OCR A Level Biology: Enzymes

What is metabolism?

The sum of all chemical reactions within cells and organisms

What is an anabolic reaction?

One which builds a molecule or molecules

What is a catabolic reaction?

One which breaks down a molecule or molecules

Why do organisms need enzymes in their bodies?

To act as catalysts for their metabolic reactions

What are some of the benefits that enzymes provide to organisms?

Mean the organism requires less energy to function, meaning it needs to consume less food and could maintain a lower body temperature. Would also save time.

Where do intracellular enzymes act?

Inside cells

Where do extracellular enzymes act?

Outside of cells

What is the structure of catalyse?

A conjugated globular protein with 4 haem groups in its quaternary structure.

Is catalyse intracellular or extracellular?

Intracellular

What is the function of catalase?

It breaks down hydrogen peroxide into water and oxygen

What is the function of amylase?

Breaks down starch into maltose

Where is amylase present, and is it intra or extracellular?

Found in the mouth and small intestine, extracellular

What is the function of trypsin?

It is a protease, and therefore breaks down proteins into peptides.

Where does trypsin work and is it intra or extracellular?

Works in the small intestine, extracellular

What is the lock and key hypothesis?

The substrate randomly moves into the enzyme's active site, which it is an exactly complementary fit for, forming an enzyme-substrate complex. The substrate is placed under stress through interaction with R-groups from the active site, breaking it/joining 2 substrates together

What determines the shape of an enzyme's active site?

Its tertiary structure

What sort of proteins are enzymes?

Globular proteins

How do enzymes lower the activation energy of anabolic reaction?

The tight fit between the two substrates in the enzyme-substrate complex reduces the repulsion between the two molecules, so they bond more easily

How do enzymes lower the activation energy of catabolic reactions?

The strain placed on the substrate by its fitting in the active site causes it to break down more easily

What is the induced fit theory?

The substrate enters the active site of the enzyme, which is not complementary but a slightly different shape to the substrate. The active site slightly changes shape, putting pressure on the substrate via its R-groups and causing the reaction.

How does a low or high pH denature enzymes?

The H+ or OH- ions affect the hydrogen and ionic bonding of the active site's tertiary structure, causing it to change shape

Can the enzyme return to its original shape after being denatured by pH?

Yes, but only if pH changes slightly and then returns to optimum

What is denaturation?

The loss of biological activitity

What is renaturation?

The regaining of biological activity

How does temperature affect enzyme activity?

At first, increased temperature increases kinetic energy for the enzyme's particles, so its function increases, up to an optimum temperature. After this, the enzymes particles have too much energy which begins to affect the bonds in its tertiary structure, denaturing it until at a high enough temperature it has no function at all

What is the optimum temperature for most enzymes?

Around 40C

What is a Q10 value?

How much the rate of reaction changes if the temperature of a reaction is raised by 10C. I.e. a Q10 value of 2 would mean the rate of reaction doubles.

What is Vmax?

The maximum rate of reaction for a reaction

What is the trend of enzyme activity with varying pH?

The enzyme activity increases up to the enzyme's optimum pH, after which it falls again, eventually reaching 0 at a high enough pH.

What Q10 value do most enzymes have?

2

What is the trend of enzyme activity with increasing substrate concentration?

The enzyme activity steadily rises for a period, after which the increase slows and eventually stops as Vmax is reached and all enzymes have a substrate molecule in their active site

Why do enzymes need regulating?

So that products do not build up too fast and substrates are not used up too quickly

How does a competitive inhibitor work?

It enters the active site of the enzyme, preventing the substrate reaching the active site. No reaction takes place.

What is the structure of a competitive inhibitor?

Similar (but not identical) to the structure of the substrate it stops being catalysed

How long do competitive inhibitors bind to the active site for?

Usually temporarily

How is the Vmax affected by competitive inhibitors?

It stays the same, but takes longer to reach

What are 2 examples of competitive inhibitors?

Aspirin and Statins

What is an example of a permanent competitive inhibitor?

Aspirin, which permanently binds to COx enzymes to block chemicals responsible for pain and fever

How do statins work in terms of enzymes?

Serve as competitive inhibitors for enzyme responsible for cholesterol synthesis, lowering cholesterol levels

What is an allosteric site?

A site elsewhere on the enzyme that is not its active site, to which a non-competitive inhibitor binds

How do non-competitive inhibitors work?

They bind to an allosteric site on the enzyme. This causes the enzyme, and its active site, to change shape, meaning that the substrate cannot bind to the enzyme

How do non-competitive inhibitors affect Vmax?

They lower it

How does increasing enzyme or substrate concentration affect the rate of reaction when non-competitive inhibitors are involved?

Neither would affect it.

What are 2 examples of non-competitive inhibitors?

Organophosphates and Proton Pump Inhibitors

How do Proton Pump Inhibitors work and what are they used to treat?

They block enzymes which are responsible for secreting H+ ions. They are use to treat stomach ulcers as this would reduce acidity in the stomach.

How do organophosphates work and what are they used for?

They block an enzyme necessary in nerve impulses, and can thus cause cramp, paralysis and death. Used in insecticides and nerve agents such as Sarin, non-reversible

What is the general principle of end-product inhibition?

The product of the reaction acts as an inhibitor for the same reaction, inhibiting the enzyme

What is a cofactor?

A non-protein component that is necessary for effective functioning of an enzyme

What are some common cofactors?

Iron, zinc, calcium and chlorine (found in amylase) ions. Inorganic

What is a coenzyme?

An organic cofactor, usually derived from vitamins.

What is a prosthetic group in the context of enzymes?

A cofactor which is a permanent feature of the enzyme and essential to its function.

What is a precursor enzyme?

An enzyme created in an inactive state and activated later on, for example digestive enzymes

What is the term for an inactive enzyme?

Apoenzyme

What is a holoenzyme?

An active enzyme

What is a zymogen?

An apoenzyme chose change into a holoenzyme is brought about by pH or temperature change

What is an example of a zymogen?

Pepsin

What is a method of activating an enzyme that doesn't involve temperature, pH or a cofactor?

By cleaving off a piece, such as in protease