lactate, glycerol, and glucogenic amino acids (especially alanine). Fatty acids cannot make net glucose in humans, because even-chain fatty acids yield acetyl-CoA, which cannot be converted to OAA. Odd-chain fatty acids yield propionyl-CoA, which enters via succinyl-CoA and is glucogenic. - **Diabetes mellitus:** Insulin deficiency or resistance causes loss of PEPCK repression, which leads to excess hepatic glucose output and fasting hyperglycemia. Metformin decreases hepatic gluconeogenesis (by inhibiting mitochondrial glycerophosphate dehydrogenase and complex I, and by activating AMPK). - **Ethanol metabolism:** Increased NADH/NAD+ ratio shunts pyruvate to lactate and OAA to malate. This reduces gluconeogenic substrate and causes fasting hypoglycemia in alcoholics with depleted glycogen. - **Inherited deficiency:** Rare cytosolic PEPCK deficiency (PCK1) causes severe fasting hypoglycemia, lactic acidosis, hepatic steatosis, and failure to thrive. Mitochondrial deficiency (PCK2) has been described in rare cases. - **Related mnemonic:** Cortisol excess (Cushing syndrome) increases gluconeogenic enzyme expression and causes hyperglycemia. - **Other:** Glycerol enters gluconeogenesis at DHAP via glycerol kinase and glycerol-3-phosphate dehydrogenase, so it bypasses PEPCK. PEPCK also has roles in glyceroneogenesis in adipose tissue, which supplies glycerol-3-phosphate for triglyceride synthesis, and in the TCA cycle for cataplerosis.

**High-yield contrast:** Pyruvate kinase (glycolysis) makes pyruvate from PEP and generates ATP. PEPCK works in the opposite direction, from OAA to PEP, and consumes GTP (equivalent to ATP). Overall gluconeogenesis from 2 pyruvate requires 4 ATP + 2 GTP (6 high-energy phosphate bonds) and 2 NADH.