An enzyme does not push a reaction. It holds two molecules still in the one position where they can react — and most of them cannot do even that without a mineral or a vitamin in hand.
~11 min read
Almost every chemical change inside you is one a beaker would refuse to make. The starch in a slice of bread will sit in warm water for years without becoming sugar. In your mouth it starts coming apart in seconds. Nothing was added but a protein.
Two molecules react only when they meet at the right angle carrying enough energy to get over a hump — the arrangement partway through, where old bonds are half broken and new ones half made. That is the activation energy, and in a beaker you pay it with heat, pressure or a strong acid. A body has none of those to spend: it runs at 37 °C, at a blood pH held between about 7.35 and 7.45, in water.
An enzyme lowers the hump, and the way it does it is disarmingly physical. It grips both molecules and holds them in the one orientation from which the reaction is easy — the right atoms facing each other, at the right distance, with the surrounding water pushed out of the way. Nothing is forced. A collision that would have happened by luck once in a decade is arranged deliberately, thousands of times a second.
The rates are worth sitting with. Carbonic anhydrase, which lets your blood carry carbon dioxide away as bicarbonate, converts in the region of a million molecules a second. A human body runs several thousand distinct enzymes, and a reaction with no enzyme for it is, for practical purposes, a reaction that does not happen in you.
And the enzyme comes out of it unchanged. Not consumed, not spent — the same molecule turns over the next pair, and the next. That is what catalyst means, and it is why the quantities involved are so small.
Diagram to come
One enzyme drawn large and simply as a folded blob with a shaped notch cut into its surface — the active site — shown across three frames left to right. Frame one: two substrate molecules approaching, drawn as small distinct shapes, plus a separate small round ion in a second colour already seated in a pocket at the base of the notch and labelled as the cofactor. Frame two: both substrates settled into the notch, touching, with the ion visibly holding one of them in position — a short line or bracket from ion to substrate makes that contact explicit. The notch is drawn slightly tightened around them compared with frame one, so induced fit reads as a change in the drawing rather than as a word. Frame three: one joined product leaving, the notch open again, and the cofactor STILL seated in its pocket. The most important thing the drawing must carry is that the enzyme and the cofactor are both unchanged at the end while the substrates are not. No chemical structures, no atom labels.
The notch that does the holding is the active site — often a few dozen amino acids out of several hundred, with everything else in the protein there to hold those few in exactly the right places relative to one another. The molecules it grips are its substrates.
Which is why specificity is so sharp. An enzyme that takes apart starch will not touch protein, and usually will not touch a sugar chain joined the other way round. The old picture for this was a lock and key; the better one is induced fit — the site closes around its substrate as that substrate arrives, more like a hand taking hold of something than a slot accepting a coin. Both make the same point. The shape is the function. There is nothing else.
And the shape is held by the weak forces rather than the strong ones. The amino acid chain is joined by covalent bonds, but the fold that turns that chain into a working tool is held by hydrogen bonds and van der Waals contacts — the forces from chemical bonding, where a hydrogen bond came in around 20 kilojoules per mole against 350 for a carbon-carbon bond. A fold held together by twenties comes apart long before the chain does.
That is denaturing, and you have watched it. Egg white is a solution of proteins; it sets somewhere around 62 °C and does not un-set on cooling. The chain is intact — nothing has been cut — but the fold is gone, and with it the site.
So every enzyme works inside a narrow band of temperature and acidity, and the band is set by where it has to work. Pepsin does its best work in the stomach near pH 2. The pancreatic enzymes that take over in the small intestine want a pH nearer 8, which is why the pancreas sends bicarbonate along with them. Two sets of enzymes, a hand’s width apart, on opposite sides of neutral.
In a beaker, warming a reaction speeds it up — as a rough rule, doubling the rate for each 10 °C. Enzymes follow that rule until they stop following it, and then fall off a cliff: warm one a little further and the fold holding the active site loosens, so the rate does not plateau, it collapses.
Acidity works the same way. The amino acids lining a site often do their work by carrying a charge — donating a proton, or holding a negative one. Shift the acidity and they gain or lose it, and the site stops working without the protein unfolding at all. Some of that reverses; push further and the fold goes too, and that does not. It is also why fresh pineapple will not set in gelatine and tinned pineapple will: the protease in the tin has been heated, and is still a chain of amino acids but no longer a tool.
Here is the fact that makes nutritional chemistry hang together: a large share of enzymes are inert as pure protein. The fold is correct, the active site is open, and nothing happens — because a non-protein part is missing.
That part is a cofactor. Protein alone is an apoenzyme; protein plus cofactor is a holoenzyme, and only the holoenzyme works. Cofactors come in two kinds, and the split maps almost exactly onto the two halves of a vitamin-and-mineral label.
Metal ions — minerals, used as they arrive
An element with a charge, seated in a pocket of the protein. No conversion is needed: the magnesium in your food is the magnesium in the enzyme. That is the whole job description of a dietary mineral, and minerals as ions explains the charge itself.
Coenzymes — organic molecules built from a vitamin
A small carbon-based molecule the enzyme uses as a tool: something to carry a chemical group from one reaction to another, or to accept a pair of electrons and hand them on. Most of these your body assembles from a vitamin you ate, in a step or two.
The mineral cases are easiest to see, because the charge is doing something you can picture. Magnesium sits in the active sites of several hundred reactions, and in most of them its two positive charges pin the negatively charged phosphates of ATP in position — holding the substrate still so the enzyme can work on it.
Iron and copper are used for a different property: both can change charge, so both can take an electron and give it back, which makes them the working parts of enzymes that move electrons about. Catalase holds iron at the centre of four haem groups; superoxide dismutase comes in two versions, one using copper and zinc and one manganese; glutathione peroxidase is built around selenium. Those three are the body’s own handling of the reactive molecules in free radicals and antioxidants, and every one has a mineral at its centre.
Why the amounts are so small
Protein arrives by the gram because it is building material — it becomes the muscle, and the enzyme itself. A cofactor arrives by the milligram or the microgram because it is a tool rather than a brick: regenerated after every use and straight back to work. That is the real content of the macronutrient and micronutrient labels in what a nutrient is — a statement about catalysis, saying nothing about importance.
A protein is built from twenty amino acids, and for all their variety they are limited chemists. None carries a strong positive charge concentrated into a small space, and none holds an electron loosely enough to pass it on and take it back. A metal ion does both, which is why enzymes recruit them.
Three jobs cover nearly all of it. Holding: a doubly charged ion grips a negatively charged part of the substrate and stops it drifting — the arranging work this lesson is about. Polarising: a charge that close to a bond drags electrons along it and leaves one atom slightly short, which makes that atom easier to attack. Zinc does exactly this to a water molecule in carbonic anhydrase, and the water becomes reactive enough to take on carbon dioxide at speed. Carrying electrons: iron, copper and manganese switch between two charge states, taking an electron on one side of a reaction and releasing it on the other.
The B vitamins have a reputation as a vague group with vague benefits, and the reason is that almost nobody is told what they are. They are coenzyme parts. Each one is converted, inside you, into a specific tool that a specific class of enzyme cannot work without.
The conversion matters. A B vitamin as swallowed is usually not the working form — it is the part your cells finish. Thiamine has two phosphate groups added to it. Riboflavin is built into something larger. This is the point vitamins as molecules makes about vitamers, seen from the other end: a label says “as” something because the thing on the label is one step short of the thing your enzymes use.
| Vitamin | Becomes | The work it does |
|---|---|---|
| B1 Thiamine | Thiamine pyrophosphate | Cuts a two-carbon fragment off a molecule and carries it |
| B2 Riboflavin | FAD and FMN | Accepts and releases electrons in pairs |
| B3 Niacin | NAD and NADP | Accepts and releases electrons; more reactions than any other coenzyme |
| B5 Pantothenic acid | Coenzyme A | Carries two-carbon acetyl groups, where the fuel routes meet |
| B6 Pyridoxine | Pyridoxal phosphate | Moves amino groups; nearly all amino acid chemistry |
| B7 Biotin | Attached to its enzyme directly | Carries carbon dioxide, so a carbon can be added to a chain |
| B9 Folate | Tetrahydrofolate | Carries single-carbon groups, the units that build DNA bases |
| B12 Cobalamin | Methylcobalamin, adenosylcobalamin | Moves a methyl group; rearranges a carbon skeleton |
Read down the third column and a pattern appears: almost every one is a courier. A coenzyme picks up a small group — two carbons, one carbon, an amino group, a pair of electrons — and puts it down where another enzyme needs it. Metabolism is not one long pipe. It is a set of reactions that hand parts to each other, and the B vitamins are what the parts are handed on.
What a missing cofactor looks like
Not a symptom you could name. A pathway slowed at one step, with everything downstream of it arriving late and everything upstream backing up. Because one coenzyme serves dozens of enzymes across different tissues, the consequence spreads thin over the whole body rather than concentrating anywhere you could point to — which is why the B group took decades to sort out, and why it resists a tidy description now.
Diagram to come
One horizontal flow in three stages, then a consequence. Stage one, far left: a small molecule labelled as the vitamin as eaten. Stage two: an arrow marked with a conversion step — a phosphate drawn as a small circled P being added — producing a slightly larger molecule labelled as the coenzyme. Stage three: that coenzyme slotted into an enzyme notch, with a small chemical group drawn being picked up on one side and put down on the other, and a circular arrow showing the coenzyme returning to do it again; the recycling must be visible. Below and to the right, a second row shows a pathway as five boxes joined by arrows, with the coenzyme-dependent step greyed out, the boxes before it drawn stacked up and the boxes after it drawn thin and empty. That lower row is the point of the figure: one missing tool is not a hole but a queue on one side and a shortage on the other. Name no specific vitamin in the drawing.
Every enzyme so far has been inside a cell. One family works outside — secreted into the tube running through you, cutting food into pieces small enough to cross a wall. Nothing you eat is absorbed as itself. It is absorbed as fragments.
Starch is a chain of glucose units and the gut wall cannot take up a chain; protein is a chain of amino acids, and the same applies; a fat is three fatty acids tied to a glycerol backbone and has to be untied. The digestive system is a sequence of enzymes doing that cutting, in order, at the acidity each one needs.
Amylase — starch
Made in the salivary glands, and again in quantity by the pancreas. It cuts starch into short chains and then into maltose.
Protease — protein
Several, in stages. Pepsin works in the stomach and is secreted in an inactive form that the acid itself switches on. Trypsin and chymotrypsin arrive from the pancreas, also inactive until they reach the intestine. That is no accident: an enzyme that takes protein apart is made inside a gland made of protein.
Lipase — fat
Mostly pancreatic, with a little from the mouth and stomach. Lipase has a problem the others do not: fat will not dissolve in water, so it arrives as large droplets with almost no surface. Bile has to break those up before lipase has anything to work on, which is where absorption and bioavailability takes over.
Lactase — milk sugar
Not secreted into the cavity at all but anchored in the surface of the intestinal lining, so lactose is split at the moment it is taken up. Across most of the world’s adult populations lactase activity falls away after infancy; a minority keep it at full strength for life. Both are ordinary human variation.
Cellulase — plant fibre
Humans make none, which is exactly why fibre passes through: no enzyme exists in us for the bonds holding cellulose together. Some of it is fermented further down by bacteria, which do have the enzyme. Cellulase in a supplement comes from fungal fermentation, because no animal source exists.
Those five classes are what you will see named on a digestive enzyme product such as DigestZen TerraZyme, and the names are readable now rather than decorative — each is the tool for one kind of bond. Which products are available differs by country across the markets this app serves, and whether any of it is worth thinking about belongs to digestive support rather than here.
Diagram to come
A vertical schematic of the digestive tract, simplified to one tube with five labelled stations down its length: mouth, stomach, duodenum, jejunum, large intestine. Alongside the tube, a narrow vertical pH scale running from about 2 at the stomach to about 8 at the duodenum and back toward neutral — the acid swing must be legible, because it is why the enzymes hand over where they do. At each station, arrows entering the tube show which enzyme is secreted there and from which gland: salivary glands at the mouth, the stomach lining, the pancreas at the duodenum. Lactase is drawn differently, embedded IN the wall rather than arriving in the cavity, because the text makes that distinction. Inside the tube, three substrates — a starch chain, a protein chain, a fat droplet — are drawn progressively shorter as they travel down, ending as single units. At the large intestine, bacteria are drawn working on a chain that has passed all the way through untouched.
One thing an essential oil is not is an enzyme. An oil is a mixture of small molecules that dissolve in fat, with no protein in it and no catalytic machinery, which is why it takes a different route into you — the route the oil effect describes. Dilute an oil in a carrier before it goes on skin, and take one internally only if the label on your bottle states it is for internal use.
Keep oils away from eyes and inner ears, and out of reach of children. Always dilute before applying to skin, and only consume an oil whose label states it is for internal use. If you are pregnant, nursing, under medical care or taking medication, speak to your healthcare practitioner before use. Nothing here is intended to diagnose, treat, cure or prevent any disease.
Three things follow, and none is about a product. An enzyme is a shape, so whatever changes the shape changes the outcome: temperature and acidity are part of a reaction’s definition rather than details of it. Most enzymes need a partner, so a mineral and a vitamin are parts of a tool rather than health accessories. And a cofactor is used again and again, which is why it is needed in quantities that look, beside a gram of protein, almost too small to matter.
It also closes a loop this course opened a long way back. A functional group behaves the same way wherever it appears — a hydroxyl is a hydroxyl in a terpene and in ascorbic acid. An active site is that idea turned around: specific groups positioned deliberately, so that one reaction becomes easy and every other one stays hard.
Where to go from here
Lesson 17 of 18 · Science