Glycolysis, Gluconeogenesis, and Pentose Phosphate Pathway

Glycolytic Steps and Regulation

Glycolysis occurs in the cytoplasm and consists of 10 enzymatic steps divided into two phases. Energy Investment Phase (Steps 1–5, consumes 2 ATP): Step 1 — Glucose → Glucose-6-phosphate (G6P) by hexokinase (or glucokinase in liver/pancreas), consuming 1 ATP. This traps glucose inside the cell (G6P cannot cross the glucose transporter). Hexokinase is inhibited by G6P (product inhibition). Glucokinase (liver/pancreatic β-cells) has a higher Km (~10 mM vs. ~0.1 mM for hexokinase) and is NOT inhibited by G6P, allowing the liver to continue phosphorylating glucose when blood glucose is high. Step 2 — G6P → Fructose-6-phosphate (F6P) by phosphoglucose isomerase. Step 3 — F6P → Fructose-1,6-bisphosphate (F1,6BP) by phosphofructokinase-1 (PFK-1), consuming 1 ATP. THIS IS THE RATE-LIMITING, COMMITTED STEP OF GLYCOLYSIS. PFK-1 regulation: Activated by AMP (low-energy signal) and fructose-2,6-bisphosphate (F2,6BP — the most potent allosteric activator). Inhibited by ATP (high-energy signal) and citrate (TCA cycle intermediate — signals abundant biosynthetic precursors). PFK-1 exhibits sigmoidal kinetics (allosteric enzyme). The insulin/glucagon axis controls F2,6BP levels: insulin activates PFK-2 (which makes F2,6BP from F6P), activating PFK-1 and promoting glycolysis; glucagon activates fructose-2,6-bisphosphatase (F2,6BPase), decreasing F2,6BP and inhibiting glycolysis. Step 4 — F1,6BP → Dihydroxyacetone phosphate (DHAP) + Glyceraldehyde-3-phosphate (G3P) by aldolase. Step 5 — DHAP ↔ G3P by triose phosphate isomerase (only G3P continues in the pathway). Energy Payoff Phase (Steps 6–10, produces 4 ATP and 2 NADH): Step 6 — G3P → 1,3-Bisphosphoglycerate (1,3-BPG) by G3P dehydrogenase, producing 1 NADH (per G3P, so 2 NADH total per glucose). This is the ONLY oxidative step in glycolysis. Step 7 — 1,3-BPG → 3-Phosphoglycerate (3PG) by phosphoglycerate kinase, producing 1 ATP by substrate-level phosphorylation (first ATP-producing step; 2 ATP per glucose). Step 8 — 3PG → 2-Phosphoglycerate (2PG) by phosphoglycerate mutase. Step 9 — 2PG → Phosphoenolpyruvate (PEP) by enolase (dehydration reaction; fluoride inhibits enolase, which is why fluoride tubes are used for blood glucose measurement). Step 10 — PEP → Pyruvate by pyruvate kinase, producing 1 ATP by substrate-level phosphorylation (second ATP-producing step; 2 ATP per glucose). THIS IS THE THIRD IRREVERSIBLE STEP. Pyruvate kinase is activated by F1,6BP (feed-forward activation) and dephosphorylated by insulin (active form). It is inhibited by ATP, alanine (signals abundant amino acids), and glucagon-mediated phosphorylation (inactive form in liver). The net yield per glucose: 2 ATP (4 produced − 2 consumed), 2 NADH, and 2 pyruvate.

Substrate-Level Phosphorylation

Substrate-level phosphorylation is the direct transfer of a high-energy phosphate group from a substrate to ADP to form ATP, catalyzed by a kinase. This is distinct from oxidative phosphorylation (which uses the ETC and ATP synthase). In glycolysis, two steps involve substrate-level phosphorylation, both occurring twice per glucose (once per 3-carbon intermediate). Step 7 (phosphoglycerate kinase): 1,3-BPG + ADP → 3PG + ATP. 1,3-BPG has a high-energy acyl phosphate bond (ΔG°' of hydrolysis ≈ −49 kJ/mol), making this transfer thermodynamically favorable. Step 10 (pyruvate kinase): PEP + ADP → Pyruvate + ATP. PEP has an even higher-energy enol phosphate bond (ΔG°' ≈ −62 kJ/mol), the highest-energy phosphate compound in the glycolytic pathway. PEP → pyruvate is essentially irreversible under cellular conditions (the free energy drop drives the reaction even more strongly than ATP hydrolysis). Both of these steps are 'substrate-level' because the phosphate donor is a metabolic intermediate (substrate), not the proton gradient. The TCA cycle also has one substrate-level phosphorylation: succinyl-CoA → succinate by succinyl-CoA synthetase, producing GTP (mammals) or ATP (plants/bacteria). The MCAT may ask you to count total substrate-level ATP produced per glucose: 2 from glycolysis + 2 GTP (≈ 2 ATP) from the TCA cycle (one per pyruvate, since each glucose yields 2 acetyl-CoA) = 4 total substrate-level phosphorylation ATP equivalents per glucose in aerobic metabolism.

Pyruvate Fates

The fate of pyruvate depends on oxygen availability and tissue type. Three major fates are tested. Aerobic conditions (most tissues): Pyruvate enters the mitochondrial matrix via the mitochondrial pyruvate carrier (MPC) and is converted to acetyl-CoA by the pyruvate dehydrogenase complex (PDH). PDH reaction: Pyruvate + CoA + NAD+ → Acetyl-CoA + CO2 + NADH + H+. This is an irreversible oxidative decarboxylation. PDH requires five cofactors: TPP (from thiamine/B1), lipoic acid, CoA (from pantothenic acid/B5), FAD (from riboflavin/B2), and NAD+ (from niacin/B3). PDH is regulated by phosphorylation (inactivated by PDH kinase) and dephosphorylation (activated by PDH phosphatase). Acetyl-CoA then enters the TCA cycle for complete oxidation. Anaerobic conditions (contracting muscle, RBCs): Pyruvate is reduced to lactate by lactate dehydrogenase (LDH). Reaction: Pyruvate + NADH + H+ → Lactate + NAD+. This regenerates NAD+, which is essential for glycolysis to continue (G3P dehydrogenase requires NAD+). LDH runs in both directions; the direction depends on the NADH/NAD+ ratio and pyruvate/lactate concentrations. The Cori cycle: lactate produced by muscle travels via blood to the liver, where it is converted back to glucose by gluconeogenesis and returned to muscle. Anaerobic glycolysis yields only 2 ATP per glucose (vs. ~30–32 with oxidative phosphorylation) but can operate without oxygen—critical for intense exercise. Fermentation (yeast and some bacteria): Pyruvate is decarboxylated to acetaldehyde by pyruvate decarboxylase (requires TPP), then reduced to ethanol by alcohol dehydrogenase, regenerating NAD+. This is anaerobic; the decarboxylation releases CO2 (causes bread to rise). Note that mammals do NOT perform ethanol fermentation—they only produce lactate. Ethanol metabolism in humans: ethanol → acetaldehyde (by alcohol dehydrogenase) → acetate (by aldehyde dehydrogenase); acetate is then converted to acetyl-CoA and used for energy or fatty acid synthesis.

Gluconeogenesis Bypass Steps

Gluconeogenesis synthesizes glucose from non-carbohydrate precursors (lactate, glycerol, most amino acids, and all TCA cycle intermediates except acetyl-CoA). It occurs primarily in the liver (90%) and to a lesser extent in the kidney cortex (10%). The pathway is essentially the reversal of glycolysis, but seven steps are shared (reversible enzymes), while the three irreversible glycolytic steps are bypassed by distinct gluconeogenic enzymes. Bypass 1 (Pyruvate → PEP, bypassing pyruvate kinase): This is a two-step process. Step 1a: Pyruvate → Oxaloacetate (OAA) by pyruvate carboxylase (mitochondrial matrix, requires biotin as a CO2 carrier and ATP). Pyruvate carboxylase is activated by acetyl-CoA—when acetyl-CoA is abundant, it signals that the TCA cycle has sufficient fuel, and excess pyruvate should be used for gluconeogenesis. Step 1b: OAA → PEP by PEP carboxykinase (PEPCK, mitochondrial or cytosolic, depending on species; consumes GTP). OAA must leave the mitochondria, but the inner mitochondrial membrane lacks an OAA transporter. OAA is reduced to malate (by mitochondrial malate dehydrogenase, consuming NADH), which can cross via the malate-aspartate shuttle. In the cytosol, malate is re-oxidized to OAA (by cytosolic malate dehydrogenase, producing NADH), then OAA → PEP by PEPCK. Bypass 2 (F1,6BP → F6P, bypassing PFK-1): Catalyzed by fructose-1,6-bisphosphatase (F1,6BPase). This is a simple hydrolytic removal of the 1-phosphate (F1,6BP + H2O → F6P + Pi). F1,6BPase is inhibited by AMP and fructose-2,6-bisphosphate (opposite of PFK-1). Bypass 3 (G6P → Glucose, bypassing hexokinase): Catalyzed by glucose-6-phosphatase (G6Pase), which is embedded in the ER membrane. G6P + H2O → Glucose + Pi. G6Pase is present ONLY in the liver and kidney cortex—not in muscle or brain. This is why muscle cannot release free glucose into the blood (muscle lacks G6Pase); muscle glycogen is for local use only. The overall energy cost of gluconeogenesis is 6 ATP equivalents per glucose (4 ATP + 2 GTP), compared to the 2 ATP produced by glycolysis. Precursors: Lactate (via Cori cycle, lactate → pyruvate → glucose), glycerol (from TAG breakdown; glycerol → glycerol-3-phosphate → DHAP → glucose), glucogenic amino acids (alanine → pyruvate; aspartate → OAA; glutamate → α-ketoglutarate → OAA → glucose). Fatty acids (even-chain) cannot be used for gluconeogenesis in animals because acetyl-CoA cannot be converted to pyruvate or OAA (the PDH reaction is irreversible and there is no mammalian pathway from acetyl-CoA to glucose). Odd-chain fatty acids yield propionyl-CoA, which can enter the TCA cycle as succinyl-CoA and contribute to gluconeogenesis. Key regulatory principle: Insulin (fed state) inhibits gluconeogenesis and promotes glycolysis. Glucagon (fasting state) promotes gluconeogenesis and inhibits glycolysis. This reciprocal regulation prevents futile cycling (simultaneous glycolysis and gluconeogenesis, which would waste ATP).

Pentose Phosphate Pathway (PPP)

The pentose phosphate pathway (also called the hexose monophosphate shunt or phosphogluconate pathway) branches from glycolysis at glucose-6-phosphate and operates in the cytoplasm. It has two distinct phases. The oxidative phase (irreversible) produces NADPH and ribulose-5-phosphate. Step 1: G6P → 6-Phosphoglucono-δ-lactone by glucose-6-phosphate dehydrogenase (G6PDH), producing 1 NADPH. This is the rate-limiting and committed step of the PPP. G6PDH is inhibited by NADPH (product inhibition). Step 2: 6-Phosphoglucono-δ-lactone → 6-Phosphogluconate by lactonase (hydrolysis). Step 3: 6-Phosphogluconate → Ribulose-5-phosphate (Ru5P) + CO2 by 6-phosphogluconate dehydrogenase, producing 1 NADPH. Net oxidative phase: G6P + 2 NADP+ + H2O → Ru5P + CO2 + 2 NADPH + 2 H+. The non-oxidative phase (reversible) interconverts pentose phosphates and glycolytic intermediates via transketolase and transaldolase reactions, producing ribose-5-phosphate (for nucleotide synthesis) and connecting back to glycolysis. Key enzymes: Transketolase transfers 2-carbon units (requires TPP/thiamine as cofactor). Transaldolase transfers 3-carbon units. The non-oxidative phase can produce ribose-5-phosphate (for DNA/RNA synthesis) OR glycolytic intermediates (F6P and G3P) depending on cellular needs. Tissue-specific PPP activity: Liver, adipose tissue, lactating mammary gland, and adrenal cortex have high PPP activity because they require large amounts of NADPH for fatty acid synthesis and steroidogenesis. Red blood cells depend heavily on the PPP for NADPH to maintain reduced glutathione (GSH), which protects against oxidative damage. G6PDH deficiency: The most common human enzyme deficiency (X-linked recessive, affecting ~400 million people worldwide). Without sufficient NADPH, RBCs cannot maintain reduced glutathione, leading to oxidative damage and hemolysis when exposed to oxidative stress (fava beans,某些 drugs like primaquine, sulfonamides, or infection). This produces Heinz bodies (denatured hemoglobin precipitates) and bite cells on blood smear. The MCAT may ask about the mode of the non-oxidative phase: 3 Ru5P ↔ 2 F6P + 1 G3P (via transketolase and transaldolase), which regenerates glycolytic intermediates. Alternatively, when ribose-5-phosphate is needed: F6P + G3P → Ru5P (reverse non-oxidative phase). The PPP is regulated primarily by the NADP+/NADPH ratio: when NADPH is consumed (e.g., during fatty acid synthesis), NADP+ levels rise and activate G6PDH, increasing PPP flux.

NADPH Production and Functions

NADPH (nicotinamide adenine dinucleotide phosphate, reduced form) is the primary reducing agent in biosynthetic reactions, distinct from NADH which feeds electrons into the ETC for ATP production. The cytosolic NADP+/NADPH ratio is maintained very low (~0.005), reflecting a highly reduced state that drives reductive biosynthesis. NADPH is produced by: (1) The PPP oxidative phase (major source — G6PDH and 6-phosphogluconate dehydrogenase each produce one NADPH; net 2 NADPH per G6P). (2) Malic enzyme (malate → pyruvate + CO2 + NADPH, cytosolic). (3) Isocitrate dehydrogenase (cytosolic isoform). Major NADPH-consuming pathways: Fatty acid synthesis — each round of fatty acid elongation by fatty acid synthase requires 2 NADPH. Synthesis of a 16-carbon palmitate requires 14 NADPH. Cholesterol synthesis — requires NADPH for the reductive steps catalyzed by HMG-CoA reductase (the target of statins). Nucleotide synthesis — ribonucleotide reductase converts ribonucleotides to deoxyribonucleotides (DNA precursors) using NADPH. Glutathione reduction — glutathione reductase uses NADPH to reduce oxidized glutathione (GSSG) back to GSH (2 GSH + NADP+ ← GSSG + NADPH + H+). Reduced glutathione (GSH) is a critical antioxidant that neutralizes reactive oxygen species (ROS) via glutathione peroxidase. Cytochrome P450 monooxygenase system — drug metabolism and steroid hydroxylation in the liver. Phagocyte oxidative burst — NADPH oxidase in neutrophils and macrophages uses NADPH to produce superoxide (O2−) for bacterial killing. Key distinction for MCAT: NADH is primarily used for ATP production (oxidative phosphorylation). NADPH is primarily used for anabolic (reductive) biosynthesis. The cell maintains separate pools of NADH (high NAD+/NADH ratio, oxidative) and NADPH (low NADP+/NADPH ratio, reductive) through the nicotinamide nucleotide transhydrogenase and by keeping the PPP separate from glycolysis/TCA cycle electron flow.

Fed/Fasting Integration

The three pathways—glycolysis, gluconeogenesis, and PPP—are reciprocally regulated by the insulin/glucagon axis and allosteric effectors that signal the energy state of the cell. Fed state (high insulin, low glucagon): Glucose is abundant. Insulin stimulates: (1) GLUT4 translocation to the plasma membrane in muscle and adipose (increasing glucose uptake). (2) PFK-2 activity → increased F2,6BP → PFK-1 activation → increased glycolysis. (3) Pyruvate dehydrogenase phosphatase → PDH activation → pyruvate → acetyl-CoA → TCA cycle + fatty acid synthesis. (4) Glycogen synthase activation (glycogen synthesis). (5) Lipogenesis (acetyl-CoA → fatty acids → triglycerides). (6) PPP activity increases (NADPH needed for fatty acid synthesis). Insulin inhibits: gluconeogenesis (represses PEPCK and G6Pase gene expression), glycogenolysis, lipolysis. Fasting state (high glucagon, low insulin): Glucose is scarce. Glucagon stimulates: (1) Fructose-2,6-bisphosphatase → decreased F2,6BP → PFK-1 inhibition → glycolysis slows. (2) F1,6BPase activation → gluconeogenesis bypass 2 activated. (3) PEPCK and G6Pase gene expression (via cAMP/PKA/CREB pathway). (4) Glycogen phosphorylase activation (glycogenolysis in liver). (5) Lipolysis in adipose (HSL activation) → free fatty acids released; fatty acid oxidation in liver produces acetyl-CoA, which allosterically activates pyruvate carboxylase (first bypass of gluconeogenesis). (6) PDH kinase activation → PDH phosphorylation and inactivation → pyruvate conserved for gluconeogenesis, not converted to acetyl-CoA. Glucagon inhibits glycolysis and lipogenesis. Fructose-2,6-bisphosphate is the key intracellular switch: it simultaneously activates PFK-1 (glycolysis ON) and inhibits F1,6BPase (gluconeogenesis OFF) in the fed state. In the fasting state, low F2,6BP removes both signals, so PFK-1 is inactive and F1,6BPase is active—gluconeogenesis proceeds. Liver-specific metabolic specialization: the liver expresses glucokinase (high Km, not inhibited by G6P) and G6Pase, enabling it to both utilize glucose when abundant (fed) and produce glucose for other tissues when scarce (fasting). Muscle lacks G6Pase, so muscle glycogen cannot contribute directly to blood glucose. The Cori cycle (muscle lactate → liver glucose → muscle) and the glucose-alanine cycle (muscle alanine → liver glucose; alanine is transaminated from pyruvate, with the amino group carried to the liver as alanine for urea cycle disposal) shuttle substrates between muscle and liver during fasting and exercise. Brain and RBCs are obligate glucose consumers (RBCs lack mitochondria entirely; brain prefers glucose, though it can adapt to ketone bodies during prolonged fasting). This is why gluconeogenesis is essential—without it, blood glucose would fall below the level required to sustain brain function within hours of fasting.

How it works

Glycolysis and gluconeogenesis are reciprocal pathways separated by three enzyme-catalyzed bottlenecks. The three irreversible steps of glycolysis (hexokinase, PFK-1, pyruvate kinase) each have a corresponding gluconeogenic bypass (G6Pase, F1,6BPase, pyruvate carboxylase + PEPCK). This architecture prevents futile cycling—both pathways cannot run simultaneously because the irreversible enzymes are never active at the same time. The insulin/glucagon axis controls this via fructose-2,6-bisphosphate: in the fed state, F2,6BP is high, PFK-1 is active, F1,6BPase is inhibited → glycolysis. In the fasting state, F2,6BP is low, PFK-1 is inactive, F1,6BPase is active → gluconeogenesis. The PPP branches at G6P and provides the reducing power (NADPH) and ribose-5-phosphate needed for biosynthesis. The MCAT will test your understanding of pathway logic: why gluconeogenesis requires 6 ATP equivalents, why muscle cannot release glucose, why PFK-1 regulation integrates ATP/AMP, citrate, and hormonal signals, and why G6PDH deficiency causes hemolytic anemia. The key is seeing these pathways not as isolated sequences but as an integrated network whose flux is determined by energy charge, hormonal state, and tissue-specific enzyme expression.

How it works

Glycolysis and gluconeogenesis are reciprocal pathways separated by three enzyme-catalyzed bottlenecks. The three irreversible steps of glycolysis (hexokinase, PFK-1, pyruvate kinase) each have a corresponding gluconeogenic bypass (G6Pase, F1,6BPase, pyruvate carboxylase + PEPCK). This architecture prevents futile cycling—both pathways cannot run simultaneously because the irreversible enzymes are never active at the same time. The insulin/glucagon axis controls this via fructose-2,6-bisphosphate: in the fed state, F2,6BP is high, PFK-1 is active, F1,6BPase is inhibited → glycolysis. In the fasting state, F2,6BP is low, PFK-1 is inactive, F1,6BPase is active → gluconeogenesis. The PPP branches at G6P and provides the reducing power (NADPH) and ribose-5-phosphate needed for biosynthesis. The MCAT will test your understanding of pathway logic: why gluconeogenesis requires 6 ATP equivalents, why muscle cannot release glucose, why PFK-1 regulation integrates ATP/AMP, citrate, and hormonal signals, and why G6PDH deficiency causes hemolytic anemia. The key is seeing these pathways not as isolated sequences but as an integrated network whose flux is determined by energy charge, hormonal state, and tissue-specific enzyme expression.

Comparisons

• B/B (PFK-1 regulation): The most tested allosteric enzyme regulation on the MCAT. Know all four effectors: ATP (inhibitor), AMP (activator), citrate (inhibitor), fructose-2,6-BP (activator). Know the sigmoidal kinetics vs. inhibitor effects.
• B/B (Irreversible enzymes): The three irreversible glycolytic enzymes and their bypasses appear in almost every metabolism passage. Know hexokinase → G6Pase, PFK-1 → F1,6BPase, pyruvate kinase → pyruvate carboxylase + PEPCK.
• B/B (Fructose-2,6-bisphosphate): The insulin/glucagon switch via PFK-2/F2,6BPase. This is a classic hormone signaling integration question.
• B/B (G6PDH deficiency): X-linked recessive hemolytic anemia. Oxidative stress → Heinz bodies → hemolysis. NADPH is needed for glutathione reduction.
• B/B (Fed/fasting): Insulin promotes glycolysis + PPP; glucagon promotes gluconeogenesis + glycogenolysis. Know which pathways run when and why.
• C/P (Substrate-level phosphorylation): ATP production without ETC. Two steps in glycolysis, one in TCA cycle. Know the high-energy phosphate donors (1,3-BPG, PEP, succinyl-CoA).
• C/P (Thermodynamics): Irreversible steps have large negative ΔG. Bypass reactions consume ATP/GTP to overcome the thermodynamic barrier. Gluconeogenesis cost = 6 ATP eq.
• B/B (Cori cycle): Lactate → liver gluconeogenesis → glucose → muscle. This integrates anaerobic glycolysis with gluconeogenesis across tissues.

Common confusions

• Confusing the direction of regulation. ATP inhibits PFK-1 but ACTIVATES pyruvate carboxylase (gluconeogenesis). High ATP means 'we have energy — stop glycolysis, start storing.' AMP activates PFK-1 — 'we need energy — make more ATP.'
• Forgetting that PFK-1 is allosteric (sigmoidal kinetics). The MCAT may show a sigmoidal V0 vs. [F6P] curve and ask you to identify the enzyme. PFK-1 = sigmoidal. Hexokinase = hyperbolic (Michaelis-Menten).
• Thinking muscle can release free glucose into the blood. Muscle lacks glucose-6-phosphatase. Only the liver and kidney cortex can release free glucose. Muscle glycogen is for local use only.
• Mixing up the bypass enzymes. Pyruvate kinase bypass = pyruvate carboxylase (pyruvate → OAA) + PEPCK (OAA → PEP). NOT the same as pyruvate dehydrogenase (pyruvate → acetyl-CoA).
• Assuming acetyl-CoA can be converted to glucose. The PDH reaction is irreversible, and animals lack the glyoxylate cycle (plants and bacteria have it). Even-chain fatty acids → acetyl-CoA → cannot make glucose.
• Confusing NADH and NADPH roles. NADH = ATP production (ETC). NADPH = reductive biosynthesis (fatty acid synthesis, glutathione). The MCAT loves asking why a G6PDH-deficient patient has hemolysis — it's because NADPH is needed to keep glutathione reduced.
• Forgetting substrate-level phosphorylation steps. Step 7 (phosphoglycerate kinase) and Step 10 (pyruvate kinase) each produce ATP. Don't count Step 3 or Step 1 — those CONSUME ATP.
• Thinking the PPP produces ATP. It does NOT. It produces NADPH and ribose-5-phosphate. No ATP is directly made in the PPP.
• Mixing up glucokinase and hexokinase. Glucokinase = liver/pancreas, high Km, not inhibited by G6P, induced by insulin. Hexokinase = most tissues, low Km, inhibited by G6P. This distinction matters in fed-state questions.

Quick review

• Glycolysis net: Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 ATP + 2 H2O. Location: cytoplasm. 10 steps, 2 phases.
• Three irreversible glycolytic enzymes: Hexokinase/Glucokinase (Step 1), PFK-1 (Step 3, rate-limiting), Pyruvate kinase (Step 10).
• PFK-1 regulation: Activated by AMP, F2,6BP. Inhibited by ATP, citrate. Sigmoidal kinetics (allosteric enzyme).
• Substrate-level phosphorylation: Step 7 (phosphoglycerate kinase, 1,3-BPG → 3PG) and Step 10 (pyruvate kinase, PEP → pyruvate). 2 ATP each per glucose.
• Pyruvate fates: Aerobic → Acetyl-CoA (PDH, mitochondria). Anaerobic (muscle/RBC) → Lactate (LDH, regenerates NAD+). Yeast → Ethanol + CO2.
• Gluconeogenesis bypasses: Pyruvate kinase bypassed by Pyruvate Carboxylase (pyruvate → OAA, biotin, ATP) + PEPCK (OAA → PEP, GTP). PFK-1 bypassed by F1,6BPase. Hexokinase bypassed by G6Pase (liver/kidney only).
• G6Pase is ONLY in liver and kidney cortex. Muscle cannot release free glucose into blood. Muscle glycogen is local use only.
• Gluconeogenesis cost: 4 ATP + 2 GTP + 2 NADH per glucose. Precursors: lactate, glycerol, glucogenic amino acids. NOT acetyl-CoA or even-chain fatty acids.
• PPP oxidative phase: G6P + 2 NADP+ → Ru5P + CO2 + 2 NADPH. Rate-limiting enzyme: G6PDH (inhibited by NADPH).
• G6PDH deficiency: X-linked recessive, hemolytic anemia with oxidative stress. NADPH needed for glutathione reduction. Heinz bodies.
• Fructose-2,6-bisphosphate: The insulin/glucagon switch. Fed (insulin) → PFK-2 active → high F2,6BP → PFK-1 ON, F1,6BPase OFF. Fasting (glucagon) → F2,6BPase active → low F2,6BP → PFK-1 OFF, F1,6BPase ON.
• Glucokinase vs. Hexokinase: Glucokinase (liver/pancreas) — high Km (~10 mM), not inhibited by G6P, induced by insulin. Hexokinase (most tissues) — low Km (~0.1 mM), inhibited by G6P.