Key Enzymes in Metabolism to Know for Biochemistry

Enzymes are vital players in metabolism, driving biochemical reactions that fuel life. This overview highlights key enzymes, their roles in energy production, and how they regulate metabolic pathways, connecting the dots between energy needs and biochemical processes in our bodies.

1. Hexokinase

   • Catalyzes the phosphorylation of glucose to glucose-6-phosphate, the first step in glycolysis.
   • Functions as a key regulatory enzyme, controlling glucose entry into cells.
   • Inhibited by its product, glucose-6-phosphate, to prevent excessive glucose metabolism.

2. Phosphofructokinase

   • Acts as the main regulatory step in glycolysis by converting fructose-6-phosphate to fructose-1,6-bisphosphate.
   • Allosterically activated by AMP and fructose-2,6-bisphosphate, indicating low energy status.
   • Inhibited by ATP and citrate, signaling sufficient energy availability.

3. Pyruvate kinase

   • Catalyzes the conversion of phosphoenolpyruvate to pyruvate, the final step of glycolysis.
   • Regulated by fructose-1,6-bisphosphate (feedforward activation) and ATP (inhibition).
   • Plays a crucial role in energy production by generating ATP.

4. Citrate synthase

   • Initiates the citric acid cycle by condensing acetyl-CoA and oxaloacetate to form citrate.
   • Regulated by the availability of substrates (acetyl-CoA and oxaloacetate) and inhibited by ATP and NADH.
   • Essential for linking carbohydrate and fat metabolism.

5. Isocitrate dehydrogenase

   • Catalyzes the oxidative decarboxylation of isocitrate to alpha-ketoglutarate in the citric acid cycle.
   • Regulated by NAD+ (activation) and NADH (inhibition), reflecting the cell's redox state.
   • Produces CO2 and NADH, contributing to energy production.

6. Alpha-ketoglutarate dehydrogenase

   • Converts alpha-ketoglutarate to succinyl-CoA, another key step in the citric acid cycle.
   • Inhibited by its products (succinyl-CoA and NADH) to regulate the cycle's flow.
   • Plays a role in amino acid metabolism and energy production.

7. Acetyl-CoA carboxylase

   • Catalyzes the carboxylation of acetyl-CoA to malonyl-CoA, the first step in fatty acid synthesis.
   • Activated by citrate and inhibited by long-chain fatty acyl-CoA, regulating fatty acid synthesis based on energy needs.
   • Plays a critical role in lipid metabolism and energy storage.

8. Fatty acid synthase

   • A multi-enzyme complex that synthesizes fatty acids from acetyl-CoA and malonyl-CoA.
   • Requires NADPH as a reducing agent, linking it to the pentose phosphate pathway.
   • Regulated by insulin and nutritional status, influencing lipid storage.

9. Carnitine palmitoyltransferase I

   • Facilitates the transport of long-chain fatty acids into the mitochondria for beta-oxidation.
   • Inhibited by malonyl-CoA, preventing simultaneous fatty acid synthesis and degradation.
   • Essential for energy production from fatty acids.

10. Glucose-6-phosphatase

    • Catalyzes the hydrolysis of glucose-6-phosphate to glucose, allowing glucose release into the bloodstream.
    • Primarily found in the liver and kidneys, playing a key role in gluconeogenesis and glycogenolysis.
    • Essential for maintaining blood glucose levels during fasting.

11. Phosphoenolpyruvate carboxykinase

    • Converts oxaloacetate to phosphoenolpyruvate in gluconeogenesis, bypassing the irreversible pyruvate kinase step.
    • Regulated by glucagon and cortisol, promoting gluconeogenesis during fasting.
    • Plays a critical role in maintaining glucose homeostasis.

12. Glycogen synthase

    • Catalyzes the addition of glucose units to glycogen, the storage form of glucose.
    • Activated by glucose-6-phosphate and insulin, promoting glycogen synthesis.
    • Inhibited by glucagon and epinephrine, regulating glycogen metabolism based on energy needs.

13. Glycogen phosphorylase

    • Catalyzes the breakdown of glycogen to glucose-1-phosphate, initiating glycogenolysis.
    • Activated by glucagon and epinephrine, signaling energy demand.
    • Inhibited by glucose-6-phosphate and ATP, preventing excessive glycogen breakdown.

14. Glutamine synthetase

    • Catalyzes the conversion of glutamate and ammonia to glutamine, playing a key role in nitrogen metabolism.
    • Regulated by feedback inhibition from its products (glutamine) and other amino acids.
    • Essential for maintaining nitrogen balance and amino acid homeostasis.

15. Carbamoyl phosphate synthetase I

    • Catalyzes the formation of carbamoyl phosphate from ammonia and bicarbonate, the first step in the urea cycle.
    • Activated by N-acetylglutamate, linking it to amino acid metabolism.
    • Plays a crucial role in detoxifying ammonia and regulating nitrogen excretion.