Comprehensive Notes on Carbohydrate Metabolism and Glycolysis

Monosaccharides, Disaccharides, and Polysaccharides

• Major monosaccharides, disaccharides, and polysaccharides in the human body and diet (overview from objectives):
  • Identify major monosaccharides, disaccharides, and polysaccharides found in body/diet.
  • Distinguish structures and glycosidic linkages of common disaccharides (e.g., lactose, sucrose, maltose).
• Absorption and transport across intestinal epithelium; transporters involved in glucose movement into blood and into tissues; tissue-specific GLUT expression.

Absorption and Transport of Carbohydrates

• Absorption of monosaccharides from the intestinal lumen to blood: monosaccharides are absorbed; disaccharides/polysaccharides are digested to yield monosaccharides.

• Main monosaccharides absorbed: ext{Glucose}
  ightarrow 90 ext{%, Galactose}, Fructose
  ightarrow 10 ext{%, all pumped into plasma by GLUT2 from basal membrane to blood}

• Secondary active transport (sodium-dependent): uptake of glucose and galactose against gradient via SGLT-1; Na+ gradient provides energy. Also found in proximal convoluted tubule.

• Facilitated diffusion transporters (GLUTs): absorption of glucose, galactose, and fructose from enterocytes to blood; transport into tissues.

• Kidney glucose reabsorption: SGLT2 major cotransporter in renal glomerulus; reabsorbs glucose from filtrate.

• GLUT2 is the basolateral transporter in intestine and pancreas; glucose sensor in pancreas; allows glucose to exit enterocytes to blood.

• Fructose transport: apical GLUT5 (facilitated diffusion).

• Transport into tissues: GLUT transporters (≈14 isoforms) with tissue-specific expression; two conformational states:

  • Outside-facing binding induces conformational change to release glucose inside cell; transporter returns to original conformation.

GLUT Isoforms: Distribution and Roles

• GLUT1: high affinity; tissue distribution – erythrocytes, blood–brain barrier, blood–retinal barrier, blood–placental barrier, blood–testis barrier. Basal high-affinity glucose uptake.

• GLUT2: liver, pancreatic beta cells; serosal surface of intestinal mucosa; renal tubular cells; high-capacity, low-affinity transporter; glucose sensor in pancreas (regulates insulin release).

• GLUT3: brain (neurons) and placenta; high affinity; basal uptake.

• GLUT4: adipose tissue, skeletal muscle, heart muscle; insulin-dependent; translocation to membrane increases glucose uptake when insulin is high.

• GLUT5: intestinal epithelium and spermatozoa; fructose transporter.

• Transport into tissues: glucose into peripheral cells via GLUTs; RBCs rely heavily on GLUT1; brain uses GLUT1 and GLUT3; liver/pancreas use GLUT2; muscle/adipose rely on insulin-regulated GLUT4.

Sorbitol Pathway (Polyol Pathway)

• Glucose is reduced to sorbitol by aldose reductase; sorbitol accumulation occurs in nerve tissues, retina, lens during prolonged hyperglycemia, contributing to diabetic complications.
• Galactitol formed from galactose by aldose reductase in lens; untreated galactosemia can cause early cataracts (E420 food additive noted).

Monosaccharides, Disaccharides, and Oligosaccharides: Structures and Bonding

• Monosaccharides: simple sugars; general formula ext{Cn(H2O)}n; cannot be hydrolyzed further.

• Oligosaccharides: 3–10 monosaccharide units; often linked to proteins; roles in protein folding and targeting to cell surface or subcellular locales; cell surface markers for cell recognition.

• Disaccharides: two monosaccharides linked by a glycosidic bond via condensation between the hydroxyl of the anomeric carbon of one monosaccharide and a hydroxyl group of another.

• Examples and linkages:

  • Lactose: \beta\text{-galactose} + \text{glucose} with a \beta(1\to4) glycosidic bond; reducing (has anomeric -OH).
  • Sucrose: \alpha\text{-glucose} + \beta\text{-fructose} with a \alpha(1\to\beta2) bond; non-reducing (no anomeric -OH).
  • Maltose: \alpha(1\to4) linkage of two D-glucose molecules; reducing because it has 1 anomeric -OH.
  • Cellobiose: two glucose units linked by a \beta(1,4) bond; not found freely in nature; degradation product of cellulose.

• Oligosaccharides: two broad classes

  • N-linked oligosaccharides: attached to polypeptides via N-glycosidic bond to the amide side chain of asparagine.
  • O-linked oligosaccharides: attached to the side chain hydroxyl of serine or threonine in polypeptides or to lipid membranes.

• Polysaccharides: high molecular weight polymers; may be linear or branched; two main classes

  • Homopolysaccharides: composed of a single monosaccharide type (e.g., starch, glycogen, cellulose).
  • Heteropolysaccharides (glycosaminoglycans, GAGs): contain two or more monosaccharides.

Polysaccharides: Detailed Structures

• A. Starch (plant storage): mixture of amylose (15–20%) and amylopectin (80–85%)

  • Amylose: unbranched, α1→4 linkages; soluble in water; ~250–300 D-glucose units; linear helix.
  • Amylopectin: highly branched; main chain via α1→4 linkages; branch points via α1→6 linkages; insoluble in cold water; chain architecture differs from glycogen.

• B. Glycogen (animal storage): highly branched; stored in liver and muscle; glucose units linked by α1→4 in the chains with α1→6 branches; more compact and higher solubility than amylopectin.

• C. Cellulose: structural polymer in plants; polymer of glucose with β1→4 glycosidic bonds; very stable and insoluble; humans lack cellulose-degrading enzymes; provides dietary bulk (roughage) stimulating peristalsis.

• C. Cellulose structural notes: 2,3-BPG and other glycolytic intermediates are not part of cellulose; cellulose is not digested by humans but contributes to stool bulk.

• 4. Polysaccharides: Summary of key architectures

  • Homopolysaccharides: starch (amylose + amylopectin) and glycogen.
  • Heteropolysaccharides (GAGs): repeated disaccharide units with various side groups; important in connective tissues, joints, etc.

Absorption Of Dietary Carbohydrates – Ingestion to Blood

• Primary monosaccharides absorbed: glucose, galactose, and fructose; order: glucose/galactose majority, fructose about 10% in absorption.
• Transport mechanisms on the intestinal luminal surface
  • SGLT-1: secondary active transport (Na+-dependent) moves glucose and galactose from lumen into enterocytes using the Na+ gradient (energy source).
  • GLUT5: facilitates fructose uptake from lumen into enterocytes by facilitated diffusion.
  • SGLT-2: major glucose reabsorption transporter in proximal tubule of kidney; not intestinal, but mentioned in context of glucose handling.
• Basolateral release
  • GLUT2: allows diffusion of glucose, galactose, and fructose from enterocytes into blood on the basolateral side.
• Transport into blood: All monosaccharides exit enterocytes via GLUT2 to bloodstream; no energy is directly used for basolateral exit (facilitated diffusion).
• Liver, pancreas, and other tissues export or regulate glucose using their GLUT transporters; brain and RBCs have high affinity transporters.

Glycolysis: Overview, Localization, and Importance

• Glycolysis definition: oxidation of glucose to pyruvate with ATP generation; can occur with or without oxygen.
• Role in cellular metabolism:
  • Generates ATP via substrate-level phosphorylation and NADH.
  • Pyruvate can enter TCA cycle with oxygen to yield additional ATP via oxidative phosphorylation; or be reduced to lactate in anaerobic conditions.
• Cellular localization: glycolysis occurs in the cytoplasm of nearly all cells.
• Fates of pyruvate and NADH:
  • Aerobic glycolysis (with oxygen): pyruvate → acetyl-CoA → TCA cycle; NADH feeds ETC.
  • Anaerobic glycolysis: pyruvate reduced to lactate by LDH; regenerates NAD+ for GAPDH to continue glycolysis; occurs in RBCs and exercising muscle.
• Energy yield:
  • Net: glucose + 2 NAD+ + 2 Pi + 2ADP → 2 pyruvate + 2 NADH + 4 H+ + 2 ATP + 2 H2O; overall ΔG°' < 0 (irreversible under physiological conditions).
  • When anaerobic: net 2 ATP per glucose (no net ATP from NADH if not reoxidized via mitochondria).
• Important note: location of glycolysis is cytosolic; LDH is cytosolic.

Reactions of Glycolysis – Stepwise (Preparative Phase 1–5; ATP-Generating Phase 6–10)

• Preparative phase (consumes ATP to trap glucose in the cell):

  • Step 1: Glucose → Glucose-6-phosphate (G6P)
  • Enzymes: Hexokinases (all tissues) and Glucokinase (liver, pancreatic beta cells)
  • Reaction: \text{Glucose} + \text{ATP} \rightarrow \text{Glucose-6-phosphate} + \text{ADP}
  • Irreversible under physiological conditions due to large negative ΔG°
  • Step 2: G6P → Fructose-6-phosphate (F6P)
  • Enzyme: phospho-hexose isomerase; can act only on α-anomer of G-6-P; reversible step
  • Step 3: F6P → Fructose-1,6-bisphosphate (F1,6BP)
  • Enzyme: Phosphofructokinase-1 (PFK-1); committed and allosterically regulated
  • Reaction: \text{F6P} + \text{ATP} \rightarrow \text{F1,6BP} + \text{ADP}
  • Irreversible under physiological conditions; key regulatory step
  • Step 4: F1,6BP → Dihydroxyacetone phosphate (DHAP) + Glyceraldehyde-3-phosphate (G3P)
  • Enzyme: Aldolase; multiple isoenzymes (A, B, C) with tissue distributions
  • DHAP and G3P interconvertible; DHAP can be used for lipid synthesis
  • Step 5: DHAP ↔ G3P
  • Enzyme: Triose phosphate isomerase (TIM); converts DHAP to G3P so that both molecules proceed downstream

• ATP-generating phase (G3P to pyruvate; produces ATP and NADH):

  • Step 6: G3P → 1,3-bisphosphoglycerate (1,3-BPG)
  • Enzyme: Glyceraldehyde-3-phosphate dehydrogenase; reduces NAD+ to NADH; adds inorganic phosphate to form 1,3-BPG
  • Arsenate can mimic phosphate and inhibit this step; NADH produced here can enter mitochondria if oxygen is available
  • Step 7: 1,3-BPG → 3-phosphoglycerate (3-PG)
  • Enzyme: Phosphoglycerate kinase; substrate-level phosphorylation yields ATP per 1,3-BPG
  • Step 8: 3-PG → 2-phosphoglycerate (2-PG)
  • Enzyme: Phosphoglycerate mutase
  • Step 9: 2-PG → Phosphoenolpyruvate (PEP)
  • Enzyme: Enolase; requires Mg2+ or Mn2+; fluoride inhibits enolase
  • Step 10: PEP → Pyruvate
  • Enzyme: Pyruvate kinase; substrate-level phosphorylation yields ATP; irreversible

• Key notes on steps and energy: two ATP consumed in preparative phase (Step 1 only uses 1 ATP here in text; overall preparative consumes 2 ATP per glucose); four ATP produced in the ATP-generating phase with net +2 ATP per glucose, plus 2 NADH from Step 6.

• Important enzymatic highlights:

  • Hexokinase vs glucokinase: different tissue distributions and regulatory properties; hexokinase inhibited by glucose-6-phosphate; glucokinase has high Km, not inhibited by G6P; induced by insulin; liver/pancreas isoforms facilitate hepatic glucose handling and storage as glycogen when energy is abundant.
  • PFK-1: allosterically activated by AMP and fructose-2,6-bisphosphate; inhibited by ATP and citrate; major regulatory control point.
  • PFK-2 / FBPase-2: bifunctional enzyme regulating fructose-2,6-bisphosphate (F2,6BP); F2,6BP stimulates PFK-1, promoting glycolysis; phosphorylation state controls activity:
  • Glucagon (fasting) via PKA phosphorylation reduces PFK-2 activity, lowers F2,6BP, inhibits glycolysis; dephosphorylation (insulin signaling) promotes glycolysis via PFK-1 activation.

• Notes on anaerobic glycolysis and lactate:

  • In absence of oxygen, NADH cannot be reoxidized via ETC; LDH regenerates NAD+ by converting pyruvate to lactate, enabling glycolysis to continue.
  • Lactate dehydrogenase (LDH) reaction recycles NAD+; elevated lactate in exercising muscle and other tissues under anaerobic conditions.

• Summary of glycolysis (net):

  • Overall: \text{Glucose} + 2\,\text{NAD}^+ + 2\,\Pi + 2\,\text{ADP} \rightarrow 2\,\text{Pyruvate} + 2\,\text{NADH} + 4\,\text{H}^+ + 2\,\text{ATP} + 2\,\text{H}_2\text{O}
  • Net ΔG°' < 0; pathway is energetically favorable but irreversible under physiological conditions.
  • Location: cytosol; RBCs rely on glycolysis for ATP due to lack of mitochondria; brain, muscles, and other tissues use glycolysis as part of energy metabolism.

Regulation of Glycolysis – Overview of Control Points

• Three rate-limiting steps:
  • Step 1: Glucose → Glucose-6-phosphate (Hexokinase/Glucokinase)
  • Step 3: Fructose-6-phosphate → Fructose-1,6-bisphosphate (PFK-1)
  • Step 10: Phosphoenolpyruvate → Pyruvate (Pyruvate kinase)
• All three steps are subject to allosteric regulation, hormonal control, and cross-talk with other metabolic pathways.

Regulation of Hexokinase vs Glucokinase

• Hexokinases (general tissues): inhibited by Glucose-6-phosphate (G6P); lower Km (~1 mM) means high affinity; Vmax lower; not induced by insulin.
• Glucokinase (liver, pancreatic beta cells): high Km (~10 mM); not inhibited by G6P; allows liver to phosphorylate glucose when blood glucose is high; induced by insulin; liver stores glucose as glycogen; liver/β-cells respond to energy state.
• Insulin signaling promotes hepatic glucokinase activity (via induction) and promotes glycolysis when energy is abundant; glucagon in liver reduces glycolysis via PFK-2 pathway and lowers F2,6-BP.

Regulation of Phosphofructokinase-1 (PFK-1)

• Allosteric activators: AMP, Fructose-2,6-bisphosphate (F2,6BP)
• Allosteric inhibitors: ATP, Citrate (from TCA cycle)
• Fructose-2,6-bisphosphate (F2,6BP) is a major regulator of glycolysis and gluconeogenesis; produced by PFK-2 and degraded by FBPase-2; acts as an allosteric activator of PFK-1.
• PFK-2 is bifunctional: kinase domain (produces F2,6BP) and phosphatase domain (degrades F2,6BP).
  • In skeletal muscle and liver, high F-6-P activates the kinase and inhibits the phosphatase, increasing F2,6BP and activating glycolysis.
  • PFK-2 is regulated by phosphorylation state:
  • Glucagon raises cAMP → PKA phosphorylation → activates phosphatase domain, reduces F2,6BP → inhibits PFK-1 → glycolysis slows; gluconeogenesis favored.
  • Insulin signaling activates protein phosphatases that dephosphorylate PFK-2, increasing kinase activity, increasing F2,6BP, and activating PFK-1 → glycolysis promoted.

Regulation of Pyruvate Kinase

• Pyruvate kinase regulation varies by tissue:
  • Brain, heart, and muscle isoforms are less regulated by phosphorylation for metabolic control.
  • Liver isoform can be inhibited by phosphorylation (glucagon pathway) and inhibited allosterically by ATP and alanine; activated by fructose-1,6-bisphosphate (F1,6BP) which acts as a feed-forward activator.
  • Alanine increases during fasting as a signal for gluconeogenesis; activates PK regulation to direct phosphoenolpyruvate toward glucose production in liver.

Pyruvate Kinase Deficiency – Clinical Impact

• Pyruvate kinase deficiency leads to hemolytic anemia due to reduced ATP in red blood cells (RBCs rely on glycolysis for ATP).
• RBC ATP depletion causes RBC fragility and hemolysis; hemoglobin levels commonly fall; high 2,3-BPG can compensate by decreasing Hb affinity for O2, aiding tissue oxygen delivery.
• Clinical features: normocytic normochromic anemia; no Heinz bodies (unlike G6PD deficiency); jaundice; variable erythrocyte morphology (echinocytes).
• Additional notes: In PK deficiency, supplementation with glucose can exacerbate hemolysis in RBCs due to increased glycolytic flux and subsequent ATP depletion in susceptible cells.

Fates of Pyruvate and NADH – Aerobic vs Anaerobic Glycolysis

• Aerobic glycolysis (with O2): Pyruvate enters mitochondria and is converted to acetyl-CoA, feeding the TCA cycle and oxidative phosphorylation to maximize ATP yield.
• Anaerobic glycolysis (without O2): Pyruvate is reduced to lactate by LDH; NADH is oxidized to NAD+, enabling glycolysis to continue in cytosol regardless of mitochondrial activity; important in exercising muscle and RBCs.
• Lactate dehydrogenase (LDH) recycles NAD+, supporting continuous glycolysis under anaerobic conditions.
• In erythrocytes, glycolysis is the primary ATP source; RBCs lack mitochondria, so they rely entirely on glycolysis and lactate production.
• 2,3-BPG is produced in RBCs from 1,3-BPG and modulates hemoglobin oxygen affinity, facilitating oxygen release to tissues during hypoxic stress.

Energy Yield and Pathway Notes

• Glycolysis energy accounting (per glucose):
  • Preparative phase consumes ATP: typically 2 ATP consumed (Step 1 and Step 3 equivalents).
  • ATP-generating phase yields 4 ATP (via substrate-level phosphorylation in Steps 7 and 10) and 2 NADH (Step 6).
  • Net gain: +2 ATP per glucose (plus 2 NADH that can yield additional ATP in mitochondria via oxidative phosphorylation if oxygen is present).
• Key inhibitors and regulators:
  • Arsenate inhibits glyceraldehyde-3-phosphate dehydrogenase by mimicking inorganic phosphate.
  • Fluoride inhibits enolase.
  • 2,3-BPG shifts oxygen delivery by RBCs under hypoxic conditions.

Connections to Physiology and Pathology

• Glucose transporters and tissue distribution underlie systemic glucose homeostasis: fasting vs fed states alter GLUT4 translocation; insulin increases GLUT4 on muscle/adipose cells, increasing glucose uptake for glycolysis/glycogen synthesis.
• SGLT-1/2 and GLUT2 regulation exemplify organ-level glucose handling: gut absorption and renal reabsorption maintain plasma glucose, while pancreatic beta cells sense glucose via GLUT2 to regulate insulin release.
• The sorbitol pathway links hyperglycemia to diabetic complications in nerves, retina, and lens due to osmotic and oxidative stress from sorbitol accumulation.
• Dietary carbohydrates classification and digestion influence nutrition, gut health (bulk), and metabolic responses; disaccharide composition and glycosidic bonds determine digestibility and glycemic response.
• The balance between glycolysis and gluconeogenesis is governed by hormonal signals (insulin, glucagon) and by regulators like F2,6BP which coordinate energy production with energy stores.

Key Equations and Numerical References (LaTeX)

• Net glycolysis reaction:
  ext{Glucose} + 2\text{NAD}^+ + 2 \Pi + 2\text{ADP} \rightarrow 2\text{pyruvate} + 2\text{NADH} + 4\text{H}^+ + 2\text{ATP} + 2\text{H}_2\text{O}
• Overall, ΔG°′ < 0 for glycolysis (irreversible without external input).
• Glucose ↔ Glucose-6-phosphate (Step 1):
  \text{Glucose} + \text{ATP} \rightarrow \text{Glucose-6-phosphate} + \text{ADP}
• Fructose-6-phosphate → Fructose-1,6-bisphosphate (Step 3):
  \text{F6P} + \text{ATP} \rightarrow \text{F1,6BP} + \text{ADP}
• 1,3-Bisphosphoglycerate formation (Step 6, NADH produced):
  \text{G3P} + \text{NAD}^+ + \text{P}_i \rightarrow \text{1,3-BPG} + \text{NADH} + \text{H}^+
• Pyruvate kinase reaction (Step 10):
  \text{PEP} + \text{ADP} \rightarrow \text{Pyruvate} + \text{ATP}
• Pyruvate fate in aerobic conditions: Pyruvate → Acetyl-CoA → TCA cycle → ETC; in anaerobic conditions: Pyruvate → Lactate via LDH with NAD+ regeneration.
• 2,3-BPG effect on Hb-O2 affinity: 2,3-BPG binds to hemoglobin reducing its affinity for O2, promoting oxygen release to tissues.

Summary Takeaways

• Carbohydrates are diverse and functionally essential in energy, structure, and signaling; digestion converts polysaccharides and disaccharides into monosaccharides for absorption via SGLT-1, GLUT transporters, and GLUT2 basolateral exit.
• Glycolysis provides rapid ATP and metabolic intermediates; tightly regulated at three key steps (HK/GK, PFK-1, PK) by allosteric effectors and hormonal signaling via PFK-2/FBPase-2.
• Tissue-specific GLUTs ensure glucose delivery where needed (brain with high affinity GLUT3/GLUT1; muscle/adipose with insulin-responsive GLUT4; liver with GLUT2 as a glucose sensor).
• Pyruvate’s fate depends on oxygen availability; anaerobic glycolysis generates ATP quickly but produces lactate and regenerates NAD+; aerobic glycolysis feeds into oxidative phosphorylation for higher ATP yield.
• PK deficiency and G6PD variability illustrate how glycolytic regulation and redox balance impact health and disease states.