Enzymes are proteins that speed up chemical reactions in living organisms. They lower the energy barrier needed for reactions to proceed, which makes the countless processes in your cells possible. Without enzymes, reactions that take milliseconds would take years.
This section covers how enzymes work at the structural level, what activates and inhibits them, and what happens when they lose their shape.
Enzyme Structure and Function
Enzymes as Biological Catalysts
A catalyst is any substance that speeds up a chemical reaction without being used up in the process. Enzymes are biological catalysts, and they're almost always proteins.
- They accelerate reactions by lowering the activation energy (more on this below)
- Because they aren't consumed, a single enzyme molecule can catalyze the same reaction over and over
- Each enzyme is highly specific to its substrate (the molecule it acts on), thanks to the unique shape of its active site
Active Sites and Substrates
The active site is a small pocket or groove on the enzyme's surface where the substrate binds and the reaction occurs. Its shape and chemical properties are complementary to the substrate, so only the right molecule fits.
This is often described using the lock and key model: the substrate (key) fits precisely into the active site (lock). A more accurate description is the induced fit model, where the active site slightly changes shape as the substrate binds, creating a tighter fit that helps catalyze the reaction.
When the substrate binds, it forms an enzyme-substrate complex. The enzyme then converts the substrate into one or more products, releases them, and is free to bind another substrate molecule.
Catalytic Efficiency
Enzymes are remarkably efficient. Some can increase reaction rates by a factor of compared to the same reaction without a catalyst.
This efficiency comes from two things:
- The active site holds substrates in the precise orientation needed for the reaction
- The enzyme stabilizes the transition state (the unstable intermediate between reactants and products), making it easier to reach
Unlike industrial catalysts that often require extreme heat or pressure, enzymes work under the mild conditions found inside cells: body temperature around 37°C and near-neutral pH.
Enzyme Activation and Inhibition
Activation Energy and Enzyme Catalysis
Activation energy is the minimum amount of energy reactant molecules need to start a reaction. Think of it as the energy "hill" molecules must get over before they can be converted into products.
Enzymes lower this hill. They don't change what products form or whether the reaction is energetically favorable. They just provide an alternative reaction pathway with a lower energy barrier. With a lower barrier, more molecules at any given moment have enough energy to react, so the overall reaction rate increases.
Cofactors and Coenzymes
Some enzymes can't function on their own. They need helper molecules called cofactors.
- Inorganic cofactors are metal ions such as , , or . These often help stabilize the enzyme's shape or participate directly in the chemical reaction.
- Coenzymes are organic cofactors, often derived from vitamins. Examples include (from niacin/vitamin B3), (from riboflavin/vitamin B2), and coenzyme A (from pantothenic acid/vitamin B5).
Both types assist the enzyme by either participating in the reaction itself or helping maintain the enzyme's proper three-dimensional structure.
Enzyme Inhibition
Enzyme inhibitors are molecules that reduce or block enzyme activity. There are two main types you need to know:
- Competitive inhibitors resemble the substrate and bind directly to the active site, physically blocking the real substrate from entering. This type of inhibition is reversible because the inhibitor and substrate compete for the same spot. Adding more substrate can overcome it.
- Non-competitive inhibitors bind to a different location on the enzyme called an allosteric site. This binding changes the enzyme's overall shape (conformation), distorting the active site so the substrate can no longer fit properly. Non-competitive inhibition can be reversible or irreversible, and adding more substrate won't overcome it since the inhibitor isn't competing for the active site.
Why this matters: Inhibitors aren't just a lab concept. Your body uses them to regulate metabolic pathways, and many drugs work as enzyme inhibitors. ACE inhibitors treat high blood pressure by blocking an enzyme in the pathway that constricts blood vessels. Protease inhibitors treat HIV by blocking an enzyme the virus needs to replicate.
Enzyme Denaturation
Factors Affecting Enzyme Structure and Function
Enzymes depend on their three-dimensional shape to function. Denaturation is the process where that shape is disrupted, causing the enzyme to unfold and lose its activity.
When an enzyme denatures, its tertiary structure (and quaternary structure, if it has one) breaks down. The active site loses its specific shape, so the substrate can no longer bind properly.
Three main factors cause denaturation:
- Extreme temperatures — High heat increases molecular vibrations enough to break the weak bonds (hydrogen bonds, ionic interactions) holding the enzyme's shape together. This is why cooking an egg turns the clear, liquid egg white opaque and solid: heat denatures and coagulates the proteins.
- Extreme pH changes — Very acidic or very basic conditions alter the charges on amino acid side chains, disrupting ionic bonds and hydrogen bonds within the protein.
- Strong chemicals — Substances like urea or detergents interfere with the interactions that maintain protein folding.
Each enzyme has an optimal temperature and optimal pH where it works best. For most human enzymes, that's around 37°C and pH 7.4. Pepsin in your stomach is a notable exception, working best at a very acidic pH of about 2.
Consequences of Enzyme Denaturation
Once an enzyme denatures, its active site no longer has the right shape to catalyze reactions. In most cases, denaturation is irreversible: the unfolded protein may clump together (aggregate) or refold incorrectly.
Some enzymes can renature (refold into their functional shape) if returned to optimal conditions, but this depends on how severe the denaturation was and how long it lasted. In living cells, irreversibly denatured enzymes are typically tagged for degradation and replaced by newly synthesized copies.