Glucose Metabolism in Biochemistry

Overview of Glucose Metabolism

  • Focuses on glucose catabolism and anabolism.

  • Key pathways:

    • Glycolysis: Breakdown of glucose to extract energy.

    • Citric Acid Cycle (TCA): Processes acetyl-CoA and produces electron carriers.

    • Oxidative Phosphorylation: Generates ATP using electron carriers.

Glycolysis Overview

  • Location: Cytosol of eukaryotes and prokaryotes.

  • 10 enzymatic steps involved:

    • Slight oxidation of glucose to extract energy.

    • Anaerobic process (doesn’t require O2).

  • Net production of:

    • 2 ATP (2 ATP invested and 4 produced).

    • 2 NADH (reduced energy carriers).

  • Key equation: Glucose + 2 ADP + 2 NAD+ + 2 Pi → 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

First Half of Glycolysis: Energy Investment Phase

  • Key Points:

    • Hexokinase increases glucose energy by phosphorylation.

    • Phosphofructokinase (PFK) has a strong negative ΔG, controlling pathway flux.

    • Aldolase reaction is unfavorable, yet subsequent reactions maintain low concentrations of products.

    • First half is anabolic, consuming 2 ATP.

Second Half of Glycolysis: Energy Payoff Phase

  • Key Points:

    • Involves 3-carbon molecules (occurs twice per glucose).

    • Dehydrogenase catalyzes redox reactions, producing NADH.

    • F-1,6-bisP activates pyruvate kinase (feed-forward activation).

    • 4 ATP produced, making this phase catabolic.

Regulation of Glycolysis

  • Glycolysis regulation relies on enzyme activity, particularly at steps with strongly negative ΔG:

    • Hexokinase (inhibited by Glc-6-P)

    • Phosphofructokinase (PFK): allosterically activated by ADP and Fructose-2,6-bisphosphate; inhibited by citrate.

    • Pyruvate kinase: regulated by feed-forward activation by F-1,6-bisP.

Fate of Pyruvate

  • Anaerobic respiration leads to lactate or ethanol production when O2 is absent:

    • Lactate produced during heavy exercise (muscle cells).

    • Ethanol produced by yeast, regenerating NAD+ for glycolysis.

  • Aerobic respiration: Pyruvate converted to Acetyl-CoA for TCA cycle.

Gluconeogenesis Overview

  • Occurs primarily in the liver to synthesize glucose when dietary intake is low.

  • Uses substrates like:

    • Pyruvate, lactate, glycerol, or citric acid cycle intermediates.

  • Not a simple reverse of glycolysis; requires 4 unique enzymes:

    • Pyruvate carboxylase

    • PEP carboxykinase

    • Fructose-1,6-bisphosphatase

    • Glucose-6-phosphatase

Glycogen Metabolism

  • Glycogen synthesis requires free energy and involves converting Glc-6-P into Glc-1-P using UTP.

  • Glycogen phosphorylase breaks down glycogen, regulated by hormones.

    • Hydrolysis of glycogen is inefficient; phosphorolysis is preferred for energy conservation.

  • Glycogen and starch synthesis follow similar mechanisms, differing mainly in the enzymes used.

Metabolism of Other Carbohydrates

  • Galactose is converted to Glc-1-P.

  • The Pentose Phosphate Pathway (PPP) processes glucose for ribose production and NADPH formation and has two phases:

    • Oxidative phase (produces NADPH and CO2).

    • Non-oxidative phase (interconverts various sugars).

Summary

  • Glucose metabolism includes diverse pathways depending on organismal needs: energy production (glycolysis), storage (glycogen synthesis), and contribution to other metabolic pathways (gluconeogenesis and PPP).

  • Regulation occurs mainly at key metabolic steps ensuring efficient energy use and storage.