Overview
Most chemical reactions in the body would happen far too slowly to sustain life on their own. Enzymes are biological catalysts, almost always proteins, that dramatically speed up reactions without being used up. A single enzyme molecule can process thousands of reactions per second, and each enzyme is highly specific, acting on only one kind of molecule or a small group of related molecules. Enzyme names often end in “-ase” and hint at their job, such as lactase (breaks down lactose) and amylase (breaks down starch, or amylose).
Activation Energy
Every reaction faces an energy barrier called the activation energy, the initial push needed to break existing bonds before new ones form. Enzymes work by lowering this barrier. Importantly, enzymes do not change whether a reaction is energetically favorable or how much energy it ultimately releases; they only make the reaction reach that outcome much faster. Because the enzyme emerges unchanged, it can be reused over and over.
The Active Site: Lock-and-Key and Induced Fit
The reactant an enzyme acts upon is its substrate. The substrate binds to a specific pocket on the enzyme called the active site. Two models describe this binding:
- Lock-and-key — the early model, in which the substrate fits the active site precisely, like a key in a lock.
- Induced fit — the modern refinement, in which the active site adjusts its shape slightly as the substrate enters, tightening its grip and straining the substrate’s bonds to help the reaction along.
Once the reaction occurs, the enzyme releases its product(s) and is free to bind another substrate.
Factors That Affect Enzyme Activity
Enzyme activity is sensitive to its environment. Each enzyme has an optimum at which it works fastest.
| Factor | Effect |
|---|---|
| Temperature | Activity rises with heat up to an optimum (about 37 degrees C in humans), then drops sharply as the enzyme denatures |
| pH | Each enzyme has an optimal pH; most body enzymes prefer near-neutral, but stomach pepsin works best around pH 2 |
| Substrate concentration | More substrate speeds the reaction until all active sites are busy (saturation), after which the rate levels off |
| Enzyme concentration | With plenty of substrate, more enzyme means a faster overall rate |
When conditions move too far from the optimum, the protein’s shape distorts. Extreme heat or pH causes denaturation, unfolding the active site so the enzyme can no longer bind its substrate.
Cofactors and Coenzymes
Many enzymes cannot work alone and need a non-protein helper.
- A cofactor is often an inorganic ion such as zinc, magnesium, or iron. For example, iron sits in enzymes and oxygen-carrying proteins.
- A coenzyme is an organic cofactor, frequently made from a vitamin. Examples include NAD+ (from niacin, vitamin B3) and FAD (from riboflavin, vitamin B2), both essential carriers in metabolism.
This is one reason vitamins and trace minerals matter in the diet: without them, key enzymes stall.
Enzyme Inhibition
Cells and drugs can slow enzymes using inhibitors.
- A competitive inhibitor resembles the substrate and competes for the active site, blocking access. Adding more substrate can outcompete it.
- A noncompetitive inhibitor binds elsewhere on the enzyme (an allosteric site), changing the active site’s shape so the substrate no longer fits well.
Inhibition is not just a nuisance; it is how the body regulates pathways and how many medicines work.
Real Examples
- Lactase breaks the disaccharide lactose into glucose and galactose in the small intestine. People with low lactase activity cannot digest lactose well, leading to lactose intolerance.
- Amylase, found in saliva and the pancreas, begins breaking starch into smaller sugars. You can taste this: chewing a plain cracker long enough makes it taste sweet as amylase releases glucose.
Clinical Relevance
Enzymes are central to medicine. Many drugs are deliberate enzyme inhibitors: aspirin blocks the enzyme cyclooxygenase to reduce pain and inflammation, and ACE inhibitors lower blood pressure by blocking an enzyme in the kidney’s pressure pathway. Genetic enzyme deficiencies cause inherited diseases, such as phenylketonuria (PKU), where a missing enzyme lets a toxic amino acid build up. Doctors also measure enzymes in blood to diagnose disease; elevated cardiac enzymes can signal a heart attack, and elevated liver enzymes can signal liver damage. Even a dangerously high fever is a threat partly because excessive heat denatures the enzymes the body depends on.