Topic 5Enzymes

Enzymes: a perfect fit

Almost every reaction in a cell would be far too slow at body temperature on its own. Enzymes speed them up — and each one works on just one kind of molecule, because of its shape.

1

Understand

What an enzyme is, and why shape matters

Biological catalysts

An enzyme is a protein that acts as a biological catalyst: it speeds up a chemical reaction in a living thing and is not changed by the reaction.

a catalyst is a substance that increases the rate of a reaction and is not used up
the molecule an enzyme works on is its substrate; what it turns into are the products
because the enzyme is unchanged, one molecule of it can be used again and again — cells need only small amounts
without enzymes, reactions such as digestion and respiration would be too slow to keep you alive

substrate + enzyme → enzyme–substrate complex → products + enzyme

The active site: lock and key

Part of the enzyme, the active site, has a shape that is complementary to its substrate, like a lock and its key.

the substrate fits into the active site and forms an enzyme–substrate complex
the reaction happens, the products leave, and the active site is free again
a different molecule does not fit, so it is not changed: each enzyme is specific to one substrate

amylase: starch → maltose
protease: protein → amino acids
lipase: fat → fatty acids + glycerol

2

Experiment

Offer each enzyme a substrate and see which ones fit

Lock and key

enzymesubstrateproducts

Controls

Enzyme

Substrate

Results

What just happened

3

Interpret

From shape to specificity

Exam tip. Write that the active site and the substrate have complementary shapes — not the “same” shape. Name the enzyme–substrate complex, and say the enzyme is unchanged at the end.

Topic 5Enzymes

Temperature: faster, then denatured

Warming an enzyme speeds it up — until the active site starts to lose its shape. Past that point the rate falls fast, and cooling it down again does not bring it back.

1

Understand

Two effects of heating, pulling in opposite directions

Up to the optimum: more collisions

As the temperature rises, enzyme and substrate molecules gain kinetic energy, so they move faster and collide more often.

more collisions → more enzyme–substrate complexes form each second → a faster rate
the rate is highest at the optimum temperature — about 37 °C for human enzymes
when cold, enzymes are inactive but not damaged: warm them up and they work again

higher T → more kinetic energy → more collisions → more complexes → faster

Past the optimum: denaturing Supp.

Above the optimum the enzyme vibrates so much that the bonds holding its shape break. The active site changes shape.

the substrate is no longer complementary, so it cannot fit and fewer complexes form
the rate falls steeply; when no active sites fit, the enzyme is denatured
denaturing is permanent: cooling does not restore the shape
enzymes are molecules, not living things — say denatured, never “killed”

too hot → active site changes shape → substrate does not fit → denatured

2

Experiment

Heat it, cool it, and watch the active site

Amylase at different temperatures

fresh enzymeafter heatingdenaturing

Controls

Temperature

37 °C

Results

What just happened

3

Interpret

Reading the temperature curve

Exam tip. Explain both halves of the curve separately: below the optimum use kinetic energy and collisions; above it use active site, shape and denatured. Read the optimum from the peak of the graph.

Topic 5Enzymes

pH: each enzyme has its own optimum

The stomach is acidic, the small intestine slightly alkaline. Each digestive enzyme has the shape that works best where it is found, and the wrong pH changes that shape.

1

Understand

Optimum pH, and what goes wrong either side

Optimum pH matches the place

Every enzyme works fastest at its optimum pH, and that is usually the pH of the place where it works.

protease in the stomach (pepsin): optimum about pH 2 — the stomach makes hydrochloric acid
amylase in saliva: optimum about pH 7 — neutral
lipase and protease in the small intestine: optimum about pH 8 — bile and pancreatic juice are alkaline and neutralise the stomach acid

stomach pH 2 · mouth pH 7 · small intestine pH 8

Away from the optimum Supp.

A pH that is too high or too low changes the shape of the active site, in the same way that too much heat does.

a small change: some active sites change shape, so the rate drops
a large change: the substrate no longer fits at all — the enzyme is denatured
the curve is bell-shaped, with the rate falling on both sides of the optimum

wrong pH → active site changes shape → substrate does not fit → slower, then denatured

2

Experiment

Pick an enzyme and change the pH

Three digestive enzymes and pH

chosen enzymethe other twopH now

Controls

Enzyme

pH

pH 7.0

Results

What just happened

3

Interpret

Matching enzymes to places

Exam tip. If a graph shows two enzymes, give each optimum as a number read from the peak, then link it to where the enzyme works. Explain the fall on either side with the shape of the active site, just as for temperature.