What is Metabolism?

• Derived from the Greek word "Metaballein" which means to turn about, to change, or to alter.
• Defined as the inter-conversion of chemical compounds in the body, involving
  • different pathways, their inter-relationship, and their regulation.
• Significance (Why metabolism matters):
  • Synthesis of essential substances (molecules).
  • Degradation (oxidation) of certain molecules.
  • Provision of energy.
  • Conversion of toxic substances to non-toxic substances for excretion.

Types of Pathways

• Anabolic Pathways
  • Synthesis of larger molecules from smaller molecules.
  • Endothermic (energy-requiring) processes (e.g., gluconeogenesis, protein synthesis).
• Catabolic Pathways
  • Breakdown of larger molecules (oxidative).
  • Exothermic (energy-yielding) processes (e.g., glycolysis).
• Amphibolic Pathways
  • Involved in both synthesis and breakdown; act as a link between anabolic and catabolic pathways (e.g., Citric acid cycle).

Carbohydrate Metabolism Overview

• Focus on the provision and fate of glucose.
• Major metabolic pathways include:
  • Glycolysis
  • Hexose monophosphate (HMP) shunt
  • Uronic acid pathway
  • Gluconeogenesis
  • Glycogenesis
  • Glycogenolysis
• Citric acid cycle is a common metabolic pathway linked to many processes.

Introduction to Glycolysis (Embden–Meyerhof Pathway)

• Location: Cytoplasm of the cell.
• Structure: Approximately ten enzyme-catalyzed reactions.
• Phases:
  • Preparatory Phase (Phase I): Converts glucose into two molecules of glyceraldehyde-3-phosphate (GA3P) at the expense of two ATP.
  • Payoff Phase (Phase II): Converts GA3P into pyruvate with production of ATP and NADH.
• Stoichiometry per glucose:
  • Produces two molecules of pyruvate, two ATP (net), and one NADH (per GA3P; total per glucose: two GA3P yield 2 NADH and 4 ATP produced? See notes below).
  • Overall for glucose: two molecules of pyruvate, two ATP net, and two NADH generated during the payoff phase.

Significance of Glycolysis

• Essential for all cells and tissues; especially important for the brain and RBCs.
• Major pathway for glucose utilization; Fructose and Galactose can also be metabolized via glycolysis.
• Can provide ATP even in the absence of oxygen (anaerobic conditions).
• Provides carbon skeletons for synthesis of non-essential amino acids.
• Provides glycerol for the synthesis of fats (lipogenesis).

Major Pathways of Glucose Utilization

• Entry points and branching from glucose include:
  • Glycolysis (to pyruvate).
  • Oxidation via the pentose phosphate pathway (PPP).
  • Extracellular matrix and cell wall polysaccharides.
  • Synthesis of structural polymers: glycogen, starch, sucrose.
• Glucose oxidation via glycolysis leads to pyruvate storage and further metabolism.

How Glucose Transports into Cells

• Glucose transporters (GLUTs) facilitate inward and outward transport of glucose across the cell membrane.
• Several GLUT isoforms exist, with tissue-specific distribution and regulation.

Glucose Transporters (GLUTs) – Overview

• GLUT1 (Blood) – Insulin-independent; present in many tissues including the blood barrier; high Km indicates low affinity.
• GLUT2 – Insulin-independent; expressed in liver, pancreas, small intestine, and others; high capacity, low affinity.
• GLUT3 – Insulin-independent; primarily in brain and neurons; high affinity (low Km).
• GLUT4 – Insulin-dependent; present in skeletal muscle, adipose tissue, heart; translocation to the membrane increases with insulin; high affinity (low Km).
• Key kinetic notes:
  • GLUT1-3 are insulin-independent (their activity depends on glucose availability).
  • GLUT4 is insulin-dependent (its number/efficiency increases in the presence of insulin).

Glycolysis Pathway (Stepwise) – Preparatory Phase

• Step 1: Phosphorylation of glucose to glucose-6-phosphate (G6P)
  • Enzymes: Hexokinase (in most tissues) and Glucokinase (in liver).
  • Cofactors: ATP → ADP.
  • Product: Glucose-6-phosphate (G6P).
  • Notation: $ext{Glucose} + ext{ATP} <br /> ightarrow ext{Glucose-6-phosphate} + ext{ADP}$
• Step 2: Isomerization of G6P to fructose-6-phosphate (F6P)
  • Enzyme: Phosphohexose isomerase (phosphoglucose isomerase).
  • Reversibility: Reversible step.
  • Product: Fructose-6-phosphate (F6P).
  • Notation: $ext{Glucose-6-phosphate} <br /> ightleftharpoons ext{Fructose-6-phosphate}$
• Step 3: Phosphorylation of F6P to fructose-1,6-bisphosphate (F1,6BP)
  • Enzyme: Phosphofructokinase-1 (PFK-1).
  • Regulation: Allosteric enzyme; irreversible step; key control point.
  • Cofactor: ATP → ADP (one ATP consumed).
  • Product: Fructose-1,6-bisphosphate (F1,6BP).
  • Notes on terminology:
  • Bisphosphate means two phosphate groups on different carbons.
  • Bi-phosphate (bi) indicates two phosphate groups linked on the same carbon atom (di- vs bi-); here F1,6BP has two phosphates on C1 and C6.
• Step 3 significance: PF-1 is a major allosteric regulator of glycolysis; integrates metabolic signals.

Glycolysis Pathway – Payoff Phase (Key Steps)

• Step 4: Cleavage of F1,6BP

  • Enzyme: Aldolase.
  • Reaction: Fructose-1,6-bisphosphate → glyceraldehyde-3-phosphate (GA3P) + dihydroxyacetone phosphate (DHAP).
  • Outcome: One hexose splits into two triose phosphates.

• Step 5: Isomerization of DHAP to GA3P

  • Enzyme: Triose phosphate isomerase.
  • Reversibility: Reversible; DHAP is converted to GA3P so that two GA3P enter payoff phase.

• End of Preparatory Phase: 2 GA3P molecules per glucose are generated.

• Step 6: Oxidation of GA3P to 1,3-bisphosphoglycerate (1,3-BPG)

  • Enzyme: Glyceraldehyde-3-phosphate dehydrogenase.
  • Redox change: NAD+ is reduced to NADH.
  • Product: 1,3-BPG.
  • Reaction (per GA3P): $ext{GA3P} + ext{NAD}^+ + ext{P}_i <br /> ightarrow 1,3 ext{-BPG} + ext{NADH} + ext{H}^+$

• Step 7: 1,3-BPG → 3-phosphoglycerate

  • Enzyme: Phosphoglycerate kinase.
  • ATP generation: Substrate-level phosphorylation; ATP produced from ADP.
  • Product: 3-phosphoglycerate.
  • Substrate-level phosphorylation (first use): $1,3 ext{-BPG} + ext{ADP} <br /> ightarrow 3 ext{-phosphoglycerate} + ext{ATP}$

• Step 8: 3-PG → 2-PG

  • Enzyme: Phosphoglycerate mutase.
  • Conversion: 3-phosphoglycerate to 2-phosphoglycerate.

• Step 9: Dehydration to form PEP

  • Enzyme: Enolase.
  • Product: Phosphoenolpyruvate (PEP).
  • Note: Release of a water molecule; formation of a high-energy phosphate.

• Step 10: PEP → Pyruvate

  • Enzyme: Pyruvate kinase.
  • ATP generation: Second substrate-level phosphorylation; ATP produced from ADP.
  • Product: Pyruvate.
  • Irreversibility: Irreversible reaction (another substrate-level phosphorylation).

Net Energy Yield from Glycolysis (Per Glucose)

• From Steps 1 and 3: ATP consumed (−2 ATP).
• From Steps 7 and 10: ATP generated ( +4 ATP ).
• From Step 6: NADH produced (2 NADH total per glucose).
• Step 8/9: No net ATP in these steps directly.
• Overall net ATP (substrate-level) per glucose: $-2 + 4 = 2 ext{ ATP}$
• NADH produced per glucose: $2 ext{ NADH}$
• Under aerobic conditions (oxidative phosphorylation), NADH can yield additional ATP (commonly cited as about 3 ATP per NADH, giving higher total energy depending on the shuttle system). According to the material:
  • NADH contribution: $2 imes 3 = 6 ext{ ATP}$
  • Substrate-level ATP: $4 ext{ ATP}$
  • ATP consumed: $-2 ext{ ATP}$
  • Net aerobic glycolysis yield: $6 + 4 - 2 = 8 ext{ ATP}$
• Under anaerobic conditions (e.g., RBCs), NADH cannot be oxidized in mitochondria; thus glycolysis yields only the substrate-level ATP:
  • Net anaerobic glycolysis yield: $2 ext{ ATP}$

Significance and Applications of Glycolysis

• Essential for energy production in tissues with limited mitochondrial access (e.g., RBCs).
• Brain requires glucose; glycolysis is a major source of ATP; RBCs rely entirely on glycolysis for energy.
• Provides carbon skeletons for amino acid synthesis when needed.
• Provides glycerol backbone for lipid synthesis from carbohydrate sources.
• Lactate production under anaerobic conditions maintains redox balance by regenerating NAD+ from NADH via lactate dehydrogenase.
  • Conversion: $ext{Pyruvate} + ext{NADH} + ext{H}^+ <br /> ightarrow ext{Lactate} + ext{NAD}^+$
  • This keeps glycolysis running when oxygen is scarce.
• Redox balance note:
  • The NAD^+ consumed in glyceraldehyde-3-phosphate dehydrogenase is regenerated by lactate dehydrogenase in tissues producing lactate; the NADH produced in the same step is consumed in the lactate DH reaction.
  • Thus, the redox balance is maintained.
  • Net equation (per glucose under anaerobic conditions):
    $ext{Glucose} + 2 ext{P}<em>i + 2 ext{ADP} ightarrow 2 ext{lactate} + 2 ext{ATP} + 2 ext{H}</em>2 ext{O} + 2 ext{H}^+$

Integration of Other Hexoses into Glycolysis

• Fructose (major liver pathway)
  • Step 1: Fructose is phosphorylated by fructokinase to fructose-1-phosphate (F1P) using ATP:
    $ext{Fructose} + ext{ATP} <br /> ightarrow ext{Fructose-1-phosphate} + ext{ADP}$
  • Step 2: Fructose-1-phosphate is split by aldolase B into glyceraldehyde and dihydroxyacetone phosphate (DHAP):
    $ext{Fructose-1-phosphate} <br /> ightarrow ext{Glyceraldehyde} + ext{DHAP}$
  • Step 3: Glyceraldehyde is phosphorylated by glyceraldehyde kinase (triose kinase) to glyceraldehyde-3-phosphate (GA3P) which then enters the glycolytic pathway:
    $ext{Glyceraldehyde} + ext{ATP} <br /> ightarrow ext{GA3P} + ext{ADP}$
  • Net effect: Fructose in liver largely bypasses the PFK-1 control point.
• Fructose (minor liver pathway)
  • Fructose can be phosphorylated by hexokinase to fructose-6-phosphate (F6P) which then enters glycolysis via the standard pathway (PFK-1 step).
  • This pathway is subject to liver hexokinase kinetics and regulatory control.
• Galactose entry into glycolysis (Galactose Metabolism)
  • Galactose is phosphorylated by galactokinase to galactose-1-phosphate (Gal-1-P).
  • Gal-1-P is converted through a UDP-glucose–dependent pathway to UDP-galactose and glucose-1-phosphate (G1P) via UDP-galactose 4-epimerase and related enzymes:
  • Galactose + ATP → Galactose-1-phosphate + ADP (Galactokinase).
  • Gal-1-P + UDP-glucose → UDP-Galactose + Glucose-1-phosphate (UDP-Gal-4-epimerase + uridyl transferase).
  • G1P is converted to Glucose-6-phosphate (G6P) via phosphoglucomutase, feeding into glycolysis.
• Overall: Glucose and hexoses can feed into glycolysis through multiple entry points, maintaining energy production and metabolic flexibility.

Galactosemia (Clinical Note)

• Condition caused by inherited defects in enzymes of galactose metabolism (e.g., galactokinase, UDP-glucose:galactose-1-phosphate uridyltransferase, or 4-epimerase).
• Consequences: accumulation of galactose and galactose-1-phosphate in blood, leading to potential liver failure and mental deterioration.
• Pathophysiology: Galactose cannot be efficiently metabolized, disrupting normal energy metabolism and detoxification pathways.

Fructose Metabolism in the Liver – Major vs Minor Pathways

• Major pathway (liver): Fructose metabolism via fructokinase to fructose-1-phosphate, bypassing PFK-1; rapid and not inhibited by insulin/fasting state.
  • Fructose-1-phosphate is split into glyceraldehyde and DHAP by aldolase B, with subsequent phosphorylation of glyceraldehyde by triose kinase to GA3P.
• Minor pathway (liver and other tissues): Fructose can be phosphorylated by hexokinase to Fructose-6-phosphate, entering glycolysis at the standard step after longer regulation.
• Metabolic fate: Fructose metabolism in liver rapidly leads to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, which feed into glycolysis, glycerol synthesis, acetyl-CoA production, and lipid synthesis pathways.
• Clinical notes: Excess fructose intake can promote acetyl-CoA production and downstream fatty acid synthesis (-ketone bodies and lipids) via Fructose-1-phosphate pathway.

Regulation of Glycolysis – Quick (Rapid & Short-Term Regulation)

• Three irreversible, rate-limiting exergonic steps regulate glycolysis:
  • Hexokinase/Glucokinase (Step 1)
  • Phosphofructokinase-1 (PFK-1) (Step 3)
  • Pyruvate kinase (Step 10)
• In the fed state: ↑ insulin → upregulates gene transcription and synthesis of glycolytic enzymes, enhancing glycolysis.
• In starvation/diabetes mellitus: ↓ insulin, ↑ glucagon → promotes gluconeogenesis and downregulates glycolysis.
• Hormonal regulation (fast, long-term effects): 1) Epinephrine/glucagon → ↑ cAMP → activates protein kinase A (PKA), which affects glycolysis and gluconeogenesis differently in liver vs muscle.
  • In liver: glucagon activation inhibits glycolysis via PKA signaling.
  • In muscle: epinephrine activation stimulates glycolysis to provide rapid energy for contraction.
    2) ↑ AMP activates PFK-1 → stimulates glycolysis.
    3) ↑ Citrate and ↑ ATP inhibit PFK-1 → downregulate glycolysis (feedback).
    4) Hypoxia → ↓ ATP and ↑ AMP → upregulates glycolysis to meet energy demand.
• Additional regulatory proteins:
  • PFK-2/FBPase-2 regulation via glucagon/insulin affects levels of fructose-2,6-bisphosphate (F-2,6-BP), a potent activator of PFK-1.
  • In liver: glucagon reduces glycolysis; in muscle: epinephrine promotes glycolysis to support contraction.

Regulation – Liver vs Muscle (Glucagon/Epinephrine Actions)

• Liver:
  • Glucagon activates PKA → increases glycogen breakdown and reduces glycolysis; promotes gluconeogenesis; decreases F-2,6-BP.
• Muscle:
  • Epinephrine activates signaling for glycolysis to supply energy for contraction.
• Effects summarized:
  • Increased F-2,6-BP in tissues where glycolysis is promoted (via PFK-2 activity) and decreased when glycolysis is inhibited (via F-2,6-BPase activity by glucagon signaling in liver).

Inhibitors of Glycolysis

• Arsenite: Competes with inorganic phosphate, forming I-arseno-3-phosphoglycerate, inhibiting glyceraldehyde-3-phosphate dehydrogenase.
• Iodoacetate / Iodoacetamide: Bind and inactivate glyceraldehyde-3-phosphate dehydrogenase, causing accumulation of GA3P.
• Fluoride: Inhibits enolase.

Entry of Other Hexoses into Glycolysis – Summary

• Galactose entry:
  • Galactose enters via the Leloir pathway, ultimately generating glucose-1-phosphate and then glucose-6-phosphate for glycolysis.
  • Key steps: Galactokinase; UDP-glucose; UDP-galactose 4-epimerase; Gal-1-P-uridyl transferase; Mutase.
• Fructose entry:
  • Major liver pathway bypasses PFK-1; minor pathway enters as Fructose-6-phosphate via hexokinase.

Galactosemia (Clinical Correlation)

• Inherited defects in galactose metabolism enzymes lead to impaired galactose processing.
• Consequences include elevated galactose and galactose-1-phosphate, possible liver failure, mental deterioration.

Fructose Metabolism in the Liver – Detailed View (Major vs Minor Pathways)

• Major pathway (liver):
  • Fructose → Fructose-1-phosphate via fructokinase (uses ATP).
  • Fructose-1-phosphate → Glyceraldehyde + DHAP via aldolase B.
  • Glyceraldehyde → Glyceraldehyde-3-phosphate via triose kinase; DHAP feeds glycolysis.
• Minor pathway (liver):
  • Fructose → Fructose-6-phosphate via hexokinase (low affinity for Fru in liver).
  • Fructose-6-phosphate continues in glycolysis downstream of the PFK-1 control point.

Overall Glycolysis Pathway – Visual Summary (Key Steps)

• Start: Glucose
• Series of enzymatic steps convert glucose to two GA3P molecules through phosphorylation, isomerization, cleavage, and oxidation steps.
• End: Two pyruvate molecules; NADH produced; net ATP produced (substrate-level): 2 ATP per glucose in glycolysis.
• Important intermediate representations include:
  • G6P, F6P, F1,6BP, GA3P, DHAP, 1,3-BPG, 3-PG, 2-PG, PEP, Pyruvate, Lactate (under anaerobic conditions).
• Note on energy carriers: NADH produced in glycolysis can feed into oxidative phosphorylation to yield additional ATP under aerobic conditions.

Energy Yield – Quick Reference

• Aerobic glycolysis (per glucose):
  • NADH: $2 imes 3 = 6 ext{ ATP (via oxidative phosphorylation)}$
  • Substrate-level ATP: $4 ext{ ATP}$
  • ATP consumed: $-2 ext{ ATP}$
  • Net ATP: $8 ext{ ATP}$
• Anaerobic glycolysis (e.g., RBC):
  • Substrate-level ATP: $4 ext{ ATP}$
  • ATP consumed: $-2 ext{ ATP}$
  • Net ATP: $2 ext{ ATP}$

Glycolysis: Summary of Key Figures

• Net production per glucose under aerobic conditions (glycolysis only): $8 ext{ ATP}$ (when accounting for NADH energy).
• Net production per glucose under anaerobic conditions (glycolysis only): $2 ext{ ATP}$.
• NADH produced per glucose: $2 ext{ NADH}$ (reducible to ATP via oxidative phosphorylation depending on cellular context).
• Pyruvate produced per glucose: $2 ext{ pyruvate}$.

2,3-Bisphosphoglycerate (2,3-BPG) and Oxygen Delivery

• 2,3-BPG forms a complex with hemoglobin (Hb) and decreases Hb's affinity for O₂.
• This promotes O₂ unloading from Hb at lower partial pressures, enhancing tissue oxygen delivery.
• Intermediary representation: 2,3-BPG derives from GA3P metabolism in glycolysis;
  • 2,3-BPG schematic conversion involves phosphatases and mutases switching from 1,3-BPG to 2,3-BPG.

Quick Glucose Metabolism Visualization (Optional Notes)

• Fructose and galactose feed into glycolysis via their respective entry points.
• The liver modulates glycolysis via hormonal signals (insulin, glucagon) and allosteric regulators (AMP, citrate, ATP).
• Oxygen availability determines whether NADH contributes to ATP production via mitochondria or is used to sustain anaerobic NAD+ recycling via lactate dehydrogenase.

References to Figures and Notes (From Transcript)

• Glycolysis steps and enzyme names: Hexokinase, Glucokinase, Phosphofructokinase-1, Aldolase, Triose phosphate isomerase, Glyceraldehyde-3-phosphate dehydrogenase, Phosphoglycerate kinase, Phosphoglycerate mutase, Enolase, Pyruvate kinase.
• Energy yield figures: Net ATP = 2 (glycolysis), Aerobic net ATP = 8 (glycolysis portion, assuming NADH ATP yield), Anaerobic net ATP = 2; NADH produced = 2.
• Regulatory concepts: Allosteric control, insulin/glucagon signaling, epinephrine effects, AMP activation, citrate/ATP inhibition, hypoxia.
• Carbohydrate entry points: Fructose (major & minor liver pathways), Galactose (Leloir pathway and entry into glycolysis).
• Pathology: Galactosemia (enzyme defects causing metabolic disturbances).
• 2,3-BPG and its role in oxygen delivery.

Note: All chemical equations and energy values are presented as LaTeX expressions within double dollar signs as requested. For example, Glucose + ATP → Glucose-6-phosphate + ADP would be written as $\text{Glucose} + \text{ATP} \rightarrow \text{Glucose-6-phosphate} + \text{ADP}$ in your final notes.