activation energyThe energy barrier a reaction must get over to start. The term was introduced in 1889. An enzyme speeds its reaction by lowering this barrier.
active siteAn enzyme's pocket, in which the reaction happens. It takes up only about 10 to 20 percent of the enzyme's volume. Most of the remaining protein is there to hold the pocket in shape.
cofactorA non-protein helper that an enzyme needs to work. Some cofactors are metal ions, such as magnesium, zinc and copper. Others are small organic molecules.
denatureTo lose the folded shape an enzyme needs to work. High heat or an extreme pH can do it. Scientists say denatured and not killed, because an enzyme was never alive.
enzymeA cell's own catalyst, nearly always a protein. The word was first used in 1877 and comes from the Greek for in yeast. A cell holds thousands of kinds of enzyme, each for its own reaction.
enzyme-substrate complexAn enzyme and its substrate while they are joined. It forms when the substrate binds. The products are then released, and the enzyme is free to bind again.
Show all 36 terms (30 more)
How enzymes work, continued
induced fitThe reshaping of an enzyme as its substrate binds. The idea dates from 1958. An enzyme is flexible, so the substrate it meets molds the active site around itself.
inhibitorA molecule that slows or stops an enzyme. Some inhibitors let go again and some bind for good. Many medicines are enzyme inhibitors.
lock-and-key modelThe idea that a substrate fits its enzyme exactly. It was proposed in 1894 to explain why each enzyme acts on so few substances. The enzyme is the lock and the substrate is the key.
productThe new substance an enzyme's reaction turns out. Catalase turns hydrogen peroxide into two products, water and oxygen.
specificityThe way each enzyme acts on only certain substrates. Each kind of enzyme usually speeds up only one kind of reaction. The active site's shape decides which molecules fit.
substrateThe starting molecule that an enzyme changes. Starch is the substrate of amylase, and hydrogen peroxide is the substrate of catalase.
Speed, conditions and control 12
allosteric siteA second site where a regulating molecule binds. It is also called a regulatory site. A molecule that binds there changes the enzyme's shape, and with it how active the enzyme is.
catalystSomething that hurries a reaction and is never spent. Because it is not used up, a very small amount of a catalyst is often enough.
competitive inhibitorA look-alike that occupies the active site. It resembles the normal substrate. Adding more substrate can overcome it.
feedback inhibitionThe slowing of a pathway by its own end product. The end product often blocks one of the first enzymes of its pathway, so a cell stops making what it already has enough of.
metabolic pathwayA chain of reactions, each with its own enzyme. The product of one enzyme is usually the substrate of the next.
noncompetitive inhibitorA blocker that leaves the active site unoccupied. It binds just as well whether or not the substrate is already in place.
optimum pHThe acidity at which an enzyme works fastest. It depends on where the enzyme works: about 2 for an enzyme of the stomach and about 7.5 for one of the small intestine.
optimum temperatureThe temperature at which an enzyme works fastest. For human enzymes it is usually between 35 and 40 degrees Celsius, close to the body's 37.
proteinThe kind of big molecule that almost every enzyme is. The order in which its amino acids are strung decides the folded shape, and the shape decides the job.
reaction rateHow fast substrate is used up or product appears. It is found by measuring how much product forms, or how much substrate is used, and dividing by the time taken.
saturationThe state when every active site is busy. Past this point more substrate does not raise the rate, because no enzyme molecule is free to take it.
zymogenAn inactive form from which an enzyme is made. It is also called a proenzyme. The stomach enzyme pepsin is released as the zymogen pepsinogen and is switched on by stomach acid.
Enzymes by name 12
amylaseThe starch-splitting enzyme found in saliva. Rice or potato chewed for a while can start to taste sweet, because amylase is turning some of the starch into sugar.
-aseThe ending shared by the names of most enzymes. It is usually added to the name of the substrate: lactase splits lactose. The ending comes from diastase, the name of the first enzyme discovered.
catalaseThe enzyme that splits hydrogen peroxide. It is found in nearly every living thing that is exposed to oxygen. One catalase molecule can deal with millions of hydrogen peroxide molecules a second.
cellulaseAny enzyme that breaks down cellulose. Fungi, bacteria and protozoans are its main makers. Cattle and sheep rely on bacteria in the gut to make it for them.
coenzymeA small organic helper, often made from a vitamin. A coenzyme ferries a chemical group from enzyme to enzyme.
enzymologyThe study of enzymes. A turning point came in 1897, when juice pressed from yeast was shown to ferment sugar with no living cell present.
lactaseThe enzyme that digests lactose. It breaks lactose into glucose and galactose, two simple sugars. Lactase is found in the small intestine of mammals.
lipaseAny enzyme that takes fats apart. It uses water to split fatty acids from a fat molecule. A lipase made by the pancreas helps digest the fat in food.
luciferaseAn enzyme whose reaction gives off light. It makes fireflies glow. The name comes from the Latin lucifer, light-bearer.
proteaseAny enzyme that cuts proteins into smaller pieces. It is also called a peptidase. Proteases are found in every form of life.
rennetThe mix of enzymes used to curdle milk for cheese. Its key enzyme, chymosin, is a protease that curdles the casein in milk. Cheese makers in North America and Europe now mostly use chymosin made by microbes.
rubiscoThe carbon-fixing enzyme of green plants. It takes carbon dioxide from the air. No enzyme on Earth is more plentiful.
How this list was compiled: Term list chosen for this site on 2026-10-12. Each meaning is written in our own words and was checked that day against the article on the term in the English Wikipedia (https://en.wikipedia.org/), whose text is under the Creative Commons Attribution-ShareAlike license: Enzyme, Catalysis, Substrate (chemistry), Active site, Product (chemistry), Activation energy, Reaction rate, Enzyme inhibitor, Competitive inhibition, Non-competitive inhibition, Allosteric regulation, Cofactor (biochemistry), Protein, Denaturation (biochemistry), Enzyme kinetics, Enzyme assay, Metabolic pathway, Zymogen, Pepsin, Amylase, Protease, Lipase, Lactase, Catalase, Cellulase, RuBisCO, Luciferase, Firefly luciferase, Rennet and -ase. Enzyme, substrate, product, active site, the lock and key, denaturing, the optimum temperature and pH, the rate of an enzyme reaction and saturation were checked a second time against BBC Bitesize (https://www.bbc.co.uk/bitesize/guides/z88hcj6/revision/1 and its next two pages, Enzymes; https://www.bbc.co.uk/bitesize/guides/zhdw7nb/revision/1, Properties of enzymes and use in industries), and enzyme against the Talking Glossary of the National Human Genome Research Institute (https://www.genome.gov/genetics-glossary/Enzyme). The fact after each meaning is from the same pages. Licence: own work
How it works
This enzyme vocabulary worksheet lists enzyme, substrate, active site, product, activation energy, denature, inhibitor and five more terms down the left, with their meanings in another order down the right. The answer key prints as the last page.
Read the full guide
Speed, conditions and control
Set opens on How enzymes work. The second set has catalyst, protein, reaction rate, saturation, optimum temperature, optimum pH, competitive inhibitor, noncompetitive inhibitor, allosteric site, feedback inhibition, metabolic pathway and zymogen.
Enzymes by name
The third set has amylase, protease, lipase, lactase, catalase, cellulase, rubisco, luciferase, rennet, coenzyme, enzymology and the ending -ase.
Catalyst is in the second set, away from enzyme: an enzyme is a catalyst, so one meaning would fit both.
Questions and answers
What is the difference between an enzyme and a catalyst?
A catalyst is any substance that speeds up a reaction without being used up. An enzyme is a catalyst made by a living cell. It is nearly always a protein, and it acts on only a few substrates.
Why do enzymes stop working at high temperatures?
Heat changes the shape of the active site, so the substrate no longer fits it. The enzyme is then denatured, and the reaction slows or stops.
Are all enzymes proteins?
Nearly all. A few of the catalysts in a cell are made of RNA and not of protein. They are called ribozymes, and they are sometimes counted as enzymes.
Do enzymes get used up in a reaction?
No. An enzyme comes out of its reaction unchanged and is used over and over, so a cell needs only a little of each one.