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Define an enzyme

A globular protein that catalyses metabolic reactions


What is a catalyst?

A molecule which speeds up a chemical reaction but remains unchanged at the end of the reaction


What is an enzyme?

A biological catalyst made of protein with a specific tertiary structure and active site


Why are enzymes essential for life?

Because virtually every metabolic reaction in a living organism is catalysed by an enzyme


What is the name ending for most enzymes?



What are intracellular enzymes?

Enzymes that operate within cells


What are extracellular enzymes?

Enzymes that are secreted by cells and that catalyse reactions outside cells
- some organisms secrete enzymes outside their bodies


Describe the structure of enzymes as globular proteins

Enzyme molecules are coiled into a precise, 3D shape, with hydrophilic R groups (side-chains) on the outside of the molecule


What make enzymes soluble?

The hydrophilic R groups on the outside of the molecule


What is the active site?

A region of the enzyme, usually a cleft or depression, to which another molecule (substrate) or molecules can bind


Describe the lock and key hypothesis

The idea that the enzyme has a particular shape, into which the substrate fits exactly
- the substrate is held in place by temporary bonds which form between the substrate and some of the R groups of the enzyme's amino acids
- this combined structure is called the enzyme-substrate complex


How is the enzyme specific for a substrate?

Each type of enzyme will usually act on only one type of substate molecule because the shape of the active site will only allow one shape of molecule to fit


What does catabolic mean?

Breaking down


What does anabolic mean?

Building up


Describe the induced fit hypothesis for enzyme action

It is mostly the same as lock and key but it adds the idea that the enzyme, and sometimes the substrate, can change shape slightly as the substrate molecule enters the enzyme, in order to ensure a perfect fit
- makes the catalysis even more efficient


What is formed briefly before the release of the product?

An enzyme-product complex


How do enzymes work?

1) Enzymes may catalyse the breakdown of a molecule as the interactions between the R groups of the enzyme and the atoms of the substrate can break bonds in the substrate molecule
2) Enzymes may catalyse the joining together of 2 molecules as the interactions can encourage formation of bonds in the substrate molecule


How fast is enzyme catalysis?

Very rapid e.g. breakdown of H2O2 into H2O and O2


What is the activation energy?

The energy temporarily given to a substrate to convert it to the product in a chemical reaction


How do enzymes affect the activation energy?

Enzymes decrease the activation energy of the reaction which they catalyse, making it easier to turn substrate into product
- they do this by holding the substrate(s) in such a way that their molecules react more easily and their bonds become weaker
- reactions catalysed by enzymes will take place rapidly at a much lower temp then they otherwise would


Why are enzymes needed when one way of increasing the rate of metabolic reactions is to increase the energy of the reactants is by heating them?

It is not enough to give most substrates the activation energy which they need to change into products


Describe what happens during the course of the reaction of the breakdown of H2O2 by catalase

1) The reaction begins very swiftly - bubbles of O2 are released quickly and a large vol of O2 is collected in the first minute
2) But as the reaction continues, the rate at which O2 is released gradually slows down
3) The reaction gets slower and slower until it eventually stops completely


Explain why the breakdown of H2O2 by catalase begins very swiftly

- When the enzyme + substrate are first mixed, there are a large number of substrate molecules - at any moment, virtually every enzyme molecule has a substrate molecule at its active site


What does the rate at which the breakdown of H2O2 depend on?

How many enzyme molecules there are and the speed at which the enzyme can convert substrate to product, release it and bind with another substrate molecule


What happens as the breakdown of H2O2 continues?

As more and more substrate is converted into product, there are fewer and fewer substrate molecules to bind with enzymes
- therefore, the reaction gets slower and slower until it stops


Describe the graph for the rate of an enzyme controlled reaction

The graph is always steepest at the beginning (the initial rate of reaction) and then increases as a curve until it flattens out


How can you measure the initial rate of reaction by the release of O2?

Measure the amount of O2 given off in the first 30s and then convert to cm3/min or just work out gradient


What are the factors that affect enzyme action?

1) Enzyme concentration
2) Substrate concentration
3) Temperature
4) pH


Describe the effect of enzyme concentration on enzyme action

- Higher enzyme conc, faster reaction
- Overall, same amount of product will be formed eventually regardless of different enzyme conc because the substrate conc is the same


How would you compare the effect of different enzyme concentrations on reaction rate and why?

- Plot enzymes concentration against initial rate of reaction - it should increase linearly and be directly proportional
- You use the initial rate of reaction because once the reaction is under way, the amount of substrate in each reaction begins to vary because substrate is converted into product at different rates - this ensures that the difference in reaction rate is only caused by the difference in enzyme concentration

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