Enzymes (OCR A Level Biology): Flashcards

Amylase – digests starch outside cells, e.g. in the gut.

True

False — enzymes are not used up or permanently changed.

The region of an enzyme with a specific shape to which a specific substrate binds, forming an enzyme-substrate complex.

The temporary structure formed when a substrate binds to the active site of an enzyme.

The temporary structure formed when the product(s) are still bound to the enzyme's active site, just before they are released.

The property whereby an enzyme catalyses only one (or a few) reactions, because the shape of its active site is complementary to only its specific substrate.

The complex tertiary structure of the protein, which is itself determined by the sequence of amino acids.

The enzyme and substrate are rigid, complementary shapes. The substrate fits precisely into the active site like a key into a lock, forming an enzyme-substrate complex.

As the substrate binds, the enzyme's active site (and sometimes the substrate) changes shape slightly – a conformational change – to fit closely around the substrate, maximising catalysis.

It treats the enzyme as flexible rather than a rigid, fixed shape.

The slight change in shape of an enzyme's active site as a substrate binds, as described by the induced-fit hypothesis.

The minimum amount of energy needed for a reaction to occur – the energy needed to make the substrate unstable enough to react.

They provide an alternative reaction pathway with a lower activation energy (e.g. by destabilising bonds in the substrate), rather than changing the overall energy change of the reaction.

By lowering the activation energy, enzymes allow reactions to proceed quickly at the mild conditions inside cells, avoiding extremes that would otherwise kill the cells.

Its tertiary structure (and active-site shape) has changed so that the substrate can no longer bind – e.g. due to extremes of temperature or pH.

Enzymes speed up reactions by providing an alternative pathway with a lower activation energy.

False — enzymes provide an alternative pathway with a lower activation energy; they do not change the overall energy change of the reaction.

True

The pH at which an enzyme catalyses a reaction at its maximum rate.

An excess of H⁺ ions (acidic) or OH⁻ ions (alkaline) breaks the hydrogen and ionic bonds holding the enzyme's tertiary structure. This alters the shape of the active site, so enzyme-substrate complexes form less easily, and eventually the enzyme is denatured.

Hydrogen bonds and ionic bonds.

It functions in the stomach, an acidic environment (due to hydrochloric acid), so it is adapted to work best at pH 2.

Each buffer maintains a specific, constant pH even if the reaction would otherwise change the pH – allowing the rate to be measured accurately at set pH values.

Samples are added to iodine at regular intervals: iodine turns blue-black when starch is still present and stays orange-brown once all the starch is digested. The time taken for the starch to disappear indicates the rate.

pH does not affect the collision rate – it disrupts the substrate's ability to bind the active site. Temperature affects both the kinetic energy/collision rate and (at high temperatures) causes denaturation.

A change in an enzyme's tertiary structure that alters the shape of the active site so that the substrate can no longer bind.

The pH at which an enzyme catalyses a reaction fastest is called its optimum pH.

False — pH does not affect collision rate; it disrupts the substrate's ability to bind to the active site.

The temperature at which an enzyme catalyses a reaction at its maximum rate.

Molecules gain more kinetic energy and move faster, giving a higher frequency of successful collisions between substrate and active site, so more enzyme-substrate complexes form. Collisions also carry more energy, making bond formation/breaking more likely.

Molecules have less kinetic energy and move slowly, giving a lower frequency of successful collisions and fewer enzyme-substrate complexes. Collisions also carry less energy.

Increased kinetic energy and vibration strain and break the hydrogen and ionic bonds holding the tertiary structure. The active site changes shape and is no longer complementary to the substrate, so the enzyme is denatured (irreversibly) and the rate drops sharply.

Enzymes are protein molecules, not living organisms; they are denatured, not killed.

The ratio between the rates of a reaction at two temperatures 10 °C apart. For many enzyme-catalysed reactions the rate doubles per 10 °C rise, giving Q₁₀ = 2.

Q₁₀ = rate of reaction at (x + 10) °C ÷ rate of reaction at x °C.

Q₁₀ = 20 ÷ 10 = 2 (no units, as it is a ratio).

Enzymes (e.g. from bacteria living in thermal springs) that can withstand and function at very high temperatures without denaturing.

For most enzyme-catalysed reactions the rate doubles for every 10 °C rise in temperature, giving a Q₁₀ of 2.

True

False — denaturation is irreversible; the active site is permanently damaged and no longer complementary to the substrate.

More enzyme molecules means more available active sites and a greater likelihood of enzyme-substrate complex formation, so the initial rate increases linearly with enzyme concentration.

If the amount of substrate is limited, substrate concentration becomes the limiting factor, so adding more enzyme has no further effect on the rate.

A factor that is in short supply and so limits the rate of a reaction (e.g. substrate concentration when enzyme is in excess).

More enzyme provides more active sites, so (provided substrate is available) more substrate molecules can bind at any one time.

A linear (straight-line) increase, because each added enzyme molecule provides more active sites and increases the rate of enzyme-substrate complex formation.

When substrate is limited, increasing the enzyme concentration no longer increases the rate because substrate has become the limiting factor.

True

False — with excess substrate the rate increases linearly with enzyme concentration.

There must be sufficient substrate available, so that substrate does not become a limiting factor.

The structure formed when a substrate binds to the active site of an enzyme.

As substrate concentration increases, more substrate molecules are available, so the likelihood of enzyme-substrate complex formation increases and the rate of reaction rises.

All the enzyme active sites become saturated (occupied), so additional substrate molecules have nowhere to bind and the rate cannot increase any further.

The point at which all available enzyme active sites are occupied by substrate, so the reaction proceeds at its maximum rate.

The maximum rate of an enzyme-catalysed reaction, reached when all active sites are working continuously (saturated with substrate).

An initial linear increase, which then plateaus as the active sites become saturated.

At high substrate concentrations the rate of reaction plateaus because the enzyme active sites have become saturated.

False — once all active sites are saturated, adding more substrate does not increase the rate.

True

The structure formed when a substrate molecule binds to the active site of an enzyme.

They have nowhere to bind, so they cannot form an enzyme-substrate complex and effectively 'queue up' for an active site to become available.

A substance that reduces or stops the activity of an enzyme.

An inhibitor with a similar shape to the substrate that competes with it for the active site, blocking the substrate from binding.

An inhibitor that binds to the enzyme at an alternative site, altering the active site's shape so the substrate can no longer bind.

By increasing the substrate concentration, so substrate molecules are more likely to occupy the active sites than the inhibitor. The maximum rate can still be reached.

The inhibitor changes the shape of the active site, so enzyme-substrate complexes cannot form no matter how much substrate is present.

Reversible inhibitors bind temporarily, so enzyme activity can be restored.

Non-reversible inhibitors form covalent bonds with the enzyme, inhibiting it permanently.

A competitive inhibitor lowers the initial rate, but the same maximum rate can eventually be reached.

A non-competitive inhibitor lowers both the initial rate and the maximum rate.

A control mechanism in which the end product of a metabolic pathway acts as a non-competitive, reversible inhibitor of an enzyme earlier in the pathway.

As the end product accumulates, it binds non-competitively and reversibly to an earlier enzyme, changing its active site and slowing production. As the end product is used up and detaches, the active site reforms and the enzyme becomes active again – a negative feedback loop.

Only by producing more of the enzyme, via transcription and translation of its gene(s) – a relatively slow process.

A competitive inhibitor has a similar shape to the substrate and competes with it for the active site.

False — the active site stays altered, so enzyme-substrate complexes still cannot form.

True

A non-protein substance that an enzyme requires in order to function properly.

An organic (carbon-containing) cofactor that helps enzymes by accepting or donating electrons or chemical groups between reactions.

A cofactor that is tightly and permanently bound to an enzyme, forming a permanent part of its structure that is essential for its function.

An inorganic ion that an enzyme needs to function (helping to stabilise its structure or take part in the reaction at the active site).

Example: chloride ions act as a cofactor for amylase.

Some enzymes are inactive until they combine with the non-protein cofactor, which changes their tertiary structure so the active site can bind the substrate correctly.

Vitamins are an important source of coenzymes.

A coenzyme is an organic (carbon-containing) cofactor that donates or accepts chemical groups or electrons.

An inorganic cofactor is an inorganic ion that stabilises the enzyme or takes part in the reaction.

A cofactor that is tightly and permanently bound to an enzyme is called a prosthetic group.

Many of the B-group vitamins are used to make important coenzymes. (You do not need to recall the specific names.)

False — cofactors can be inorganic ions (e.g. chloride ions) or organic molecules (coenzymes).

False — a prosthetic group is tightly and permanently bound to the enzyme.

Coenzymes carry electrons or chemical groups between enzymes, linking reactions into sequences during processes such as respiration and photosynthesis.

• Measuring the rate of formation of a product

• Measuring the rate of disappearance of a substrate

Catalase breaks down hydrogen peroxide into water and oxygen. The volume of oxygen produced in a set time is measured and used to calculate the rate of reaction.

Amylase hydrolyses starch to maltose. Samples are tested with iodine at intervals – blue-black shows starch remains, yellow-brown shows it has been digested – and the time taken for starch to disappear is measured.

To measure the light absorbance or transmission of a solution. As a coloured substrate is broken down, transmission increases (absorbance decreases), allowing the reaction rate to be followed.

A stepwise dilution of a stock solution to produce a range of known, decreasing concentrations.

It sets a reference point (e.g. 100% transmission) so that absorbance or transmission readings can be compared accurately between samples.

Starch concentration (x-axis) against percentage absorbance or percentage transmission (y-axis).

A stepwise dilution used to make a range of known concentrations from a stock solution is called a serial dilution.

True

False — a colorimeter measures light absorbance or transmission, not mass.