As Biology - Enzymes

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Biological catalysts

Catalyse metabolic reactions in your body
Examples of these:

  • Digestion
  • Respiration

Phenotype = physical appearance - due to enzymes that catalyse the reactions causing growth and development
Enzyme action:

  • Intracellular - within cells
  • Extracellular - outside cells e.g. places like the blood and digestive system

Enzymes are globular proteins
They have an active site which has a specific shape. It is the part of the enzyme where the substrate molecules (the substance that enzymes interact with) bind to.
Enzymes are highly specific due to their tertiary structure 

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Lowering the activation energy of a reaction

Activation energy = the amount of energy needing to be supplied to the chemicals before the reaction will start; most commonly provided as heat

Enzymes lower the amount of activation energy needed, often making reactions happen at lower temperatures
This speeds up the rate of reaction

Enzyme-substrate-complex = when a substrate fits into the enzyme's active site; lowers the activation energy
WHY?
(1) If 2 substrate molecules need to be joined, being attached to the enzyme holds them closer together, reducing any repulsion between the molecules so they can bond more easily

(2) If the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate so the substrate molecules break up more easily 

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Lock and key model

Enzymes only work with the substrate that fits their active site

The substrate fits into the enzyme the same way as a key fits a lock

Further developments - although the enzyme and substrate do need to fit together originially, in addition the enzyme-substrate complex changed shape slightly to complete the fit

This modified the original lock and key model to become the 'induced fit' model 
'Induced fit' model explains why enzymes are so specific and only bond to one particular substrate

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Properties/Tertiary structure

Highly specific - usually catalyse only one reaction
Examples:

  • Maltase only breaks down maltose
  •  Suerase only breaks down suerose

Only 1 substrate will fit into the active site
Tertiary structure - determines the shape of the active site and is itself determined by the enzyme's primary structure

enzyme has a different tertiary structure -------> different shaped active site

if the substrate doesn't fit the active site the reaction won't be catalysed 

if the tertiary structure of a protein is altered in any way, the shape of the active site will change -----> the substrate won't fit and the enzyme will no longer be able to carry out its function

Tertiary structure of an enzyme can be altered by changes in:

  • temperature
  • pH balance

Primary structure (amino acid sequence) of a protein is determined by a gene -----> if a mutation occurs in that gene, it could change the tertiary structure of the enzyme produced

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Factors affecting enzyme activity

ENZYME INHIBITION - molecules that bind to the enzyme that they inhibit

4 types:

  • Competitive
  • Non-competitive
  • Reversible
  • Non-reversible

COMPETITIVE

  • have a similar shape to substrate molecules
  • compete with substrate molecules to bind to the active site; no reaction takes place
  • they block the active site so no substrate molecules can fit in it
  • inhibition of an enzyme depends on the relative concentrations of the inhibitor and substrate
  • high concentration means it'll take up nearly all the active sites and hardly any of the substrate will get to the enzyme

NON-COMPETITIVE

  • bind to the enzyme away from its active site
  • this causes the active site to change shape so the substrate molecules can no longer bind to it
  • they don't 'compete' with the substrate molecules to bind to the active site because they are a different shape
  • increasing the concentration of a substrate won't make any difference - enzyme activity will still be inhibited

REVERSIBLE vs NON-REVERSIBLE

Depends on the strength of the bonds between the enzyme and the inhibitor

  • Strong - covalent bonds, the inhibitor can't be removed easily and the inhibition is irreversible
  • Weak - hydrogen bonds or weak ionic bonds, the inhibitor can be removed easily and therefore is reversible
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Metabolic poisons

Often enzyme inhibitors although they Interfere with metabolic reactions (reactions that occur in cells) and can cause:

  • damage
  • illness
  • death

Examples:

  • Cyanide = irreversible inhibitor of cytochrome C oxidase, an enzyme that catalyses respiration reactions; cells that can't respire die. 
  • Malonate = inhibits succinate dehydrogenase which also catalyses respiration reactions
  • Arsenic = inhibits the action of pyruvate dehydrogenase yet another enzyme that catalyses respiration reactions
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Drugs inhibiting enzymes

Some medicinal drugs are enzyme inhibitors

EXAMPLES

  • Antiviral drugs (drugs that stop viruses such as HIV) e.g. reverse transcriptaseinhibitors inhibit the enzymereverse transcriptase, which catalyses the replication of DNA. This prevents the virus from replicating
  • Antibiotics e.g penicillin inhibits the enzymetranspeptidase, which catalyses the formation of proteins in bacterial cell walls. This weakens the call wall and prevents the bacterium from regulating its osmotic pressure. As a result the cell bursts and the bacterium is killed. 
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Cofactors & Coenzymes

Some enzymes will only work if there is another non-protein substance bound to them.

Non-protein substances = cofactors

(1)

  • Some cofactors are inorganic molecules
  • They work by helping the enzyme and substrate to bind together.
  • They don't directly participate in the reaction so aren't used up or changed in any way. 
  • EXAMPLE
    -Manganese ions are cofactors found in hydrolase enzymes (enzymes that catalyse the hydrolysis of chemical bonds

(2)

  • Some cofactors are organic molecules - these are called coenzymes.
  • They participate in the reaction and are changed by it (they are just like a second substrate but not called that)
  • They often act as carriers, moving chemical groups between different enzymes.
  • They're continually recycled during this process.
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Comments

katie

this is really good and really helped :)

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