From The Student Room
TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Biology > Edexcel Biology Synoptic Paper - Enzymes
Inhibitors
- An enzyme inhibitor is a substance that slow down the rate at which an enzyme-catalyzed reaction take place.
Competitive
- Competitive inhibitors (a.k.a active site-directed inhibitors) have rather like the enzyme’s normal substrate.
- They bind at the active site of the enzyme.
- Therefore, substrate cannot bind with the enzyme to form substrate- enzyme complex.
- The reaction eventually stopped, because some active site-directed inhibitors bind permanently to the active sites of the enzymes.
- Other bind temporarily, so this will only slow down the rate of reaction.
Non- competitive
- They are also known as non- active site- directed inhibitors.
- They are completely different in shape from the substrate molecules, and they do not compete with them for the active sites.
- They bind to the allosteric site of the enzymes.
- This will cause altering in the shape of the active site.
- Therefore, the substrate no longer binds with the enzyme.
- No reaction occurred.
- This distorts the active site, so that it can no longer bind effectively with a substrate molecule.
Active site-shape-specificity
- An active site on enzyme where substrate can bind.
- The active site us precisely the right shape for the substrate molecule to slot into.
- Particular amino acids at the enzyme’s active site have R group that are able to form temporary bonds with the substrate.
- Different enzymes have different shape of active sites, with different R groups in them, and so each kind of enzyme can only react with particular substrate.
- For example, the enzyme amylase can only bind with polymers of alpha 1-4,glycosidic glucose, that is starch or glycogen.
- This property of enzyme is known as enzyme specificity.
Lock and key
- This is whereby the enzyme’s active site has the same shape as its substrate.
- The substrate can directly bind to the active site.
Induced fit
- This is whereby enzyme has different shape of active site with its substrate.
- It will alter its shape to enable its substrate to bind on.
Catalyst (Ea)
- Catalyst provides an alternative route with lower activation energy for a reaction to occur at faster rate.
- For example, starch will not just break down into maltose unless you give it some energy.
- Heating starch solution with little dilute acid can do this.
Factors that affecting activities
Temperature
- As the temperature rises, all the molecules in a solution – including enzyme molecules and substrate molecules – move around faster than they do at lower temperatures.
- So reactions tend to take place faster, because the enzymes and substrates collide more frequently and with more energy.
- But when temperature rises too high, enzyme tends to denature.
- The bonds – H bonds, that hold the tertiary structure of the enzyme molecule in shape tend to break.
- When the shape of active site altered, it is no longer able to bind with its substrate.
- So the conversion of substrate to product cannot take place.
- The temperature at which enzyme works most rapidly is called its optimum temperature.
pH
- The pH of a solution is a measure of how acidic or alkaline it is.
- The pH affects the ionization of the R groups in any protein, including enzyme molecules.
- This affects the ionic and hydrogen bonds that help to hold the enzyme molecule in its precise three-dimensional shape.
- Extremes of pH can therefore denature enzymes, and stop them functioning as catalysts.
- Many enzyme have an optimum pH of around 7(neutral).
- However, some enzymes work in very acidic or very alkaline conditions.
- For example, the enzyme pepsin works in stomach, where HCl produces a very acidic solution.
- Pepsin has an optimum pH of 2.
Substrate and Enzyme
- So long as there are plenty of substrate molecules in a solution, then the more enzyme molecules there are the faster the reaction can take place.
- This is because it is more likely that a substrate molecule collide successfully with enzymes.
- So, increasing the enzyme concentration will increase the rate if the reaction.
- However, at very high enzyme concentration there may be not enough substrate to keep all enzyme busy all the time,
- So the graph eventually flattens out.
- Similarly, so long as there are plenty of enzyme molecules in a solution, adding more substrate molecules will increase the chances of enzyme- substrate collisions, and so increase the rate of reaction.
- However, unless the enzyme concentration is extremely high, there comes a point when every enzyme are saturated with substrate.
- The curve therefore flattens out at high substrate concentrations.
Commercial Uses of Enzyme
Pectinases(food modification)
Proteases(biological detergents)
Immobilised lactase(low-lactose milk production)
Pectinases
- Pectin is a polysaccharide found in the cell wall and middle lamella of plant cells. They have a branching structure which readily traps water to form a gel. This property is used in jams,
- But if we want to make fruit juice or wine the pectin holds the juice in the gel and the juice appears very cloudy. This is called the pectin haze.
- Pectinase is added to crushed fruits to hydrolyzed the pectin molecules and make it easier to extract the juice. The breaking down of cell wall and addition of pectin gel to the juice increases the yield.
- Pectinase can also clear pectin haze by hydrolyzing pectin molecules into soluble sugars.
- Pectinase is also used in the production of olive oil, to soften the olives and make extraction of oil easier. *It also can be used to clear pectin haze and make olive oil clear.
- Other uses: improve colour derived from fruit skins.
- Commercial sources: fungi Aspergillus, Penicillium
Proteases
- Proteases are used in biological detergents to remove stains like blood, grass stains, fruit juice, gravy, egg stains, etc. which contain insoluble proteins.
- Protease hydrolyses large insoluble proteins into soluble amino acids by breaking of peptide bonds. This helps to remove protein stains easily (by dissolving them) at lower temperatures. Previously, proteins stains had to be removed by boiling the clothes.
- Early biological washing powders caused allergies when inhaled or when it came in contact with the skin. *This problem was over come by immobilization of the enzymes in capsules. The enzymes used are obtained from bacteria, which work best in alkaline medium and over a wide range of temperatures.
- Commercial sources: bacteria(Bacillus sp.) and fungi (Aspergillus sp. Rhizopus sp. Mucor sp.)
Immobilised lactase
- Lactose in milk is hydrolysed so that it can be consumed by lactose intolerant people who cannot digest lactose (as they do not produce lactase).
- It sweetens the milk without adding additional sugars, because glucose and galactose are sweeter than lactose. This is useful in ice cream production. Lactose crystallizes at low temperatures. This would give ice-cream a sandy texture. Use of lactase in ice cream production will remove lactose, so that no crystals form and the ice cream is creamy and smooth.
- Whey is the liquid obtained from milk during cheese production. This is usually discarded as a waste product. However; treatment of whey with lactase can yield sweet syrup, containing glucose and galactose.
- Commercial source: fungi Aspergillus sp. Kluyveromyces sp.
Enzymes are globular protein
Polypeptide chains are coiled and folded into globular shape and is maintained by hydrogen bonds, ionic bonds, disulphite bonds and hydrophobic interaction.= important for the shape of the enzymes, and thus, its functions.
Involvement of enzyme in metabolic pathway
Spec:
Metabolic pathways understand the concept of a metabolic pathway as a sequence of
enzyme-controlled reactions; appreciate the roles of enzymes in the control of such pathways, illustrated by oxidoreductases and hydrolases; anabolism and catabolism;
Aerobic respirationdescribe and understand the role of mitochondria as the site of Krebs cycle and electron-transport chain; understand the location of enzymes and electron carriers; understand the role of oxidoreductases.
Metabolic pathway
- a sequence of reactions controlled by enzymes.
- the products of one reaction are the substrates for the next.
All metabolic pathway involve enzymes
One simple reaction:
Starch Amylase(hydrolase) Maltose Maltase(hydrolase) Glucose
(the above equation needs sorting out)
- enzyme present controls the direction of the metabolic pathway, this allows pathways to *branch.example:pyruvate takes a different pathway in aerobic respiration and anaerobic respiration.
- site of a metabolic pathway is also controlled by enzymes.example: Oxidoreductases are located in the *inner mitochondrial membrane, where oxidative phosphorylation occurs. Starch digestion does not occur in *the stomach because the enzymes for starch digestion amylase, maltase(hydrolases)..are not present in the stomach.
Location of enzymes on a mitochondrion
Redox reactions=reduction and oxidation of the hydrogen carrier. These redox reactions are catalysed by oxidoreductase enzymes present along with the electron transport chain, in the inner membrane of mitochondria.
Comments
