EnzymeA biological catalyst, almost always a protein. It speeds up one reaction and comes out unchanged, ready to be used again.
Active siteThe place on the enzyme where the substrate binds. The reaction happens there. Its shape fits the substrate.
SubstrateThe molecule the enzyme acts on: the reactant of the reaction. It is drawn in two halves, which this enzyme splits apart.
Enzyme–substrate complexThe enzyme with the substrate bound in its active site. Binding changes the shape of both a little so that they fit better, which is called induced fit.
ProductsWhat the substrate has become. The products leave the active site, and the enzyme is free to take another substrate.
Inhibitors 3
Competitive inhibitorA molecule shaped like the substrate. It takes the substrate's place in the active site. The enzyme cannot act on it. Adding more substrate can overcome it.
Noncompetitive inhibitorA molecule that binds the enzyme away from the active site. It changes the enzyme's shape so that it works poorly, however much substrate there is.
Allosteric siteA pocket on the enzyme that is not the active site. A molecule that binds here changes the enzyme's shape, and with it how well the enzyme works.
Activation energy 4
ReactantsThe starting level: the energy of the substrate before it reacts. It is the same with or without the enzyme.
Activation energy with the enzymeWith the enzyme the hill is lower. So far more molecules get over it and the reaction runs much faster.
Activation energy without the enzymeThe energy reactants need to get over the hill with no enzyme.
Transition stateThe top of the hill: the highest-energy point between reactants and products. An enzyme works by stabilizing it, which lowers the hill.
How this list was compiled: Every name was checked on 2026-09-30 in two places, for facts only and with nothing copied. (1) University of Wisconsin-Madison, Department of Chemistry, Biomolecules: Enzymes (a seven-page tutorial), https://www2.chem.wisc.edu/deptfiles/genchem/netorial/modules/biomolecules/modules/enzymes/enzyme3.htm. (2) Wikipedia, text only: Enzyme, Active site, Non-competitive inhibition, Allosteric regulation and Activation energy, for example https://en.wikipedia.org/wiki/Enzyme. (3) Nature Education, Scitable: Protein Function, https://www.nature.com/scitable/topicpage/protein-function-14123348/, and the NHGRI Talking Glossary entry Enzyme, https://www.genome.gov/genetics-glossary/Enzyme. (4) For the allosteric site, a StatPearls abstract read through Europe PMC, https://europepmc.org/article/MED/31424826. Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. Licence: own work
How it works
Label the enzyme in three views: the enzyme turning a substrate into products, a competitive and a noncompetitive inhibitor, and the activation energy graph. Tap a name to read about it, or press Quiz me.
Read the full guide
The enzyme at work
Read the first view from left to right. The substrate fits the active site, the two form the enzyme-substrate complex, and the products leave. The enzyme comes out unchanged and is used again.
Inhibitors and energy, for an enzyme labeling worksheet
A competitive inhibitor sits in the active site. A noncompetitive inhibitor binds at the allosteric site and changes the enzyme's shape. On the graph the enzyme lowers the activation energy, while the reactants and the products stay where they were.
Adding more substrate overcomes a competitive inhibitor. It does not overcome a noncompetitive one.
ES complex is accepted for enzyme-substrate complex, and non-competitive with a hyphen.
Questions and answers
What are the parts of an enzyme diagram?
The enzyme, its active site, the substrate, the enzyme-substrate complex and the products. Many diagrams add a competitive inhibitor, a noncompetitive inhibitor and the allosteric site.
What is the active site of an enzyme?
The place on the enzyme where the substrate binds and the reaction takes place. Its shape fits the substrate the way a lock fits its key.
How does an enzyme affect activation energy?
It lowers it, so the reaction goes thousands or even millions of times faster. The enzyme does not change the energy of the reactants or of the products.
What is the difference between a competitive and a noncompetitive inhibitor?
A competitive inhibitor resembles the substrate and takes its place in the active site. A noncompetitive inhibitor binds somewhere else and changes the enzyme's shape, so the enzyme works poorly even with substrate bound.