Metabolism: Basic Concepts and Design

You will not memorize a metabolic map in this chapter. You will learn the grammar that every later map obeys. Glycolysis, the citric acid cycle, fatty-acid oxidation — each is a long sentence, and this chapter teaches the parts of speech. Get the grammar and the rest is reading. Skip it and you will be memorizing arrows for the rest of the semester.

Here is the organizing idea, and it is worth holding onto for the next ten chapters: metabolism is an energy economy. A cell earns energy by burning fuel and spends it to build molecules, move itself, and pump ions against gradients. Earning is catabolism. Spending is anabolism. Almost everything else in this chapter is a detail about how the books are kept.

Why all roads lead to acetyl-CoA

Cells need energy for three jobs: mechanical work (a muscle contracting, a cell crawling), active transport (the Na⁺/K⁺ pump fighting a gradient), and biosynthesis (building proteins, lipids, nucleic acids). Plants and other phototrophs capture sunlight. We are chemotrophs: we oxidize carbon fuels. Energy is never created or destroyed here, only transformed — from the bonds of food into the currency the cell can spend.

Food becomes usable energy in three stages. First, digestion breaks large molecules into small ones: proteins to amino acids, starch to sugars, fats to fatty acids. Second, those small molecules funnel into a handful of key intermediates, above all acetyl-CoA. Third, acetyl-CoA is oxidized completely, and that oxidation drives ATP synthesis.

The striking thing is the convergence. Whether you eat a steak, a potato, or a spoonful of olive oil, your mitochondria see the same molecule: acetyl-CoA. Different highways, all leading to the same city center. That single fact is why you can learn one central pathway (the citric acid cycle, which gets its own lesson later in the course) and have it apply to every fuel you will ever eat.

Coupling: how the cell pays for unfavorable reactions

Building a molecule is usually uphill — endergonic, with a positive ΔG. So how does it happen? Not by magic and not by reversing some enzyme. The cell pays for it.

A thermodynamically unfavorable reaction proceeds when it is coupled to a favorable one through a shared intermediate. The trick is that ΔG values are additive. If A → B has ΔG°’ = +12 kJ/mol and ATP hydrolysis has ΔG°’ = −30.5 kJ/mol, then running them together through a shared intermediate gives a net −18.5 kJ/mol, and the reaction goes. Picture pulling a light wagon uphill by hitching it to a heavier wagon rolling down. The two are not happening at separate times; they are linked, and the combined system rolls in the favorable direction.

This is why catabolism and anabolism are coupled rather than independent. Catabolism is your paycheck; anabolism is your spending. Some pathways, like the citric acid cycle, are amphibolic: they serve both sides of the ledger, oxidizing fuel while also supplying the starting materials for biosynthesis. And when the cell needs to run a route in reverse, as it does when it makes glucose instead of breaking it down, it never does so by simply running the same enzymes backward. It installs distinct, separately regulated, irreversible enzymes for each direction. Income and spending use the same bank account, but different passwords.

Try the first module of the bench below: set a positive ΔG°’ for a target reaction, then couple it to ATP hydrolysis and watch the equilibrium constant leap.

ATP, and the myth of the high-energy bond

ATP is the universal energy currency. Its structure is adenine plus ribose plus three phosphates, named α, β, and γ from the ribose outward; the γ-phosphate is the one transferred. Hydrolysis of that terminal phosphate is exergonic, with ΔG°’ ≈ −30.5 kJ/mol.

Now correct a common misconception. There is no “high-energy bond” storing the energy. Energy is not in a bond waiting to spring out. Hydrolysis is favorable because the products are more stable than the reactants: ADP and Pi carry less crowded negative charge (less electrostatic repulsion), the released phosphate gains resonance stabilization, two molecules have more entropy than one, and the products are better hydrated than ATP was. The free energy comes from the difference in stability, not from a special bond.

That ΔG°’ is what lets ATP drive other reactions. Recall ΔG°’ = −RT ln K’eq: a ΔG°’ near −30.5 kJ/mol corresponds to coupling that shifts an unfavorable reaction’s effective K’eq by roughly a factor of 10⁵. Couple to ATP and a reaction that essentially would not happen now runs to near completion.

The deeper point is where ATP sits on the phosphoryl-transfer ladder — a ranking of how eagerly each phosphorylated compound gives up its phosphate. Phosphoenolpyruvate, 1,3-bisphosphoglycerate, and creatine phosphate all rank above ATP; glucose-6-phosphate ranks below. ATP sits in the middle, and that is exactly what makes it good currency. A compound at the top can recharge ADP into ATP; ATP in turn can phosphorylate things lower down, like glucose. A bill of intermediate denomination is the one you can both receive and spend. Drag the phosphoryl token in the ladder module of the bench above and you will discover the rule yourself: transfers only run downhill.

One more property earns ATP its job. ATP, and the phosphate esters it creates on proteins, are thermodynamically unstable but kinetically stable: they want to hydrolyze, but they will not do it on their own at any useful rate without an enzyme. So ATP waits in the cytosol until an enzyme spends it, and a phosphate put on a protein stays put until an enzyme removes it. That mismatch makes phosphorylation an ideal on/off switch: a kinase adds a phosphate to flip a protein on, a phosphatase removes it to flip it off, and nothing happens in between.

You can feel ATP’s timescale in your own muscles. Stored ATP lasts seconds. Then creatine phosphate, sitting above ATP on the ladder, recharges ADP for another 30 to 60 seconds via creatine kinase. (When muscle is injured, creatine kinase leaks into the blood, which is why elevated serum CK is a clinical marker of muscle damage.) After that, glycolysis takes over, and finally aerobic respiration for the long haul.

Why is fat the fuel of choice for the long haul? Because of how reduced its carbons are. Burning a fuel means stripping electrons from its carbons and handing them, ultimately, to oxygen. A carbon loaded with hydrogens (the long chain of a fatty acid) has many electrons to give; a carbon already bonded to oxygen (every carbon in glucose) has fewer; CO₂ has none left. That is why fat yields about 9 kcal per gram and carbohydrate about 4. Click through the fuels on the ladder below and watch energy density climb the farther a fuel sits from CO₂.

Activated carriers and recurring motifs

Beneath the thousands of named reactions, metabolism uses only about six reaction types — redox, ligation (ATP-driven joining), isomerization, group transfer, hydrolysis, and lyase reactions. Learn the verbs and the sentences get short.

It also runs on a small set of activated carriers — think delivery trucks. They are kinetically stable, few in number, and most are built from vitamins, which is why a vitamin deficiency hits metabolism hard. ATP carries phosphoryl groups. NADH carries electrons from carbon oxidations (from niacin, vitamin B3) and feeds them to the electron transport chain, worth roughly 2.5 ATP each. FADH₂ also carries electrons (from riboflavin, B2; the working part is the isoalloxazine ring) but enters lower in the chain at Complex II, worth about 1.5 ATP. NADPH is the carrier for reductive biosynthesis — same electron-carrying chemistry, dedicated to building rather than burning. And coenzyme A carries acyl groups through a thioester linkage; that bond has high transfer potential because a thioester is less stabilized by resonance than an ordinary oxygen ester, so breaking it releases more free energy. CoA’s core is pantothenate, vitamin B5.

The clinical tie-ins write themselves. Pantothenate kinase deficiency causes PKAN, with brain iron accumulation and neurodegeneration. Niacin deficiency is pellagra; thiamine deficiency (the carrier TPP) is beriberi; riboflavin deficiency is ariboflavinosis. Each disease is a missing truck.

Regulation: amount, activity, access

A cell controls flux three ways, on three timescales. It changes the amount of an enzyme by transcription (hours to days). It changes the activity of existing enzymes by allostery and reversible covalent modification (seconds to minutes), which is how hormones like insulin coordinate the whole body fast. And it changes substrate accessibility by compartmentation — keeping fatty-acid synthesis in the cytosol and fatty-acid oxidation in the mitochondrion so the two never run at once in a futile cycle.

The master gauge is energy charge: ([ATP] + 0.5[ADP]) / ([ATP] + [ADP] + [AMP]), normally held near 0.85–0.90. It is the cell’s battery meter. Adenylate kinase (2 ADP ⇌ ATP + AMP) makes AMP a hypersensitive alarm: a small dip in ATP produces a large spike in AMP, which activates AMPK to switch on catabolism and shut down anabolism. Read the battery in the third module of the bench above and watch the needle redline as ATP falls.

When this regulation fails, you get disease — and this is the frame to carry through the whole course: pathophysiology is biochemistry in a clinical context. Diabetes is regulation failure at the level of insulin signaling. Cancer cells run aerobic glycolysis (the Warburg effect), burning glucose for biosynthetic building blocks rather than maximal ATP, which is why tumors light up on a PET scan. Mitochondrial disease is electron-transport failure, often presenting as lactic acidosis.

That is the grammar. In Chapter 16, Glycolysis, you will watch every one of these ideas — coupling, substrate-level phosphorylation, activated carriers, energy-charge regulation — come alive in a single ten-step pathway.