Biochemistry: learn metabolism as stories with inputs, outputs, regulators and the diseases when a step fails

Open a biochemistry textbook to the chapter on carbohydrate metabolism and you see a page that looks like a London Underground map drawn by someone angry. Arrows, enzymes, cofactors, dotted lines for feedback. You copy it out once, stare at it, and a week later you can draw the first three steps and then it dissolves.

The problem is not your memory. The problem is that you're trying to memorise a diagram, and a diagram has no plot. Your brain holds on to stories far better than it holds on to maps. A pathway is a story in a very literal sense: something enters, it is changed in stages, something leaves, someone is in charge of the speed, and if a stage fails, characters downstream suffer.

This article is a way of reading every pathway in that shape. It won't replace your textbook, and the facts in it are standard ones you should confirm there. What it gives you is a frame to hang the facts on.

The five-line story

For every pathway, write five lines. Only five.

1. Why does the body do this? The purpose, in plain words. Make ATP, make glucose when you're fasting, dispose of nitrogen, build a membrane lipid.
2. Where does it happen? The organ and the compartment. Cytosol, mitochondria, liver, muscle.
3. What goes in and what comes out? The main input, the main product, and any key by-products or energy carriers.
4. Who is in charge? The rate-limiting enzyme, and what switches it on or off.
5. What happens when a step fails? The named diseases, and the one reason each looks the way it does.

If you can fill those five lines without looking, you know the pathway at the level an examiner usually tests. The intermediate names matter much less than people fear. You'll remember some of them anyway, because they become the middle of the story.

Line one is the one people skip

Learners often start with the enzymes. Start with the purpose instead, because purpose predicts location and regulation.

Take gluconeogenesis. Why does the body make glucose from non-sugar precursors? Because the brain and red blood cells need a steady supply, and during a fast the glycogen runs out. That one sentence tells you the pathway runs mainly in the liver (with some contribution from the kidney), that it should be switched on when insulin is low and glucagon is high, and that it should not run at the same time as glycolysis in the same cell, or you'd burn energy going round in a circle.

Now the "bypass" enzymes make sense. Glycolysis has a few steps that are essentially one-way. Gluconeogenesis needs different enzymes to get round those: pyruvate carboxylase and PEP carboxykinase for the pyruvate step, fructose-1,6-bisphosphatase, and glucose-6-phosphatase. Check the exact list in your textbook, but notice how it's a story: the road has one-way gates, so you build a second road with different gates.

Line four: regulation is the plot twist

Every pathway has a gatekeeper, and exam questions love gatekeepers. Rather than memorising "rate-limiting enzymes" as a separate table, attach each one to its story.

• Glycolysis: phosphofructokinase-1 is the committed step. It is stimulated by fructose-2,6-bisphosphate and AMP (the cell is short of energy) and inhibited by ATP and citrate (the cell is well supplied).
• Glycogen synthesis and breakdown: glycogen synthase and glycogen phosphorylase are regulated in opposite directions, by hormones. Insulin favours storage; glucagon (in the liver) and adrenaline favour breakdown.
• Fatty acid synthesis: acetyl-CoA carboxylase, which makes malonyl-CoA.
• Fatty acid oxidation: malonyl-CoA inhibits carnitine palmitoyltransferase-1, so the cell doesn't make fat and burn it at the same time.
• Cholesterol synthesis: HMG-CoA reductase, which is where statins act.
• Urea cycle: carbamoyl phosphate synthetase I, activated by N-acetylglutamate.
• Haem synthesis: ALA synthase.

Look at the logic running through that list. The cell turns pathways on when the product is needed and off when it has plenty. When you see a regulator, ask "does the cell need more of this product, or less?" and you can usually guess the direction without memorising it.

Also notice a pattern about opposing pathways. Synthesis and breakdown of the same molecule are regulated in opposite directions so they don't run together. That one idea answers a surprising number of "which hormone stimulates which enzyme" questions.

Line five: the disease is a diagnostic story

Here is where the story form pays off most. In an enzyme deficiency, three things follow in a predictable order. The substrate before the block accumulates. The product after the block is missing. And whatever alternative route is available gets overused. Almost every inborn error of metabolism can be reasoned out from those three.

Take phenylketonuria. The enzyme phenylalanine hydroxylase (or, rarely, its cofactor tetrahydrobiopterin pathway) is deficient. So phenylalanine accumulates. Tyrosine, the product, becomes relatively deficient, and tyrosine is needed for melanin, so affected children can be fair-skinned. Excess phenylalanine goes down a side route to phenylketones in urine. And the build-up damages the developing brain, which is why early detection and a dietary approach matter. One block, three consequences, all derived.

Now do the same with a few others, always checking details against your textbook.

Glucose-6-phosphatase deficiency (von Gierke disease). This enzyme is the last step releasing free glucose from the liver, in both gluconeogenesis and glycogen breakdown. So glucose can't be released, and the person gets fasting hypoglycaemia. Glucose-6-phosphate accumulates and is pushed into other routes, so lactic acid and uric acid rise and the liver enlarges with glycogen. A single missing step explains the whole picture.

Muscle glycogen phosphorylase deficiency (McArdle disease). Muscle can't break down its own glycogen, so exercise causes cramps and fatigue, and the blood lactate doesn't rise as it should with exertion because glycolysis is starved of fuel. Compare that with the liver form, where the main problem is blood glucose. Same pathway, different tissue, different symptom.

Pyruvate dehydrogenase deficiency. Pyruvate can't enter the TCA cycle as acetyl-CoA, so it is diverted to lactate and alanine. You get lactic acidosis, and brain energy supply suffers. The complex needs thiamine, lipoic acid, CoA, FAD and NAD, which is why thiamine deficiency and arsenic poisoning (which affects lipoic acid) can look similar in their metabolic effects.

Urea cycle defects. If the cycle fails, ammonia builds up, causing neurological symptoms. Ornithine transcarbamylase deficiency is the commonest, is X-linked, and typically shows raised orotic acid, because carbamoyl phosphate spills over into pyrimidine synthesis. That's a textbook example of the "overused alternative route".

Fatty acid oxidation defects such as MCAD deficiency. The body can't burn fat during a fast, so it relies on glucose, which runs out, and the person becomes hypoglycaemic without the usual ketone rise ("hypoketotic hypoglycaemia"). Ask why: ketones come from fat oxidation, so no oxidation, no ketones.

In each case the memorable thing isn't the enzyme name. It's the reasoning chain. If you forget which enzyme is missing, the chain still helps you find the right option, because the options often differ in exactly these consequences.

Cofactors and vitamins: attach them to the stage that uses them

Cofactors are a common source of lost marks because they're typically learned as a separate list. Instead, tie each vitamin to a stage.

Thiamine works with pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase and transketolase. Riboflavin forms FAD and FMN, niacin forms NAD and NADP, pantothenic acid is part of CoA. Biotin works with carboxylases, such as pyruvate carboxylase and acetyl-CoA carboxylase. Pyridoxine (B6) is central to transamination. Folate and B12 are involved in one-carbon transfers and in the methylation cycle.

Then, when a question describes a patient with a particular deficiency, you can ask which pathway stalls and what accumulates. That's the same five-line story, entered from the side.

A worked example: the TCA cycle in five lines

Let's do one full pathway.

Purpose. Oxidise acetyl-CoA completely to carbon dioxide, capturing the energy as reduced carriers (NADH and FADH2) and a little GTP, and supply intermediates to other pathways.

Where. Mitochondrial matrix, in nearly all tissues with mitochondria. Red blood cells have none, so they can't use it.

In and out. Acetyl-CoA enters by combining with oxaloacetate. Out come CO2, NADH, FADH2, GTP/ATP. The cycle needs oxygen indirectly, because NADH and FADH2 must be re-oxidised by the electron transport chain.

In charge. Citrate synthase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase are the main control points, slowed by high ATP and NADH, and sped up by ADP. Calcium also stimulates some of them. The cell tunes the cycle to its energy state.

When it fails. Thiamine deficiency impairs alpha-ketoglutarate dehydrogenase and pyruvate dehydrogenase, with effects on tissues that depend on aerobic metabolism. Arsenic interferes with lipoic acid. Fluoroacetate is a classic example of a toxin that is converted to a compound inhibiting aconitase, so citrate builds up. Check each of these against your textbook, and notice that the reasoning is always "which step, what piles up".

Five lines, and you've covered ground that would otherwise take two pages of diagram.

Group the pathways by what they have in common

Once you've done a few stories, you start to see a smaller number of themes.

Fasting versus fed. Insulin tells the body to store, glucagon and adrenaline tell it to release. Most hormonal regulation questions are this one idea in different clothes.

Organ specialisation. The liver makes glucose and ketones, and releases them for others. Muscle burns its own glycogen and can't release glucose to the blood. The brain mostly uses glucose, with ketones in prolonged fasting. Red cells depend on glycolysis. Learn a one-line "job description" for each organ and a lot of pathway questions become reasoning.

Compartments. Fatty acid synthesis in the cytosol, oxidation in the mitochondria, the urea cycle split between both. When a molecule has to cross a membrane, there is usually a shuttle, such as the carnitine shuttle, and a disease associated with it.

Energy carriers. NADH and FADH2 feed the electron transport chain. NADPH is the reducing power for synthesis and for defence against oxidative damage. Knowing which pathways produce NADPH (the pentose phosphate pathway) and why red cells care (glutathione) links several topics at once. Glucose-6-phosphate dehydrogenase deficiency then follows naturally: less NADPH, less reduced glutathione, red cells vulnerable to oxidative stress.

How to revise stories so they stay

Reading the five lines once is a start. Keeping them is a different task.

Draw from memory, not from the page. Take a blank sheet and write the five lines for a pathway. Where you stall is the part you don't know. This is far more useful than rereading a diagram.

Make cards that ask "why". "What accumulates in von Gierke disease, and why?" is a better card than "Enzyme deficient in von Gierke disease?" The first tests the story, the second tests a label. Our own flashcards can hold cards like that, though any tool, or index cards, will do.

Use contrast. A great many questions test pairs: glycogen storage diseases that look similar, urea cycle disorders, the different causes of hypoglycaemia. Put similar diseases side by side in a small table, with the one feature that tells them apart.

Test with questions early. After you've read a pathway, try a few practice questions on it the same day. Notice which of the five lines the question touches. Over time you'll see that examiners keep returning to regulation and to the consequences of a block.

Space it out. Return to each story after a few days, then after a couple of weeks. A short recall each time is enough.

The short version

• A pathway is a story: purpose, place, inputs and outputs, regulator, failure.
• Start from the purpose; it predicts where, when and how it is regulated.
• Opposing pathways are regulated in opposite directions, so they don't run together.
• For any enzyme block: substrate piles up, product is missing, side routes get overused.
• Tie vitamins and cofactors to the stage that uses them.
• Revise by drawing from memory and by answering "why" cards, not by rereading maps.

What to do this week

1. Pick three pathways you find most confusing, and write the five-line story for each on one page, from memory first, then check against your textbook.
2. Choose one hormonal contrast (fed versus fasting) and write which enzymes it switches on and off in the liver. Check it against your book.
3. Build a small table of the glycogen storage diseases you need, with columns for enzyme, tissue, what accumulates, and the main symptom. Keep the table to what your textbook covers.
4. Turn five of your lines into "why" cards and review them on three separate days this week.
5. Do one short timed set of biochemistry questions on a pathway you studied, and for each miss, note which of the five lines you were missing.