Gluconeogenesis, PPP, and Regulation of Glycolysis & Gluconeogenesis

Glycolysis, Respiration, and Overview of Gluconeogenesis

• Glycolysis (general): oxidation of glucose to pyruvate with formation of ATP and NADH; occurs in cytosol.

  • Anaerobic glycolysis: glucose (C6) → 2 pyruvate (2 × C3) → 2 lactate (in RBCs or exercising muscle) under anaerobic conditions; location: cytosol; cofactor balance: NADH ↔ NAD⁺.

  • In presence of O₂: glycolysis feeds into the TCA cycle via pyruvate oxidation to acetyl-CoA (PDH step) and subsequent oxidative phosphorylation in mitochondria; overall node: C6H12O6 → 2 CO₂ + 2 ATP (substrate-level) + NADH + FADH₂ → ATP via oxidative phosphorylation.

  • Net statement: Glycolysis and TCA together oxidize glucose to CO₂ and H₂O with ATP yield through respiration; spatial separation: glycolysis in cytosol; PDH and TCA in mitochondria; O₂-dependent steps in mitochondria.

• Respiratory chain and oxidative phosphorylation (OXPHOS):

  • Complexes of the electron transport chain (ETC):

  • Complex I: NADH dehydrogenase

  • Complex II: Succinate dehydrogenase

  • Complex III: Cytochrome bc1 complex

  • Complex IV: Cytochrome c oxidase

  • Complex I, III, IV act as proton pumps; ATP synthase acts as an ATP-dependent proton pump.

  • Electron flow: NADH → Complex I → ubiquinone (Q) → Complex III → cytochrome c → Complex IV → O₂; FADH₂ feeds at Complex II to the chain.

  • Oxidative phosphorylation uses the proton gradient to drive ATP synthesis: ADP + Pi → ATP.

  • Key definitions:

  • Oxidation: a substance loses electrons (often with uptake of oxygen).

  • Reduction: a substance gains electrons (often with uptake of hydrogen).

Gluconeogenesis: Definition, Purpose, and Core Features

• Definition: biosynthesis of glucose from non-carbohydrate precursors; maintains blood glucose during fasting.

• Primary tissues: Liver and kidneys are the main gluconeogenic tissues; muscle does not contribute to circulating glucose via G6Pase.

• Intracellular compartments: gluconeogenesis largely occurs in both mitochondria and cytosol.

• Not a simple reversal of glycolysis; involves bypass reactions at irreversible glycolytic steps.

• Common gluconeogenic precursors:

  • Pyruvate

  • Lactate

  • Glycerol

  • Propionate

  • Glucogenic amino acids (AA):

  • Converted to Pyruvate: Ala, Ser, Gly, Thr, Cys, Trp

  • Converted to Oxaloacetate: Asp

  • Converted to α-Ketoglutarate: Glu, Gln, Pro, His, Arg

• Common precursors (summary): Pyruvate, Lactate, Glycerol, Propionate, Glucogenic AAs; all funnel into gluconeogenesis via distinct pathways.

Major Enzymes, Steps, and Bypass Reactions in Gluconeogenesis

• Key glycolytic irreversible steps (three):

  • Hexokinase/Glucokinase: glucose → glucose-6-phosphate; uses ATP.

  • 6-Phosphofructo-1-kinase (PFK-1): Fructose-6-phosphate → Fructose-1,6-bisphosphate; uses ATP.

  • Pyruvate kinase: Phosphoenolpyruvate (PEP) → Pyruvate; generates ATP.

  • Note: These steps are irreversible in glycolysis and require bypass via gluconeogenesis in the reverse direction.

• Bypass reactions in gluconeogenesis (three main bypasses):

  • Pyruvate → PEP bypass:

  • Pyruvate carboxylase (mitochondria): Pyruvate + CO₂ + ATP → Oxaloacetate + ADP + Pi

  • Phosphoenolpyruvate carboxykinase (PEPCK): Oxaloacetate + GTP → Phosphoenolpyruvate + CO₂ + GDP

  • Fructose-1,6-bisphosphate → Fructose-6-phosphate bypass:

  • Fructose-1,6-bisphosphatase: Fructose-1,6-bisphosphate + H₂O → Fructose-6-phosphate + Pi

  • Glucose-6-phosphate → Glucose bypass:

  • Glucose-6-phosphatase: Glucose-6-phosphate + H₂O → Glucose + Pi

• Compartmentalization of bypass enzymes:

  • Pyruvate → PEP bypass involves mitochondrial steps (pyruvate carboxylase) and cytosolic steps (PEPCK).

  • Fructose-1,6-bisphosphatase operates in the cytosol.

  • Glucose-6-phosphatase is in liver (and kidney) cytosol; its activity determines whether a tissue can contribute to circulating glucose.

Gluconeogenic Pathways by Precursors

• Pyruvate as precursor:

  • In mitochondria: conversion of Pyruvate to Oxaloacetate via Pyruvate Carboxylase (PC) and related steps.

  • In cytosol: Oxaloacetate converted to Phosphoenolpyruvate (PEP) by PEPCK.

  • Net effect: Pyruvate → Oxaloacetate → PEP → glucose.

• Lactate as precursor (Cori cycle):

  • Lactate produced from glycolysis in muscle and RBCs is transported to liver.

  • In liver: Lactate → Pyruvate → Oxaloacetate → PEP → glucose; four key reactions/enzymes are involved in this gluconeogenic route.

  • Outcome: Glucose released into circulation.

• Alanine as precursor (Alanine cycle):

  • Alanine released by muscle; in liver, deamination yields Pyruvate → glucose.

  • Muscle: alanine transamination to pyruvate; Liver: alanine deamination regenerates pyruvate for gluconeogenesis.

  • Four key reactions/enzymes are involved in this pathway to glucose.

• Glutamate, Aspartate as gluconeogenic sources:

  • Glutamate can be converted to Oxaloacetate via TCA or related pathways.

  • Aspartate can feed into oxaloacetate or other intermediates for gluconeogenic flux.

• Glycerol as a precursor:

  • glycerol enters gluconeogenesis after conversion to dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate, feeding into the pathway with two key reactions.

• Summary of precursors and their gluconeogenic routes:

  • Lactate, Pyruvate, Alanine, Glutamate, Aspartate, Glycerol as major precursors feed into gluconeogenesis via distinct sequences, with PEPCK, Pyruvate Carboxylase, Fructose-1,6-bisphosphatase, and Glucose-6-phosphatase as core control points.

Gluconeogenesis: Subcellular Localization and Pathway Layout

• Occurs in both mitochondria and cytosol:

  • Mitochondria: generation of PEP and the two key reactions/enzymes in this compartment.

  • Cytosol: conversion of PEP to glucose; two key reactions and two key enzymes.

• PEP generation and consumption: two key reactions/enzymes in mitochondria and subsequent steps in cytosol.

• Glycerol, lactate, alanine, glutamate, and aspartate provide four, four, four, three, three, and two key reactions, respectively, in their branching routes toward glucose synthesis.

Gluconeogenesis in the Liver and Kidney: Role of Glucose-6-Phosphatase

• Glucose-6-phosphatase (G6Pase) is the enzyme that catalyzes the final step of gluconeogenesis in the cytosol: Glucose-6-phosphate → Glucose + Pi.

• Presence of G6Pase determines whether a tissue can contribute glucose to circulation.

• Tissue expression for G6Pase: predominantly liver and kidney; not expressed in muscle.

• Conversion steps highlighted:

  • F-1,6-BisP to F-6-P (bypass in gluconeogenesis vs glycolysis)

  • G-6-P to Glucose (final glucose production)

• The interplay of these steps with glycolysis highlights tissue-specific contributions to circulating glucose (liver/kidney) vs tissue glycolytic activity (muscle).

Key Gluconeogenic Pathways Involving the Cori Cycle and Alanine Cycle

• Cori cycle (lactate-to-glucose):

  • Lactate produced in muscle and red blood cells is transported to the liver.

  • Four key reactions/enzymes are involved in gluconeogenesis from lactate to glucose.

  • Glucose then re-enters circulation.

• Alanine cycle (alanine-to-glucose):

  • Alanine released from muscle is transported to liver.

  • Liver converts alanine to pyruvate (via alanine transamination and deamination steps) and then to glucose.

  • Gluconeogenesis involves four key reactions/enzymes in this pathway.

• Conceptual takeaway: These cycles enable maintenance of blood glucose during fasting and extended exercise, by reallocating carbon from muscular activity to hepatic glucose production.

Gluconeogenesis via TCA Cycle Intermediates and Pyruvate Carboxylase bypass

• GNG can proceed via TCA cycle intermediates with bypassing pyruvate kinase-dependent steps.

• Glutamate as a precursor can feed into oxaloacetate via TCA cycle, providing oxaloacetate for PEP formation (via PEPCK).

• Pyruvate carboxylase and PEPCK are central to bypassing the pyruvate kinase step in glycolysis, enabling gluconeogenic flux from pyruvate toward phosphoenolpyruvate (PEP).

• This pathway illustrates the close integration between amino acid metabolism, the TCA cycle, and gluconeogenesis.

Gluconeogenesis Pathway in Cytosol vs Mitochondria and the Glycerol Pathway

• Cytosolic portion:

  • Conversion of PEP to glucose occurs in the cytosol via two key reactions and enzymes.

  • Glycerol as a gluconeogenic precursor enters gluconeogenesis after conversion to DHAP and glyceraldehyde-3-phosphate.

• Liver-specific aspects of gluconeogenesis (summary): Glucose produced in liver can be released to blood; liver plays a major role in maintaining euglycemia during fasting.

• Kidney contribution: also contributes to gluconeogenesis, especially during prolonged fasting.

• Overall liver-kidney cooperation ensures adequate glucose supply during fasting states.

Regulation of Glycolysis and Gluconeogenesis: Substrate Cycling and Hormonal Control

• Substrate cycling (futile cycles):

  • Substrate levels of key metabolites can control rate of glycolysis and gluconeogenesis.

  • Substrate cycles coordinate glycolysis and gluconeogenesis to optimize glucose homeostasis.

• Hormonal regulation:

  • Hormones alter enzyme activity and gene expression to tune the metabolic state.

  • Key hormones: Insulin, Glucagon, Epinephrine; counterregulatory hormones (to insulin) include glucagon and epinephrine, with glucocorticoids also involved.

• Enzyme activity, allosteric regulation, and gene expression collectively regulate glycolysis and gluconeogenesis.

• Key regulatory enzymes (glycolysis):

  • Irreversible steps include Glucokinase/Hexokinase, Phosphofructokinase-1, and Pyruvate kinase.

• Key regulatory enzymes (gluconeogenesis):

  • Phosphoenolpyruvate carboxykinase (PEPCK)

  • Pyruvate carboxylase

  • Fructose-1,6-bisphosphatase

  • Glucose-6-phosphatase

• Substrates that regulate these pathways (examples): Fructose-6-phosphate, Fructose-1-phosphate, Citrate, ATP/AMP; these substrates derive from metabolic flux and energy status.

• Glucose transport and transporter regulation (GLUTs):

  • GLUT4 is regulated by insulin and is important for uptake into skeletal muscle and adipose tissue postprandially.

  • Other GLUTs (e.g., GLUT1-3, GLUT2) have tissue-specific distributions and roles in glucose handling.

• Gene expression and signaling pathways: Insulin, Glucagon, Epinephrine influence expression of glycolytic and gluconeogenic enzymes; acute regulation via phosphorylation and longer-term regulation via transcription.

• Phosphorylation (short-term regulation):

  • Reversible addition/removal of phosphate groups by kinases/phosphatases modulates enzyme activity and conformational state.

  • This is a central mechanism for the rapid regulation of metabolic fluxes in response to hormonal signals.

• Consequences of hormonal signaling:

  • Fed state (high insulin): stimulates glycolysis and glycogen storage; inhibits gluconeogenesis and glycogenolysis; upregulates glucokinase; downregulates PEPCK; promotes glucose uptake into skeletal muscle and adipose tissue; net effect: lower blood glucose.

  • Fasting state (high glucagon/epinephrine): stimulates glycogenolysis and gluconeogenesis; increases hepatic glucose output; upregulates PEPCK; promotes maintenance of blood glucose during fast; net effect: higher blood glucose when fasting.

• Counterregulatory hormones (to insulin): Glucagon, Glucocorticoids, and Epinephrine contribute to fasting responses by promoting glucose production and mobilization.

• Regulation of the glucose transporter family and their tissue distribution influences glucose uptake and utilization in different states (fed vs fasted).

Practice Questions and Study Aids

• Recommended readings and resources for deeper understanding:

  • Harper’s Chapter 12 (Respiratory Chain & Phosphorylation)

  • Stipanuk’s Chapter 12: Gluconeogenesis (pp. 294-299), Pentose Phosphate Pathway (pp. 286-290), Regulation of glycolysis & gluconeogenesis (pp. 301-304), substrate cycle, hormonal regulation, and related topics.

  • Journal article on the key role of anaplerosis and cataplerosis for TCA cycle function (course website).

• Study guide questions (Gluconeogenesis):
  1) In what physiological conditions (fed or fasted) and in what tissues does gluconeogenesis occur? Where does it occur in the cell?
  2) What are the precursors for gluconeogenesis? Do different precursors follow the same gluconeogenic pathway?
  3) Which steps in gluconeogenesis are regulated? Which enzymes catalyze these steps? How are they regulated? (Emphasize bypass reactions and enzymes: PEPCK, Pyruvate carboxylase, Fructose-1,6-bisphosphatase, Glucose-6-phosphatase)
  4) Which tissues can release glucose to circulation during fasting? What enzyme enables this?
  5) What is the role of the liver and kidneys in gluconeogenesis? Compare gluconeogenesis in liver vs kidney.

• True/False self-assessment (as presented in course materials):

  • Glycolysis statements:
    1) Glucose is oxidized to lactate or pyruvate via glycolysis. (True)
    2) Oxygen is not required for glycolysis. (True)
    3) All cells can do glycolysis for energy from glucose. (True)
    4) Glycolysis is the first step of glucose metabolism; it generates intermediates for other pathways (e.g., lipogenesis). (True)
    5) The rate-limiting step of glycolysis is the glucokinase-catalyzed phosphorylation of glucose to glucose-6-phosphate. (True in slide; note conventional biochemistry texts consider PFK-1 as the canonical rate-limiting step; the slide presents GK as rate-limiting)

  • TCA cycle statements:
    1) TCA cycle generates ATPs. (True)
    2) TCA cycle is common to oxidation of carbohydrates, fats, and proteins. (True)
    3) Pyruvate is the starting molecule of TCA cycle. (False)
    4) TCA cycle is the only pathway that produces reducing equivalents (NADH and FADH₂). (False)
    5) TCA cycle is the ending oxidative pathway for all three macronutrients. (True)
    6) The first reaction in TCA cycle is the condensation of oxaloacetate and acetyl-CoA, forming citrate. (True)

• Additional TCA-cycle true/false (condensed):
  1) The end products of the TCA cycle are CO₂ and H₂O. (True)
  2) Oxaloacetate and citrate are intermediates that can be removed from the TCA cycle for gluconeogenesis and lipogenesis. (True)

Quick formulas and reactions (selected)

• Glycolysis irreversible steps (by enzyme):

  • $ext{Glucose} + ext{ATP} <br>ightarrow ext{Glucose-6-phosphate} + ext{ADP}$

  • $ext{Fructose-6-phosphate} + ext{ATP} <br>ightarrow ext{Fructose-1,6-bisphosphate} + ext{ADP}$

  • $ext{Phosphoenolpyruvate} + ext{ADP} <br>ightarrow ext{Pyruvate} + ext{ATP}$

• Gluconeogenesis bypass reactions (key):

  • Pyruvate carboxylase: $ext{Pyruvate} + ext{CO}_2 + ext{ATP} <br>ightarrow ext{Oxaloacetate} + ext{ADP} + ext{Pi}$

  • PEP carboxykinase: $ext{Oxaloacetate} + ext{GTP} <br>ightarrow ext{Phosphoenolpyruvate} + ext{CO}_2 + ext{GDP}$

  • Fructose-1,6-bisphosphatase: $ext{Fructose-1,6-bisphosphate} + ext{H}_2 ext{O} <br>ightarrow ext{Fructose-6-phosphate} + ext{Pi}$

  • Glucose-6-phosphatase: $ext{Glucose-6-phosphate} + ext{H}_2 ext{O} <br>ightarrow ext{Glucose} + ext{Pi}$

• PPP steps (simplified):

  • $ext{Glucose-6-phosphate} + ext{NADP}^+ <br>ightarrow ext{6-phosphogluconolactone} + ext{NADPH} + ext{H}^+$

  • (via 6-phosphogluconolactonase) 6-
    phosphogluconolactone → 6-phosphogluconate

  • $ext{6-phosphogluconate} + ext{NADP}^+ <br>ightarrow ext{Ribulose-5-phosphate} + ext{CO}_2 + ext{NADPH} + ext{H}^+$

• PDH and TCA yields (as per slide):

  • Pyruvate dehydrogenase (PDH): $ext{Pyruvate} + ext{NAD}^+ + ext{CoA} <br>ightarrow ext{Acetyl-CoA} + ext{CO}_2 + ext{NADH}$

  • TCA cycle yields: per acetyl-CoA, typically ~10 ATP equivalents (NADH, FADH₂, GTP), with total ~24 ATP per glucose from TCA + ETC in aerobic metabolism (as per course notes).

Notes on Real-World Relevance and Interconnections

• Gluconeogenesis is essential during prolonged fasting and in tissues like liver and kidney to maintain euglycemia.

• The Cori and Alanine cycles illustrate how muscle energy metabolism communicates with liver metabolism to sustain blood glucose and manage nitrogen balance.

• The Pentose Phosphate Pathway provides NADPH for biosynthesis and ribose-5-phosphate for nucleotide synthesis, linking carbohydrate metabolism to lipid biosynthesis and antioxidant defense (via NADPH).

• Regulation integrates hormonal signaling (insulin, glucagon, epinephrine), allosteric control, phosphorylation, substrate availability, and gene expression to ensure metabolic flexibility.

• Understanding these pathways clarifies pathophysiology of metabolic diseases (e.g., hypoglycemia due to PEPCK deficiency, lactic acidosis, glycogen storage issues) and informs nutrition strategies in fasting, diabetes, and metabolic syndrome contexts.