Enzymes
Define the term "temperature coefficient, Q10" and state its usual value for enzyme controlled reactions
A measure of how much the rate of reaction increases with a 10°C temperature increase. Enzyme reactions normally 2 (rate doubles every 10°C increase)
Define the term "inactive precursor enzyme" and explain why enzymes may be produced in this form
A protein precursor, is an inactive protein (or peptide) that can be turned into an active form by post-translational modification, such as breaking off a piece of the molecule or adding on another molecule. Some enzymes can cause damage to cells and must only be activated under certain conditions.
Explain why an enzyme only catalyses one type of reaction
A substrate will only fit into an active site if it is complementary in both shape and charge and the reaction is only catalysed if the substrate binds with the active site. This means that each enzyme only catalyses one type of reaction.
Define the term "activation energy" and "rate of reaction"
Activation energy - the energy required to initiate a reaction Rate of reaction - how quickly or slowly the reaction takes place - measure of the change in concentration of the reactants or the change in concentration of the products per unit time
Define the terms "active site", "complementary shape", and "specific"
Active site - Area of an enzyme with a shape complementary to a specific substrate allowing the enzyme to bind a substrate with specificity Complementary shape - The shape of the active site and the substrate match so that they fit together Specific - Each enzyme has a single substrate that it works on and that will fit its active site (or group of substrates - group specificity)
Describe three ways in which inactive precursor may be activated
Addition of a cofactor Action of another enzyme e.g. protease removing part of the structure Change in conditions e.g. pH
Define the terms "anabolic reactions", "catabolic reactions", "digestion" and "metabolism"
Anabolic reactions - Reactions of metabolism that construct molecules from smaller units, requiring energy from the hydrolysis of ATP Catabolic reactions - Reactions of metabolism that break down molecules to form smaller units releasing energy. Digestion - The catabolic process in the digestive tract where ingested food is converted into simpler, soluble and diffusible substances that can be assimilated by the body. Metabolism - the chemical processes that occur within a living organism in order to maintain life
Define the terms "apoenzyme", "holoenzyme" and "zymogens" or "proenzymes"
Apoenzyme - An apoenzyme is an inactive enzyme, activation of the enzyme occurs upon binding of an organic or inorganic cofactor Holoenzyme - An enzyme with its required cofactor Zymogen/proenzyme - an inactive substance which is converted into an enzyme when activated by another enzyme
Explain how increasing the enzyme concentration affects the initial rate of an enzyme-controlled reaction
As enzyme concentration increases, rate increases as there is free substrate to react. Rate will continue to increase until there is no longer an excess of substrate and any extra enzymes are left free as there is no substrate to bind with.
Explain how increasing the substrate concentration affects the initial rate of an enzyme-controlled reaction
As substrate concentration increases, the rate of reaction also increases as there are free enzyme molecules to combine with the extra substrate. At the saturation point rate cannot increase further due to the enzymes working 'flat out' - there are no longer any free enzymes to combine with extra substrate so increasing the concentration has no effect.
Explain why increasing the temperature from below the optimum up towards the optimum increases the rate of reaction
As temperature rises, enzymes and substrate molecules have more kinetic energy and move faster, forming more enzyme-substrate complexes
State the substrates and products for the enzymes catalase, amylase and trypsin
Catalase - Hydrogen peroxide into oxygen and water Amylase - Starch into sugars Trypsin - Proteins into polypeptides (which are further hydrolysed into amino acids by other enzymes)
Define the term "cofactor" and "coenzyme" and give two possible roles of cofactors/coenzymes
Cofactor - non-protein components necessary for the effective functioning of an enzyme Coenzyme - If the cofactor is an organic molecule it is known as a coenzyme They may form part of the active site or transfer atoms or groups from one reaction to another in a multi-step pathway
Describe the similarities and differences between cofactors, coenzymes and prosthetic groups
Cofactors are inorganic and are not necessarily permanently bound to the enzyme Coenzymes are organic Prosthetic groups are permanently bound to the enzyme
Draw graph showing how substrate concentration affects the rate of an enzyme-controlled reaction if a competitive or non-competitive inhibitor is present. Explain the effect of competitive and non-competitive inhibitors on the Vmax of an enzyme-controlled reaction
Competitive inhibitor does not lower maximum rate as once substrate concentration is increased the effect is lessened as substrate more likely to bind to active site than inhibitor. Non-competitive inhibitors do lower the maximum rate as they take a certain number of enzymes out of action regardless of substrate concentration.
Explain the role of extracellular enzymes in general. Summarise the digestion of starch and of proteins as examples of the role of extracellular enzymes
Components for reactions often come from larger molecules that cannot enter cells but must be broken down so that the monomers can be used for metabolism. Starch is broken down into individual individual glucose monomers that can be used for respiration. Proteins are broken down into individual amino acids which can be used within cells to build the specific proteins needed.
State 5 factors that affect the rate of an enzyme controlled reaction
Temperature, pH, substrate concentration, enzyme concentration, inhibitors
Explain how a non-competitive inhibitor affects the rate of an enzyme-controlled reaction
Do not bind to active site but to a different part of the enzyme, distorting the tertiary structure which changes the shape of the active site so that the substrate can't fit.
Give examples of inactive precursor enzymes and describe how they are activated
During blood clotting factor X is released by platelets which catalyses prothrombin into thrombin which is a protease which catalyses fibrinogen into fibrin - this series of activations is called the coagulation cascade Pepsinogen released into stomach becomes pepsin due to pH - this protects body tissues from digestive action of pepsin
Define the terms "enzyme", "substrate", and "product"
Enzyme - Biological catalysts that interact with substrate molecules to facilitate chemical reactions. Usually globular proteins. Substrate - A substance used, or acted on, by another process or substance such as a reactant in an enzyme catalysed reaction. Product - The end result of the reaction; enzymes act on substrates and convert them into products.
Explain why increasing the temperature up from the optimum decreases the rate of reaction abruptly
Enzyme denatures which changes the shape of the active site so that the substrate cannot fit.
Define the terms "enzyme inhibitor", "competitive inhibitor", "non-competitive inhibitor", "reversible inhibitor", "irreversible inhibitor" and "allosteric site"
Enzyme inhibitor - a factor that prevents or reduces the rate of an enzyme catalysed reaction Competitive inhibitor - an inhibitor that competes with the substrate to bind to the active site of an enzyme Non-competitive inhibitor - an inhibitor that binds to an enzyme at an allosteric site Reversible inhibitor - An inhibitor that can bind temporarily to the enzyme but also dissociate itself so the enzyme returns to being functional Irreversible inhibitor - An inhibitor that once bound to the enzyme cannot dissociate and permanently stops the enzyme functioning Allosteric site - The place on an enzyme where a molecule that is not a substrate may bind, thus changing the shape of the enzyme and influencing its ability to be active.
Explain why a pH change away from the optimum decreases the rate of reaction
Enzyme structure depends on interactions of R groups. pH alters charges and therefore bonds, changing the denaturing the enzyme and changing the shape of the active site.
Explain why enzymes are necessary to life
Enzymes control the processes and reactions of metabolism. Without them many reactions would take place too slowly to keep organisms alive.
Explain how enzymes can affect both the structure and function of cells and whole organisms
Enzymes control the processes inside cells, if those chemical reactions cannot take place, the cell cannot function. If many cells are affected then the organism will feel the effects.
State what the presence of an enzyme does to the activation energy for the reaction and explain why this increases the rate of reaction
Enzymes lower the activation energy required for the reaction making it easier for it to take place and therefore happen faster.
Describe how ATP is involved in end-product inhibition of the enzyme phosphofructokinase
First step of break down of glucose involves adding two phosphate groups to the molecule. Addition of second catalysed by PFK which is competitively inhibited by ATP so it regulates its own production.
Describe 4 ways in which multi-step reaction pathways can be regulated by cells
Inhibitors - competitive, non-competitive End product inhibition Inactive precursor enzymes
Define the terms "intracellular enzyme" and "extracellular enzyme". Give one example of an intracellular enzyme and two examples of extracellular enzymes
Intracellular enzyme - Enzymes that act inside the cell e.g. DNA polymerase Extracellular enzyme - Enzymes that are secreted and act outside cells e.g. amylase, lipase (digestive enzymes)
Explain why the zinc ion that forms an important part of the structure of carbonic anhydrase (an enzyme necessary of the metabolism of carbon dioxide) is called a prosthetic group not a cofactor or coenzyme.
It is an inorganic ion permanently bound to the enzyme.
Explain why the chloride ion necessary for the correct formation of the active site in amylase is called a cofactor not a coenzyme or prosthetic group
It is not permanently bound to the enzyme. It is an inorganic ion.
Define the term "Vmax"
Maximum initial velocity or rate of an enzyme-catalysed reaction
Give two examples of coenzymes that are synthesised from vitamins in our diet
NAD is synthesised from vitamin B3 and used in respiration to transfer hydrogen Coenzyme A is synthesised from vitamin B5 and used in the breakdown of carbohydrates and fatty acids in respiration.
State two examples of non-competitive inhibitors and describe their action
Organophosphates used as insecticides inhibit enzymes used in nervous impulse transmission causing paralysis Proton Pump Inhibitors block an enzyme responsible for secreting hydrogen ions into the stomach preventing build up of acid which could lead to ulcers
State two examples of competitive inhibitors and describe their action
Statins are competitive inhibitors of the enzymes used to produce cholesterol so are prescribed to help lower cholesterol Aspirin inhibits the production of chemicals responsible for pain and fever
State the sequence of events in an enzyme-controlled reaction
Substrate fits into active site to form an enzyme-substrate complex. The active site changes shape to accommodate the substrate and the complementary charges help the 'fit'. The active site changes shape to convert the substrate into products. This is now an enzyme-product complex. The products no longer complement the active site and are released. The active site is now free for another substrate.
Describe the "lock and key" hypothesis of enzyme action
The active site is the lock, the substrate the key. One key fits one lock. This assumes that both active site and substrate have a complementary fixed shape.
Suggest how the R-groups of amino acids are involved in catalysing reactions
The active site may involve only a small number of amino acids but the R groups are key to the function. The interactions between them cause the shape which must be complementary to the substrate. They also may be charged which must be complementary.
Define the term "end-product inhibition" and describe its usefulness in controlling metabolic pathways
The product of a reaction inhibits the enzyme required for the reaction Useful as if there is little product, there is little inhibition therefore more is made, but if there is lots of product, there is lots of inhibition and production slows down. Negative feedback controls the level of product.
Explain why Siamese cats are white with black tails, ears, paws and faces
They have a mutation in the enzyme that catalyses melanin production that means it is denatured at normal body temperature. The extremities of the cat such as the tail however are cooler so melanin is produced and the fur is a darker colour.
Explain how a competitive inhibitor affects the rate of an enzyme-controlled reaction.
They have a similar shape to the substrate so enter the active site forming an enzyme-inhibitor complex. No product is formed but while the inhibitor is in the active site, the substrate can't enter.
Describe the "induced-fit" hypothesis of enzyme action
This has replaced lock and key. It assumes the active site changes shape slightly to fit around the substrate. The analogy used is a hand and a glove. The glove (active site) moulds around the hand (substrate).
Describe and explain how to investigate any of the factors that affect the rate of enzyme-controlled reactions
Using catalase, hydrogen peroxide and measuring the rate by measuring the volume of oxygen produced in a certain amount of time. Change the conditions by using buffer solutions or water baths etc.
