Glycolysis Study Guide

📖 Core Concepts Glycolysis – Cytosolic pathway that converts one glucose → 2 pyruvate, 2 ATP (net), 2 NADH, and 2 H₂O. Two phases – Investment (uses 2 ATP) and pay‑off (produces 4 ATP & 2 NADH). Regulatory “gate‑keepers” – Hexokinase/Glucokinase, Phosphofructokinase‑1 (PFK‑1), Pyruvate kinase. Their activity determines flux. Irreversible steps – Hexokinase, PFK‑1, Pyruvate kinase (large negative ΔG). All other steps are near‑equilibrium. NAD⁺/NADH balance – Oxidation of G3P produces NADH; NAD⁺ must be regenerated by fermentation (anaerobic) or mitochondrial shuttles (aerobic). Hormonal control – Insulin → dephosphorylation/activation; Glucagon/Epinephrine → cAMP‑PKA phosphorylation → inhibition (mainly liver). Metabolic branching – G6P → glycogen or PPP; DHAP → glycerol‑3‑P (lipid synthesis); PEP → pyruvate or gluconeogenesis. 📌 Must Remember Net yield per glucose: 2 ATP, 2 NADH (cytosolic). Irreversible enzymes: Hexokinase/Glucokinase, PFK‑1, Pyruvate kinase. PFK‑1 allosteric regulators – ↑AMP, ↑F2,6BP = activation; ↑ATP, ↑Citrate = inhibition. Hexokinase product inhibition – Glucose‑6‑phosphate inhibits hexokinase (not glucokinase). Pyruvate kinase regulation – Liver PK phosphorylated (inactive) by PKA; muscle PK insensitive to PKA. Fermentative ATP – Only 2 ATP/glucose (≈5 % of total oxidative potential). Mitochondrial shuttles – Malate‑aspartate (NADH → NAD⁺), Glycerol‑3‑P (NADH → FADH₂). ATP per NADH – ≈2.5 ATP via oxidative phosphorylation. 🔄 Key Processes Investment Phase Hexokinase/Glucokinase: Glucose + ATP → Glc‑6‑P + ADP. G6P Isomerase: Glc‑6‑P ↔ Fru‑6‑P. PFK‑1: Fru‑6‑P + ATP → Fru‑1,6‑bisP + ADP (irreversible). Aldolase: Fru‑1,6‑bisP → DHAP + G3P. Triose‑phosphate isomerase: DHAP ↔ G3P. Pay‑off Phase (per G3P, ×2) GAPDH: G3P + NAD⁺ + Pi → 1,3‑BPG + NADH. Phosphoglycerate kinase: 1,3‑BPG + ADP → 3‑PG + ATP. Phosphoglycerate mutase: 3‑PG → 2‑PG. Enolase: 2‑PG → PEP + H₂O. Pyruvate kinase: PEP + ADP → Pyruvate + ATP. NAD⁺ Regeneration Anaerobic: Lactate dehydrogenase (pyruvate → lactate + NAD⁺) or ethanol fermentation (yeast). Aerobic: Malate‑aspartate shuttle or glycerol‑3‑P shuttle → mitochondria → ETC → O₂ → H₂O. Hormonal Regulation Loop High glucose → insulin → protein phosphatase‑1 → dephosphorylate (activate) HK, PFK‑1, PK. Low glucose → glucagon/epinephrine → ↑cAMP → PKA → phosphorylate (inhibit) PK & PFK‑2 (↓F2,6BP) → ↓PFK‑1 activity. 🔍 Key Comparisons Hexokinase vs. Glucokinase – Affinity: Hexokinase (low Km, high affinity); Glucokinase (high Km, low affinity). Product inhibition: Hexokinase inhibited by G6P; Glucokinase not inhibited. Tissue: Hexokinase – most cells; Glucokinase – liver & β‑cells. PFK‑1 vs. PFK‑2/FBPase‑2 – PFK‑1: Directly phosphorylates Fru‑6‑P; regulated by AMP, ATP, citrate, F2,6BP. PFK‑2: Synthesizes F2,6BP (activator of PFK‑1); phosphorylated by PKA → becomes FBPase‑2 (degrades F2,6BP). Muscle PK vs. Liver PK – Muscle: Insensitive to PKA phosphorylation; stays active during exercise. Liver: Phosphorylated (inactive) when glucagon/epinephrine high, preventing futile cycle with gluconeogenesis. ⚠️ Common Misunderstandings “Glycolysis always makes 4 ATP” – 2 ATP are consumed first; net gain is 2 ATP. “All NADH from glycolysis yields 3 ATP” – Cytosolic NADH must be shuttled; actual yield ≈2.5 ATP (malate‑aspartate) or ≈1.5 ATP (glycerol‑3‑P). “PFK‑1 is only regulated by ATP” – It is a hub: AMP and F2,6BP activate, ATP and citrate inhibit. “Glucokinase works like hexokinase” – It lacks product inhibition and only functions at high glucose concentrations. 🧠 Mental Models / Intuition “Gate‑keeper model” – Imagine three security doors (HK, PFK‑1, PK). If any door is locked (inactive), the whole line stalls. “Energy cliff” – Irreversible steps are cliffs; once you go over, you can’t turn back. This makes them perfect control points. “Redox balance seesaw” – NAD⁺ ↔ NADH must stay level; fermentation is a quick “reset” when the seesaw tips under anaerobic stress. 🚩 Exceptions & Edge Cases Liver glucokinase – Not inhibited by G6P; acts only when glucose is high (post‑prandial). Glycerol‑3‑P shuttle – Transfers electrons to FAD, yielding fewer ATP per NADH (≈1.5 ATP). Cancer (Warburg effect) – Cells preferentially use glycolysis even with oxygen; they often overexpress PKM2 (less active, supports biosynthesis). 📍 When to Use Which Choose malate‑aspartate shuttle when high‑energy demand tissues (heart, liver) need maximal ATP from cytosolic NADH. Choose glycerol‑3‑P shuttle in brain or fast‑twitch muscle where rapid NADH reoxidation is needed despite lower ATP yield. Apply insulin activation in metabolic calculations for fed state (dephosphorylated, active HK, PFK‑1, PK). Apply glucagon/epinephrine inhibition in fasting/exercise calculations for liver (PK phosphorylated, PFK‑2 in FBPase‑2 mode). 👀 Patterns to Recognize “ATP → ADP → ATP” pattern – Investment steps consume ATP; payoff steps generate ATP from high‑energy intermediates (1,3‑BPG, PEP). “NAD⁺ → NADH → ETC” vs. “NADH → Lactate/Ethanol” – Presence of O₂ → shuttle → ETC; absence → fermentation. “↑AMP → ↑PFK‑1 activity” – Energy shortage always drives the pathway forward via AMP activation. 🗂️ Exam Traps Trap: Selecting “4 ATP net” – remember 2 are used early, net = 2. Trap: Assuming all NADH yields 3 ATP – cytosolic NADH uses shuttles; answer depends on shuttle mentioned. Trap: Saying “hexokinase is inhibited by glucose” – only product inhibition by G6P, not glucose itself. Trap: Confusing PFK‑2 with PFK‑1 – PFK‑2 controls F2,6BP levels, not the direct phosphorylation of Fru‑6‑P. Trap: Believing pyruvate kinase is always active – liver PK is phosphorylated (inactive) during glucagon signaling. --- Use this guide for a rapid, high‑yield review before your exam. Focus on the three regulatory enzymes, the energy accounting, and the hormonal/allosteric control logic – they are the most frequently tested concepts.