BY1 - Enzymes and biological reactions

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  • Created by: zopetre_
  • Created on: 29-04-17 12:24

What are the two types of metabolic pathway?

Anabolic reactions - building up molecules. e.g. protein synthesis

Catabolic reactions - breaking down molecules. e.g. digestion

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Describe enzyme structure

Enzymes are globular proteins with tertiary structure. They have hydrophillic R groups on the outside of the molecule making it soluble. The elements in the R groups determine the bonds the amino acids make with each other. These are hydrogen bonds, disulphide bridges and ionic bonds, holding the enzyme in tertiary form. They have a small area with a specific 3D shape, the active site.

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What are the properties of enzymes in reactions?

They speed up reactions

They aren't used up

They aren't changed

They have a high turn-over number

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What reactions do enzymes catalyse?

They only catalyse reactions that are energetically favourable, and which would happen anyway.

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What are the sites of enzyme action?

Extracellular - some enzymes are secreted from cells by exocytosis and catalyse extracellular reactions. e.g. Amylase, made in the salivary glands, moves down the salivary ducts to the mouth.

Intracellular, in solution - intracellular enzymes act in solution inside cells, e.g. enzymes in solution in the stroma of the chloroplasts ctalyse the synthesis of glucose.

Intracellular, membrane-bound - intracellular enzymes may be attached to membranes, e.g. on the cristae of mitochondria

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How does an enzyme form an enzyme-substrate comple

An enzyme acts on its substrate, and makes temporary bonds at the active site to form an enzyme-substrate complex.

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Describe the lock-and-key model

The active site is unique, so an enzyme can only catalyse one type of reaction. The enzyme is specific for its substrate 'enzyme specificity'.

The substrate fits into the active site as a key fits into a lock.

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Describe the induced fit model

Observations were made that the enzymes shape was flexible, not rigid as the enzyme's shape was altered by binding its substrate. This suggests that the enzyme shape alters to accommodate the substrate.

e.g lysozyme

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How do enzymes increase the rate of reaction?

They lower the activation energy. When a substrate enters the active site of an enzyme, the shape of the molecule alters to allow reactions to occur at lower rates.

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How does temperature effect the rate of reaction?

Increases temperature, increases kinetic energy so substrate and enzyme collide with suffucient energy to increase the rate of reaction. Usually increases to 40 degrees, above this increasing vibration breaks hydrogen bonds, changes tertiary structure and alters shape of the active site, denaturing the enzyme.

At low temperatures, the enzyme is inactivated as molecules have very low kinetic energy. 

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How does pH effect the rate of reaction?

Charges on amino acid side-chains of the enzymes active site are affected by H+ or OH- ions.

At low pH, excess H+ ions are attracted to negative charges, neutralising them.

At high pH, excess OH- ions neutralise the positive charges.

This disrupts the ionic and hydrogen bonds, changing the active site shape and denaturing the enzyme so no enzyme-substrate complex forms.

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How does substrate concentration effect the rate o

If the enzyme concentration is constant, the rate of reaction increases as substrate concentration increases. However, as more substrate is added, the active sites are occupied therefore the rate of reaction stops increasing.

Substrate concentration is a limiting factor.

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What is the effect of enzyme concentration on rate

As the enzyme concentration increases, there are more active sites available and therefore the rate of reaction increases.

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What is competitive inhibition?

Competitive inhibitors have a molecular shape complementary to the active site and similar to the substrate, they compete for the active site.

Increasing the concentration of the substrate, it reduces the effect of the inhibitor because the more substrate molecules present, the greater their chance of binding to the active sites so less are available for the inhibitor.

Increasing the concentration of inhibitor, it binds to more active sites so the rate of reaction decreases.

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What is non-competitive inhibition?

Non-competitive inhibitors bind to the enzyme at an allosteric site, so they don't compete for the substrate.

They affect bonds within the enzyme and alter its overall shape, so the substrate can't bind and no enzyme-substrate complexes form.

As the inhibitor concentration increases, more enzyme molecules are denatured, so the rate of reaction decreases.

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What is an immobilised enzyme?

Immobilised enzymes are fixed, bound or trapped on an inert matrix such as sodium alginate beads or cellulose microfibrils.

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What does immobilising enzymes allow?

Immobilising enzymes makes them more stable as it creates a microenvironment, which allows reactions to occur at higher temperatures and more extreme pHs. 

It prevents the shape change that would denature it. 

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Describe different immobilised enzymes

Enzymes immobilised in beads have a lower rate of reaction than those on a membrane, as enzymes on a membrane are readily avaialble for binding, whereas the substrate has to diffuse into a bead.

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What are the advantages of immobilised enzymes?

Increases stability and function over a wider range of temperature and pH

Products aren't contaminated

Enzymes are easily recovered for reuse

A sequence of columns can be used, several enzymes in one process

Enzymes can be easily added or removed

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Describe the uses of immobilised enzymes

Lactose-free milk: milk is passed down a column containg immobolised lactase, lactose binds to its active sites and is hydrolysed into components, glucose and galactose.

Biosensors: rapidly detect, identify and measure concentrations of molecules. They combine with a biomolecule and turn a chemical signal into an electrical signal. 

HFCS: high-fructose corn syrup manufacture, a multi-step process from starch.

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