Enzymes

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Enzymes are bilogical catalysts

Enzymes are biological catalysts  Enzymes speed up chemical reactions by acting as bilogical catalysts.     

They catalyse metabolic reactions in the body e.g digestion and respiration.      Enzymes are proteins, these proteins have an active site, which has a specific shape. The active site is the part of the enzyme where the substrate molecules bind to.     

Enzymes are highly specific due to their tertiary structure.  (A catalyst is a substance that increases the rate of a reaction without actually being used up in the reaction itself)

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Enzymes lower the Activation Enthalpy of a reactio

In a chemical reaction, a certain amount of energy needs to be supplied to the chemicals before the reaction will start. This is called the activation energy.

Enzymes lower the amount of activation energy that's required, often making reactions occur at a lower temperature than they could without an enzyme. Therefore, this speeds up the rate of reaction.

When a substrate fits into the enzymes active site it forms an enzyme-substrate complex - it is this that lowers the activation energy:

  • If two substrate molecules need to be joined, being attached to the enzyme holds them close together, reducing any repulsion between the molecules so they can bond more easily.
  • If the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate, so the substrate molecule breaks up more easily.
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Lock and Key and the Induced fit model.

LOCK AND KEY MODEL - This the idea that the substrate fits into the enzyme, in the same way that a key fits into a lock. However, this theory didnt show the full story of the enzyme-substrate complex  - as it has to changed shape slightly inorder to complete the fit.

ENZYME + SUBSTRATE  ->     ENZYME-SUBSTRATE COMPLEX      -> PRODUCTS ..                                                                                                      (enzyme unchanged)

INDUCED FIT MODEL - The induced fit model is a better theory as it helps to explain why enzymes are so specific and only bond to one particular substrate. The substrate doesnt have to be the right shape to fit the active site, it has to make the active site change shape in the correct way too.

ENZYME + SUBSTRATE ->   ENZYME SUBSTRATE COMPLEX    ->     PRODUCTS  .                                (As the substrate binds the active site changes shape slightly)

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Enzyme Properties - relative to their tertiary str

Enzymes are very specific - usually only catalyse one reaction, e.g maltase only breaks down maltose

Only one substrate will fit to the active site

The enzymes active site's shape is determined by the enzymes tertiary structure (determined by the primary structure)

Each enzyme has a different tertiary structure - therefore different shaped active site.

If the tertiary structure is altered, the shape  of the active site will change - meaning the substrate will no longer fit and the enzyme cannot carry out its function. Tertiary structure may be altered by pH or temperature.

The primary structure of a protein is determined by a gene. If a mutation occurs in that gene it could cause the tertiary structure of the enzyme produced.

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Inhibitors

Competetive inhibition:- CI molecules have a similar shape to that of the substrate. They compete with the substrate to bind to the active site, but no reaction takes place. Instead they block the active site so no substrate molecules can fit in.

How much the enzyme is inhibited depends on the relative concentrations of inhibitor and substrate. If there is a higher concentration of the inhibitor, it'll take up nearly all the active sites and hardly any of the substrate will get to the enzyme.

Non-competitive inhibition:- NCI molecules bind to the enzyme away from its active site. Causing the active site to change shape so the substrate can longer bind to it.They dont compete with the substrate as they are a different shape.

Increasing the concentration of substrate wont make any difference - enzyme activity will still be inhibited.

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