Macromolecules, protein structure and enzymes: active sites, induced fit, inhibition

An enzyme is a protein that speeds up one reaction by holding its substrate in an active site shaped for it, lowering the energy the reaction needs to get started, and coming out unchanged. Its shape is the whole story: the four levels of protein structure build the active site, heat or the wrong pH unfold it, and an inhibitor either sits in it (competitive) or bends it from elsewhere (non-competitive). Below, the four macromolecules and the bond each is built by, the four levels, one catalytic cycle traced step by step, and the rate graph that tells the two kinds of inhibition apart.

The four macromolecules and the one bond each is built by

Living things are built from four classes of large molecule. Three of them are true polymers, long chains of repeating units called monomers; lipids are the exception, and the table says so.

Class	Monomer or building block	Bond	Example	What it does
Carbohydrates	Monosaccharides (glucose)	Glycosidic	Starch, glycogen, cellulose	Short-term energy, structure
Lipids (not true polymers)	Glycerol and fatty acids	Ester	Fats, oils, phospholipids	Long-term energy, membranes
Proteins	Amino acids	Peptide	Enzymes, haemoglobin, keratin	Catalysis, transport, structure
Nucleic acids	Nucleotides	Phosphodiester	DNA, RNA	Information

One piece of chemistry builds all four. Dehydration synthesis joins two units and removes one water molecule: the hydrogen of one unit and the hydroxyl of the other leave together as H₂O and a covalent bond forms where they were. Hydrolysis is the reverse: one water is added and the bond is split. Digestion is hydrolysis; growth is dehydration synthesis. Both are run by enzymes, which is why this page's first table and its last section belong together.

The count that comes with it: joining n monomers in a chain makes n − 1 bonds and releases n − 1 waters. A polypeptide of 150 amino acids has 150 − 1 = 149 peptide bonds and released 149 water molecules as it was built. Two amino acids make one bond; three make two. The first unit has nothing before it to bond to, which is where the "minus one" comes from.

Protein structure: the four levels of protein structure, and which bonds hold each

A protein's function comes from its shape, and its shape is described at four levels.

Primary. The sequence of amino acids, joined by peptide bonds. There are 20 kinds of amino acid, told apart by their R groups (side chains), and the order is set by the gene.

Secondary. Local folding of the backbone into α-helices and β-sheets, held by hydrogen bonds between the backbone's C=O and N–H groups. The R groups are not involved yet.

Tertiary. The overall three-dimensional fold of one chain, held by interactions between the R groups: hydrogen bonds, ionic bonds between charged side chains, hydrophobic side chains clustering away from water, and disulfide bridges between two cysteines. Hydrogen bonding and polarity is the chemistry under these folds.

Quaternary. Some proteins are more than one chain. Haemoglobin is four, and the arrangement of the chains is the quaternary level. A protein of one chain has no quaternary structure.

Why this matters for everything below: an enzyme's active site is a tertiary-structure feature, a pocket whose shape comes from how the R groups hold the fold. Anything that breaks R-group interactions, heat or the wrong pH, changes the pocket, and a changed pocket no longer fits its substrate.

How an enzyme works: one cycle traced, with induced fit

Take one named example and follow one cycle. Lactase splits lactose (a disaccharide) into glucose and galactose, with one water added: a hydrolysis.

1. Approach. A lactose molecule diffuses toward the active site, the pocket in lactase whose shape and charges match lactose and little else. This is why an enzyme is specific.
2. Binding and induced fit. Weak bonds form between the substrate and the site, and the site closes slightly around the substrate, adjusting its shape to grip it. That adjustment is induced fit. The older lock-and-key model is the rigid version of the same idea: a site that already fits perfectly and does not move.
3. Catalysis. The enzyme strains the bond to be broken, holds the substrate and the water in the right orientation, and offers a micro-environment (its own acidic and basic side chains) that helps the reaction along. All of this lowers the activation energy, the energy barrier the reaction must cross to get started.
4. The transition state passes. The bond breaks, the water is added, and the products, glucose and galactose, form inside the site.
5. Release. The products no longer fit the site well, they leave, the site relaxes to its resting shape, and the enzyme is unchanged and ready for the next lactose.

Two sentences lose marks here, so write them the right way round. First, an enzyme lowers the activation energy; it does not change the overall energy released or absorbed by the reaction. The reaction gives out (or takes in) the same energy with or without the enzyme; the enzyme only changes how fast the barrier is crossed. Second, an enzyme speeds a reaction toward its equilibrium; it does not move the equilibrium. More product is not made in the end, it is made sooner.

The rate graph: substrate concentration, temperature, pH

Three graphs, each described in words so you can draw it.

Rate against substrate concentration. The curve rises steeply at first, then bends and flattens to a ceiling. At low substrate, active sites are often empty and adding substrate adds rate. At high substrate every active site is busy and the enzyme is working as fast as it can: this ceiling is V_max, and the enzyme is saturated. The substrate concentration at which the rate is half of V_max is a useful mark on the graph: it says how much substrate it takes to run the enzyme at half speed, which is a measure of how tightly the enzyme binds its substrate.

Rate against temperature. The rate rises with temperature, because molecules collide more often and harder, up to an optimum. Past it the rate falls fast, because the heat breaks the weak bonds of the tertiary structure and the protein unfolds. That is denaturation, and at this level it is usually not reversible.

Rate against pH. A peak at the enzyme's pH optimum and a fall on either side, because the charges on the R groups in and around the active site depend on pH, and a changed charge changes the fold. A stomach enzyme and a small-intestine enzyme have different optima.

Enzyme inhibition: competitive vs noncompetitive inhibition, told apart on the graph

An inhibitor is a molecule that binds an intact enzyme and slows it. There are two kinds the course asks for, and one question tells them apart.

Competitive inhibition. The inhibitor resembles the substrate and sits in the active site, so substrate molecules cannot bind while it is there. It competes. Add more substrate and the substrate wins more often, so the enzyme can still reach V_max eventually. On the graph, the ceiling is the same height but the curve is pushed to the right: more substrate is needed to reach half speed.

Non-competitive inhibition. The inhibitor binds somewhere other than the active site, an allosteric site, and the binding changes the enzyme's shape so the active site no longer works well. No amount of substrate can fix a bent site, so V_max falls. On the graph, the ceiling is lower.

The swap students make is saying that competitive inhibition lowers V_max. It does not; it raises the amount of substrate needed to get there. The one-question test: would adding more substrate restore the full rate? Yes means competitive. No means non-competitive.

Feedback regulation: the cell's own inhibitors

Cells use inhibition on purpose. In a metabolic pathway of several enzymes, the end product often binds the first enzyme at an allosteric site and slows it. When enough product has built up, the pathway slows; as the product is used up, the inhibition lifts and the pathway restarts. This is feedback inhibition, it is reversible by design, and it is a negative feedback loop at the molecular scale, the same logic that homeostasis uses for body temperature and blood glucose at the scale of the whole body.

Denatured is not inhibited

The third swap. An inhibitor is a molecule that binds an enzyme, often reversibly, and the enzyme's own structure is intact. Denaturation is the protein's own shape lost to heat or pH, no second molecule involved, and usually not reversible at this level. On a test, "the enzyme was denatured by the inhibitor" is a wrong sentence: an inhibitor inhibits, heat and pH denature. Keep the two words for the two causes.

Check: redraw it cold, then one sibling

With the page closed: write the four-row macromolecule table from memory; list the four levels of protein structure with the bonds that hold each; sketch three rate-against-substrate curves on one set of axes (no inhibitor, competitive, non-competitive) and label V_max on each; write the lactase cycle in five lines. Open the page and mark it. Karpicke and Blunt (2011) found that recalling and checking produced more learning than re-studying, and how to study biology has the vocabulary-first routine for a unit like this one. A card per row of the table and one per level keeps the facts coming back on schedule.

Then a sibling, cold. A polypeptide has 300 amino acids. How many peptide bonds does it have, and how many water molecules were released as it was built? And: an enzyme's rate is measured with and without a certain inhibitor, and with the inhibitor present the curve reaches the same V_max once the substrate concentration is high enough. Which kind of inhibitor is it?

Answer. 300 − 1 = 299 peptide bonds, and 299 water molecules, one per bond. The inhibitor is competitive: more substrate restored the full rate, so it was being outcompeted at the active site. A non-competitive inhibitor would have lowered the ceiling whatever the substrate concentration.

Where this sits in the course

These are ideas 1, 3, 4 and 5 of the 27 in the SBI4U course map: The four classes of biological macromolecule, Protein structure: primary to quaternary, Enzymes, active sites and the induced-fit model, and Enzyme inhibition and feedback regulation, the opening of strand B. Between the first and the rest sits idea 2, Water, hydrogen bonding and the chemistry of life. After idea 5 come idea 6, Membrane structure and transport across membranes, and idea 7, ATP and the energy currency of the cell, which opens strand C. That strand then spends the whole of metabolism on enzymes doing exactly what this page traces: every arrow in cellular respiration is one.

Enzymes and protein structure on one screen

Class	Monomer	Bond	Example
Carbohydrates	Monosaccharides	Glycosidic	Starch
Lipids (not true polymers)	Glycerol, fatty acids	Ester	Fats
Proteins	Amino acids	Peptide	Enzymes
Nucleic acids	Nucleotides	Phosphodiester	DNA

• Dehydration synthesis joins and releases one water per bond; hydrolysis splits by adding one. n monomers, n − 1 bonds.
• Four levels: primary (sequence, peptide bonds); secondary (helices and sheets, backbone hydrogen bonds); tertiary (the fold, R-group interactions); quaternary (more than one chain).
• The cycle: approach, bind with induced fit, lower the activation energy, products form, release unchanged. Activation energy down; overall energy and equilibrium unchanged.
• The one-question test: does more substrate restore the full rate? Yes, competitive; no, non-competitive.
• The three swaps: competitive does not lower V_max; denatured is not inhibited; an enzyme does not change how much energy a reaction releases.