2.2 Enzymes Flashcards
Why are enzymes effective in small amounts?
They can be used repeatedly.
What are enzymes?
Enzymes are globular proteins which act as catalysts. They speed up reactions without being chemically changed.
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…
Time-course
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.
