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Flashcards in 2.2 Enzymes Deck (25):

What are enzymes?

Enzymes are globular proteins which act as catalysts. They speed up reactions without being chemically changed.


Why are enzymes effective in small amounts?

They can be used repeatedly.


How do enzymes work?

They lower the activation energy of a reaction.


What is the active site?

It is made up of a relatively small number of amino acids (only a small region of the globular protein (enzyme) is functional). The active site forms a small, hollow depression which is able to form an enzyme-substrate complex with the complementary substrate which the enzyme acts on.


How is the substrate held in the active site?

By bonds that temporarily form between certain amino acids of the active site and groups on the substrate molecule.


What is the lock and key model?

A substrate will only fit the active site of one particular enzyme. The shape of the substrate (key) exactly fits the active site of the enzyme (lock).


What are the limitations of the lock and key model?

The enzyme is not a rigid structure like a lock.

Other molecules can bind to enzymes at sites other than the active site. In doing so they altered the activity of the enzyme.

The enzyme shape was altered by the binding molecule - the enzyme is flexible.


What is the induced fit model?

As opposed to being a rigid lock, it proposes that the enzyme actually changes it's shape (flexible) slightly to fit (mould) the profile of the substance.

The enzyme has a general shape but this alters in the presence of a substrate.

As it changes shape the enzyme puts a strain on the substrate molecule.

This strain distorts a particular bond and consequently lowers the activation energy needed to break the bond.


Why is the induced fit model better than the lock and key model?

It explains:
How other molecules can affect enzyme activity.
How the activation energy is lowered.


For an enzyme to work it must...

Come into physical contact with its substrate
Have an active site which fits the substrate


To measure the progress of an enzyme-catalysed reaction we usually measure it's...



What are the two most frequently measured events in drawing graphs of enzyme action?

The formation of the products of the reaction e.g. Volume of oxygen when catalase acts on hydrogen peroxide.
The disappearance of the substrate e.g. The reduction in concentration of starch when it is acted upon by amylase.


Explain the shapes of the graphs of enzymes acting on substrates...

At first there is a lot of substrate and no product

It is very easy for substrate molecules to come into contact with the empty active sites on the enzyme molecules.

All the active sites are filled and the substrate is broken down rapidly.

The amount of substrate decreases as it is broken down and more product is formed.

With less substrate molecules, fewer come into contact with the enzyme active sites and product molecules also get in the way. The rate of breakdown decreases. Levelling off.

Now there is so little substrate that any further decrease in its concentration cannot be measured.

The graphs flatten out - all the substrate has been used up and so no new product can be formed.


What effect does increased temperature have on enzyme action?

Increases kinetic energy - molecules move more rapidly and collide more often.

Too high a temperature begins to cause the hydrogen and other bonds in the enzyme molecule to break. This results in the enzyme (including it's active site) to change shape.

At first the substrate fits less easily into the changed active site, slowing the rate of reaction.

At some point the enzyme is so disrupted that it stops working all together. It is denatured.


Why have human body temperatures evolved to be 37 degrees when the optimum temperature for our enzymes is 40 degrees?

The advantages are offset by the additional energy/food required.

Other proteins may be denatured at higher temperatures.

Any further rise in temperature e.g during illness might denature the enzymes.


What is the effect of pH on enzyme action?

Changes charges on active site: Alters charges on amino acids that make up the active sites of the enzyme (Affects hydrogen and ionic bonding of -NH2 and -COOH). The substrate can no longer attach to the active site and enzyme-substrate complexes are not formed.

Changes shape/tertiary structure: Causes the bonds which maintain tertiary structure to break. The enzyme changes shape and and the substrate no longer fits in it - denatured.


What is the pH of a solution?

A measure of its hydrogen ion concentration.


In organisms, fluctuations of pH are most likely to:
A) denature it
B) reduce an enzymes activity

Reduce an enzymes activity.


Effects of substrate concentration on the rate of enzyme action.

Substrate concentration:
Low - enzymes only have a limited amount of substrate to collide with. Active sites of enzymes are not working to full capacity.
Equal - each site filled - rate of reaction is maximum (Vmax)
High/excess - no effect on reaction rate ( rate levels off).


What is a competitive inhibitor?

Inhibitors which bind to the active site of the enzyme.


What is a non-competitive inhibitor?

Inhibitors which bind to the enzyme at a position other than the active site.


What is a reversible inhibitor and give two examples.

Inhibitors (most) which only make temporary attachments to the active site as opposed to permanent inhibitors.

Competitive inhibitors/non-competitive inhibitor


How can competitive inhibitors occupy an enzymes active site?

They have a similar molecular shape to the substrate. They are complementary to the active site of the enzyme. They compete with the substrate for the available active sites.


How does substrate and competitive inhibitor concentration effect rate of reaction?

If the substrate concentration is increased it is more likely that a substrate molecule will take the place of a competitive inhibitor after it leaves the active site. The effect of the inhibitor is reduced. Sooner or later all the substrate molecules will occupy an active site it just depends on the concentration as to how long this takes. E.g saccinic acid and malonic acid.


What does a non-competitive inhibitor do that slows rate of reaction?

The inhibitor alters the shape of the active site in such a way that substrate molecules can no longer occupy it so the enzyme cannot function. Increase of concentration of substrate does not overcome this because they are not competing for the same site.