Enzymes topic in OCR Biology unit 2 Exam.


Activation Energy.

Activation energy = the amount of energy needed to start a reaction

It can be increased by heating the solution or using enzymes to give the biological molecules enough energy to destabilise the molecules and allow them to react.

Catalysts reduce the amount of activation energy needed to start the reaction.

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Lock and key:

  • Active site is specifically shaped complementary to the substrate.
  • Substrate fits enzyme like a key fits the matching lock.
  • They can also sometimes catalyse the reverse reaction.

Induced fit:

  • Substrates molecules collides with an enzymes active site.
  • Enzyme changes shape slightly
  • Active site fits more closely.
  • Substrate held in place by oppositely charged groups on the substrate and active site.
  • Change in enzyme shape changes the strain on the substrate - destabilised the molecule and bonds break producing a product which doesn't fit the active site and so it leaves.
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Temperature and enzyme function.

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PH and Enzymes

PH= the amount of hydrogen ions in a concentration. Lower PH (more acidic) the more hydrogen ions. PH in general:

  • Hydrogen ions are atrracted to negitively charged molecules because they have a positive charge.
  • Bonds holding the teritiary structure together are caused by oposite attractions of groups of aminoacids.
  • Hydrogen bonds and ionic bonds are interfeered with by the ions this alters the tertiary structure.
  • Changes the shape of the active site and so the rate of reaction. 

PH and induced fit:

  • cataylsts rely on the charged groups on the R group of the active site
  • PH alters the charges as hydrogen ions are attracted to negitively charged groups in the active site
  • Examples of enymes and optimum Ph (which gives best tertiary structure to the active site) Pepsin at PH2 and Trypsin and PH7.
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Concentration and Enzymes

Increasing substrate concentration increases rate of reaction to an optimum where all the active sites are full and so it cannot increase any more.

Enzyme concentration works similarly:

The more enzymes the more active sites available so more substrates fit in and more product is produced. Because of more enzyme-substrate complexes. It only increases until all the substrates are in an active site. You cannot get  a higher reaction rate than this.

Plateau (leveling off) occurs when enzyme or substrate concentration becomes the limiting factor. However enzymes usually remain at a relatively constant level in cells because they can be used again and again.

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Inhibitors - competitive

Competitive inhibitors have a similar shape to the substrate molecules.

So they occupy the active site and form enzyme inhibitor complexes.

They don't form products but prevent the substrate reaching the active site.

Number of enzyme substrate complexes is reduced.

Level of inhibition dependent on the concentrations of inhibitors

If more substrate is added inhibition reduces because it is more likely that an enzyme and a substrate will collide.

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Inhibitors - non competative

Do not compete for the active site

Instead they attach to the molecule in a region away from the active site this distorts the tertiary structure of the enzyme molecule and changes the shape of the active site.

Enzyme-substrate molecules can no longer form and so no product is produced and the reaction rate decreases.

The level of inhibition depends on the number of inhibitors present. Can stop reaction completely.

Permanent iinhibitors:

Most iinhibitors aren't permanent and are reversible but some aren't and they permanently denature the enzyme.

Inhibition is not always a bad thing. Can control reaction rates.

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