Carbohydrate Metabolism

Carbohydrate Metabolism

Part 1 of 4 — Glycolysis: Steps, Enzymes & Logic

The Big Picture

Glycolysis converts one glucose (6C) into two pyruvate (3C each) in the cytosol of every cell, with no oxygen required:

Glucose + 2 NAD$^+$ + 2 ADP + 2 P$_i$ → 2 pyruvate + 2 NADH + 2 ATP + 2 H$_2$O

Two phases: an investment phase (2 ATP spent) and a payoff phase (4 ATP + 2 NADH made) — net 2 ATP.

The Three Irreversible (Regulated) Steps

Fructose-2,6-bisphosphate is the master switch: made by PFK-2 when insulin dominates, destroyed when glucagon (via PKA) flips the bifunctional PFK-2/FBPase-2 enzyme. High F2,6-BP = glycolysis ON in liver.

The Payoff Chemistry You Must Recognize

• Glyceraldehyde-3-phosphate dehydrogenase (GAPDH): the ONLY oxidation in glycolysis; makes NADH and the high-energy 1,3-BPG.
• Phosphoglycerate kinase: 1,3-BPG + ADP → 3-PG + ATP — first substrate-level phosphorylation.
• Pyruvate kinase: PEP + ADP → pyruvate + ATP — second substrate-level phosphorylation.
• Aldolase splits F1,6-BP into DHAP + G3P; triose phosphate isomerase converts DHAP to G3P so BOTH halves continue (this is why payoff numbers are doubled).

Fates of Pyruvate

Red blood cells have no mitochondria: glycolysis to lactate is their ONLY ATP source, 100% of the time. RBCs also run the 2,3-BPG shunt (bypassing the PGK ATP step) to make the hemoglobin modulator 2,3-BPG.

Galactose & Fructose Enter Here

• Fructose (liver): fructokinase → F1P; aldolase B splits it → enters at G3P/DHAP — downstream of PFK-1, so fructose skips the main control point.
• Galactose: galactokinase → gal-1-P; GALT (uridyltransferase) exchanges it with UDP-glucose, releasing G1P → G6P. Deficiencies: classic galactosemia (GALT) is severe; aldolase B deficiency = hereditary fructose intolerance.

Glycolysis 🎯

Key Takeaways — Part 1

• Cytosolic, O$_2$-independent; net 2 ATP + 2 NADH + 2 pyruvate per glucose
• Three irreversible steps = the control points: hexokinase/glucokinase, PFK-1 (rate-limiting; F2,6-BP is the master activator), pyruvate kinase
• GAPDH is the lone oxidation (needs NAD$^+$); PGK and PK are the substrate-level phosphorylations
• Anaerobic fates exist to regenerate NAD$^+$ (lactate in humans, ethanol in yeast); RBCs are obligate glycolyzers
• Fructose enters BELOW PFK-1 (unregulated); galactose converges at G6P via GALT

Worked Examples — Glycolysis Logic

Question: A cell converts 3 molecules of glyceraldehyde-3-phosphate (already formed) through the rest of glycolysis to pyruvate. How many ATP and NADH result?

Solution:

1. Per G3P: GAPDH makes 1 NADH; PGK makes 1 ATP; pyruvate kinase makes 1 ATP → 2 ATP + 1 NADH each.
2. For 3 G3P: $3 \times 2 = 6$ ATP and $3$ NADH.
3. Note there is no investment cost here — the hexokinase and PFK-1 ATP were spent upstream of G3P.

MCAT Strategy: "Per G3P: 2 ATP, 1 NADH" is worth memorizing; whole-glucose numbers are just this doubled, minus the 2 invested ATP.

Question: A patient has pyruvate kinase deficiency confined to red blood cells, causing hemolytic anemia. Explain why RBCs are selectively vulnerable, and predict the level of 2,3-BPG in these cells.

Solution:

1. RBCs rely solely on glycolysis; losing its final ATP-generating step starves them of ATP, membrane pumps fail, and cells lyse. Other tissues survive on oxidative metabolism and other isozymes.
2. Intermediates upstream of the block accumulate — including the 3-phosphoglycerate/2,3-BPG pool. 2,3-BPG rises.
3. Elevated 2,3-BPG right-shifts the hemoglobin curve, improving O$_2$ unloading — partially compensating for the anemia (a favorite MCAT twist).

MCAT Strategy: For any glycolytic enzyme deficiency: upstream intermediates accumulate, downstream products fall, and RBCs suffer first.

Question: Glucose labeled at C-6 undergoes anaerobic glycolysis. Where does the label appear in lactate?

Solution:

1. Aldolase splits F1,6-BP: C-1 to C-3 become DHAP, C-4 to C-6 become G3P (C-6 ends up as G3P's phosphate-bearing C-3 position).
2. TPI converts DHAP to G3P too, so both halves proceed; by symmetry C-1 and C-6 both map to the same position of G3P.
3. G3P's C-3 becomes pyruvate's C-3 (the methyl carbon), and LDH reduction does not move carbons: the label lands on lactate's methyl carbon (C-3).

MCAT Strategy: Remember the symmetry: after aldolase + TPI, glucose C-1 and C-6 become equivalent (pyruvate C-3); C-3 and C-4 become pyruvate's carboxyl carbon.

Carbohydrate Metabolism

Part 2 of 4 — Gluconeogenesis & Reciprocal Regulation

Why a Separate Pathway?

Gluconeogenesis builds glucose from noncarbohydrate precursors during fasting — mainly in the liver (kidney cortex helps in prolonged fasts). It is NOT glycolysis run backward: the three irreversible glycolytic steps are bypassed by four dedicated enzymes.

Because OAA cannot cross the inner mitochondrial membrane, it leaves as malate (or aspartate) and is reconverted in the cytosol.

The Substrates

• Lactate (Cori cycle: RBC/muscle lactate → liver glucose → back out)
• Alanine (glucose-alanine cycle from muscle protein)
• Glycerol (from triglyceride breakdown; enters as DHAP; needs liver glycerol kinase)
• Propionyl-CoA from odd-chain fatty acids (converted to the TCA intermediate succinyl-CoA)

• The route runs propionyl-CoA → methylmalonyl-CoA → succinyl-CoA; the last step is catalyzed by methylmalonyl-CoA mutase, which requires vitamin B12.

NEVER acetyl-CoA: PDH is irreversible, and the TCA cycle loses two carbons as CO$_2$ per acetyl group — even-chain fatty acids cannot yield net glucose.

The Price Tag

2 pyruvate → 1 glucose costs 4 ATP + 2 GTP + 2 NADH (6 high-energy phosphates). Compare: glycolysis earned only 2 ATP. Running both simultaneously would be a futile cycle burning 4 ATP per round — hence reciprocal regulation.

Reciprocal Regulation: One Signal, Opposite Effects

The acetyl-CoA logic is elegant: when the fasting liver burns fat, abundant acetyl-CoA simultaneously says "stop oxidizing pyruvate" (PDH off) and "start carboxylating it toward glucose" (PC on).

Ethanol Footnote

Ethanol oxidation floods the liver with NADH, pushing pyruvate → lactate and OAA → malate — starving gluconeogenesis of substrates. This is why heavy drinking while fasting causes hypoglycemia.

Gluconeogenesis 🎯

Key Takeaways — Part 2

• Four bypass enzymes: pyruvate carboxylase (biotin, mito, needs acetyl-CoA), PEPCK (GTP), F1,6-BPase (rate-limiting), G6Pase (liver/kidney ER only)
• Substrates: lactate (Cori), alanine, glycerol, odd-chain propionyl-CoA — NEVER net glucose from acetyl-CoA/even-chain fat
• Cost: 6 high-energy bonds per glucose (vs. 2 earned by glycolysis) — futile cycling is prevented by reciprocal regulation
• F2,6-BP is the fed/fasting toggle: activates PFK-1, inhibits F1,6-BPase; glucagon destroys it, insulin raises it
• Ethanol → high NADH → substrate steal → fasting hypoglycemia

• Low-yield extras: odd-chain propionyl-CoA reaches succinyl-CoA via methylmalonyl-CoA, using the vitamin B12-dependent methylmalonyl-CoA mutase

Worked Examples — Gluconeogenic Reasoning

Question: Muscle exports 2 lactate made anaerobically (2 ATP gained in muscle). The liver reconverts them to 1 glucose. What is the whole-body ATP balance for one full loop, and why is the cycle still worthwhile?

Solution:

1. Muscle: +2 ATP (anaerobic glycolysis).
2. Liver: −6 ATP-equivalents (4 ATP + 2 GTP) for gluconeogenesis from 2 lactate.
3. Net: −4 ATP per loop for the body. Worthwhile because it moves the energy cost from an O$_2$-starved, working muscle (or an RBC that can never oxidize) to the well-oxygenated liver, and it recycles carbon instead of losing it as lactate in urine.

MCAT Strategy: The Cori cycle is an energy TRANSFER, not an energy source — expect a question asking where the "missing" 4 ATP went (liver fat oxidation pays the bill).

Question: A fasting patient's liver receives: (a) glycerol from adipose lipolysis, (b) palmitate (C16), (c) alanine from muscle, (d) propionate from gut bacteria. Which contribute NET carbon to new glucose?

Solution:

1. (a) Glycerol → glycerol-3-P → DHAP: YES, direct gluconeogenic entry.
2. (b) Palmitate → 8 acetyl-CoA: NO net glucose (carbons lost as CO$_2$); it only provides ATP and the acetyl-CoA activation signal.
3. (c) Alanine → pyruvate (transamination): YES.
4. (d) Propionate → propionyl-CoA → succinyl-CoA (a TCA intermediate that becomes OAA): YES — the odd-carbon exception.

MCAT Strategy: Sort every precursor into "becomes pyruvate/OAA/TCA intermediate/DHAP" (glucogenic) vs. "becomes acetyl-CoA only" (not).

Question: Hepatocytes engineered to express a PFK-1 variant insensitive to ATP and citrate inhibition are incubated in fasting conditions with glucagon. Compared with normal cells, what happens to ATP consumption and heat production?

Solution:

1. Glucagon activates gluconeogenesis (F1,6-BPase working), but the mutant PFK-1 cannot be turned down and keeps rephosphorylating F6P → F1,6-BP.
2. The two opposing reactions cycle: each round hydrolyzes ATP (PFK-1) and P$_i$ is released (F1,6-BPase) with no net flux — a futile (substrate) cycle.
3. ATP consumption and heat output rise; net glucose production falls.

MCAT Strategy: Whenever paired opposing enzymes are BOTH active, count the ATP burned per cycle and expect "heat" as the answer to where the energy goes (some organisms use this deliberately for thermogenesis).

Carbohydrate Metabolism

Part 3 of 4 — Glycogen Metabolism & the Pentose Phosphate Pathway

Glycogen Architecture

Glycogen is a branched glucose polymer: $\alpha$-1,4 linkages in chains, $\alpha$-1,6 linkages at branch points (every ~8-12 residues). Branching creates many nonreducing ends for FAST simultaneous synthesis/degradation and improves solubility. Stored mainly in liver (for blood glucose) and muscle (for its own fuel).

Synthesis (Glycogenesis)

1. G6P → G1P (phosphoglucomutase)
2. G1P + UTP → UDP-glucose (the activated donor)
3. Glycogen synthase (rate-limiting) adds glucose to nonreducing ends via $\alpha$-1,4 bonds; needs a primer (an existing chain)
4. Branching enzyme transfers a block of ~7 residues to create an $\alpha$-1,6 branch

• New granules start on glycogenin, a protein that attaches the first few glucose residues to itself and so serves as the primer.

Degradation (Glycogenolysis)

1. Glycogen phosphorylase (rate-limiting; uses pyridoxal phosphate/B6) cleaves $\alpha$-1,4 bonds by phosphorolysis → G1P (no ATP needed!)
2. Phosphorylase stalls 4 residues from a branch; the debranching enzyme has two activities: a transferase moves three of the four remaining residues onto another chain, then $\alpha$-1,6-glucosidase releases the last branch residue as FREE glucose
3. G1P → G6P; liver exports glucose via G6Pase, muscle burns it

About 90% of released residues are G1P; ~10% (branch points) are free glucose.

Hormonal Control — The Same Cascade, Opposite Enzymes

• Glucagon (liver) / epinephrine (muscle + liver) → G$_s$ → adenylyl cyclase → cAMP → PKA → phosphorylase kinase → phosphorylase b becomes phosphorylase a (ACTIVE); PKA also phosphorylates glycogen synthase (INACTIVE). Phosphorylation = breakdown mode.
• Insulin → activates phosphatases (PP1) that strip these phosphates: synthase ON, phosphorylase OFF. Dephosphorylation = storage mode.
• Allosteric shortcuts: in muscle, AMP activates phosphorylase directly and Ca$^{2+}$ activates phosphorylase kinase (contraction couples directly to fuel); ATP and G6P inhibit phosphorylase; liver phosphorylase a is inhibited by glucose itself.

• Ca$^{2+}$ acts through calmodulin, which is built into phosphorylase kinase as one of its subunits.

Glycogen storage diseases to recognize: von Gierke (G6Pase — severe fasting hypoglycemia + hepatomegaly), Cori (debrancher — abnormal short-branched glycogen), McArdle (muscle phosphorylase — exercise cramps, no rise in blood lactate on exercise), Pompe (lysosomal acid $\alpha$-glucosidase — cardiomegaly).

The Pentose Phosphate Pathway (HMP Shunt)

Cytosolic, parallel to glycolysis, no ATP made or used. Two products, two phases:

NADPH jobs: fatty acid & cholesterol synthesis, respiratory burst (NADPH oxidase), cytochrome P450, and reducing glutathione — the RBC's main defense against peroxides.

G6PD deficiency: the most common human enzyme deficiency. Oxidant stress (fava beans, sulfa drugs, antimalarials, infection) overwhelms glutathione regeneration → hemoglobin precipitates (Heinz bodies) → hemolytic anemia with bite cells. Mild advantage against malaria explains its prevalence.

Glycogen & the PPP 🎯

Key Takeaways — Part 3

• Glycogen: $\alpha$-1,4 chains, $\alpha$-1,6 branches; UDP-glucose builds it (synthase + brancher); phosphorylase (B6, phosphorolysis → G1P) + debrancher tear it down
• Phosphorylation = breakdown mode (phosphorylase ON, synthase OFF); insulin/PP1 reverses; muscle adds AMP and Ca$^{2+}$ shortcuts, liver phosphorylase senses glucose
• Disease anchors: von Gierke = G6Pase, McArdle = muscle phosphorylase (no exercise lactate rise), Cori = debrancher, Pompe = lysosomal acid $\alpha$-glucosidase (cardiomegaly)
• PPP: G6PD (rate-limiting, NADPH-inhibited) makes 2 NADPH + R5P; nonoxidative phase reversible (transketolase needs TPP)
• NADPH = biosynthesis + glutathione defense; G6PD deficiency = oxidant-drug hemolysis with Heinz bodies

• Low-yield extras: glycogenin primes new granules; Ca$^{2+}$ works through phosphorylase kinase's calmodulin subunit; Andersen disease is the name for branching-enzyme deficiency; McArdle extras (ammonia rise, myoglobinuria, the "second wind")

Worked Examples — Storage & Shunt Problems

Question: During an ischemic forearm exercise test, a healthy subject's venous lactate rises several-fold; a patient's lactate stays flat, and the patient cramps early. Which enzyme is deficient, and why does lactate fail to rise?

Solution:

1. Flat lactate despite exercise = muscle cannot mobilize glycogen → muscle glycogen phosphorylase (McArdle disease). Glycolysis from blood glucose still works, but glycogen (the rapid fuel) is locked.
2. Lactate fails to rise because the glycogen-fed burst of anaerobic glycolysis never happens; the early cramps reflect the resulting ATP shortfall in working fibers.

MCAT Strategy: "Exercise intolerance + no lactate rise" is the McArdle fingerprint; contrast with von Gierke, where lactate is chronically HIGH.

• McArdle extras: in the forearm test, ammonia rises even though lactate stays flat (AMP is deaminated as ATP runs short), and severe cramping can release myoglobin into the urine. Patients also report a "second wind": with continued gentle exercise, blood glucose and free fatty acids arrive and oxidative metabolism substitutes for the missing glycogenolysis, easing symptoms.

Question: Describe the glycogen produced when (a) branching enzyme is deficient, and (b) debranching enzyme is deficient.

Solution:

1. (a) Without the brancher, synthase still elongates $\alpha$-1,4 chains: very LONG, UNBRANCHED chains — poorly soluble, fewer ends, and the abnormal polysaccharide damages the liver.
2. (b) Without the debrancher, phosphorylase strips chains down to 4 residues from each branch and stops: a "limit dextrin"-like glycogen with SHORT outer branches accumulates (Cori disease); fasting hypoglycemia is milder than von Gierke because the outer $\alpha$-1,4 residues are still releasable.
3. Structure predicts pathology: both are storage diseases, but the stored material differs in architecture.

MCAT Strategy: Tie each enzyme to a structural signature — brancher missing = long straight chains; debrancher missing = short-branched limit dextrins.

• Branching-enzyme deficiency is called Andersen disease.

Question: A patient of Mediterranean descent develops dark urine, jaundice, and bite cells on smear two days after starting an antimalarial. Trace the biochemical chain from drug to hemolysis.

Solution:

1. The drug generates oxidative stress (peroxides, superoxide) inside RBCs.
2. Detoxification requires reduced glutathione (GSH); regenerating GSH requires NADPH via glutathione reductase.
3. With deficient G6PD, the oxidative PPP cannot supply NADPH on demand; GSH is exhausted.
4. Peroxides oxidize hemoglobin → denatured Hb precipitates as Heinz bodies; splenic macrophages "bite" them out; membranes fail → hemolysis, hemoglobinuria, jaundice.

MCAT Strategy: The tested chain is always drug → oxidant stress → NADPH/GSH shortfall → Heinz bodies/bite cells. Remember G6PD is X-linked and the deficiency is protective against malaria.

Carbohydrate Metabolism

Part 4 of 4 — Hormonal Integration & MCAT-Style Experiments

The Insulin/Glucagon Axis in One Table

Epinephrine mirrors glucagon (also G$_s$/cAMP in liver and muscle) during stress/exercise — and muscle DOES have epinephrine receptors.

Glucose Transporters the MCAT Names

Beta-Cell Glucose Sensing (A Favorite Passage)

Glucose enters via GLUT2 → glucokinase (high $K_m$, no product inhibition — the true "glucostat") → glycolysis + oxidation raise ATP/ADP → ATP-sensitive K$^+$ channels CLOSE → membrane depolarizes → voltage-gated Ca$^{2+}$ channels open → insulin granule exocytosis. Sulfonylurea drugs (used in type 2 diabetes) close the K$_{ATP}$ channel directly → depolarization → Ca$^{2+}$ entry → insulin release, even without a glucose signal.

Timeline of a Fast

Diabetes in Biochemical Terms

• Type 1: no insulin → unrestrained lipolysis and glucagon action → hyperglycemia + diabetic ketoacidosis possible.
• Type 2: insulin resistance → GLUT4 translocation impaired, hepatic gluconeogenesis inadequately suppressed → hyperglycemia; ketosis rare (some insulin remains).
• Chronic hyperglycemia drives nonenzymatic glycation (HbA1c as the 3-month average).

• In lens and nerve cells, aldose reductase converts excess glucose to sorbitol, which accumulates and contributes to cataracts and neuropathy.

Passage-Reading Drill

When a carbohydrate passage shows a knockout, drug, or hormone clamp, ask: (1) Which of the three regulated steps (or transporters) does it touch? (2) What happens to F2,6-BP? (3) Which tissue is being discussed — liver rules blood glucose; muscle keeps its glucose; brain and RBC are entitled consumers.

Hormonal Integration 🎯

Key Takeaways — Part 4

• Insulin = RTK, anabolic, GLUT4 translocation; glucagon = GPCR/cAMP, liver-only, catabolic; epinephrine covers muscle in stress
• Transporter map: GLUT1 basal, GLUT2 high-$K_m$ sensor (liver/beta cell), GLUT3 neurons, GLUT4 insulin/exercise, SGLT secondary active
• Beta-cell sensing: GLUT2 → glucokinase → ATP up → K$_{ATP}$ channels close → depolarize → Ca$^{2+}$ → insulin; sulfonylureas close K$_{ATP}$ directly
• Fast timeline: glycogen carries ~24 h, then gluconeogenesis; ketones spare protein in prolonged fasting
• T1DM = absent insulin (DKA risk); T2DM = resistance; HbA1c = nonenzymatic glycation clock

• Low-yield extras: aldose reductase makes sorbitol in lens and nerves (cataracts, neuropathy); the glucokinase set-point mutation is called MODY2

Worked Examples — Integration Passages

Question: A family carries a heterozygous glucokinase mutation raising its $K_m$ for glucose twofold. Predict the effect on insulin secretion and fasting blood glucose.

Solution:

1. The beta cell's "glucostat" now needs HIGHER glucose to generate the same glycolytic ATP signal.
2. Insulin secretion still occurs but at a raised set point: mild, stable fasting hyperglycemia (e.g., ~6-8 mM instead of ~5).
3. Liver glucokinase is also less active, modestly reducing postprandial glucose uptake. The phenotype is mild because the whole curve shifts rather than collapsing.

MCAT Strategy: A $K_m$ shift in a SENSOR enzyme moves the set point; a $V_{max}$ loss would cap the response. Distinguish those two outcomes whenever a passage mutates glucokinase.

• In clinical terms, heterozygous glucokinase loss is MODY2 (a monogenic form of diabetes).

Question: In an insulin clamp study, insulin is infused while glucose is infused at whatever rate keeps blood glucose constant. Subject A requires a high glucose infusion rate; subject B requires very little. Who is insulin resistant, and which tissues explain the difference?

Solution:

1. The glucose infusion rate measures how much glucose the body DISPOSES of under insulin: high rate = insulin-sensitive.
2. Subject B (low rate) is insulin resistant: muscle/adipose GLUT4-mediated uptake responds poorly, and the liver keeps releasing glucose despite insulin.
3. The clamp isolates insulin action because glucose itself is held constant — removing the confound of hyperglycemia driving uptake by mass action.

MCAT Strategy: In clamp passages, the infusion rate IS the readout. Map "less glucose needed" to "less insulin effect."

Question: A type 1 diabetic misses insulin doses and presents with glucose of 450 mg/dL, deep rapid breathing, and fruity breath. Connect the hormone state to each finding.

Solution:

1. No insulin + high glucagon: liver runs gluconeogenesis/glycogenolysis at full tilt while peripheral uptake fails → severe hyperglycemia (osmotic diuresis, dehydration).
2. Unrestrained adipose lipolysis floods the liver with fatty acids; beta-oxidation acetyl-CoA exceeds TCA capacity (OAA drawn off for gluconeogenesis) → ketone bodies (acetoacetate, beta-hydroxybutyrate) accumulate.
3. Ketoacids consume bicarbonate → metabolic acidosis → compensatory hyperventilation (Kussmaul breathing); acetone (from spontaneous acetoacetate decarboxylation) gives the fruity breath.

MCAT Strategy: DKA questions are pathway questions in disguise: absent insulin = simultaneous maximal gluconeogenesis AND ketogenesis. Type 2 patients usually retain enough insulin to suppress ketogenesis.