Enzymes Flashcards
(33 cards)
What are enzymes?
Enzymes catalyse reactions that affect metabolism at a cellular and whole organism level. Enzymes are globular proteins and act as biological catalysts. They speed up reactions but are not used in the reactions, themselves.
Describe the role of enzymes.
The enzyme has an active site with a specific shape and only a substrate with a complementary shape can bind to the active site
What are intracellular reactions?
Enzymes catalyse reactions inside the cell – i.e. hydrolases in lysosomes.
Give an example of an intracellular reaction.
Catalase is an enzyme that catalyses intracellular reactions – it is found in vesicles, called peroxisomes, inside eukaryotic cells. Catalase breaks down hydrogen peroxide (H2O2), a harmful by-product of metabolic reactions, and protects the cell. It also is used to kill pathogens, when they are ingested by white blood cells.
What are extracellular reactions?
Enzymes catalyse reactions outside the cell – i.e. digestive enzymes that are released into the alimentary canal.
Give an example of an extracellular reaction.
Amylase and trypsin are enzymes that catalyse extracellular reactions. Amylase digests starch into maltose and is produced in the salivary gland. Trypsin digests proteins into smaller polypeptides, by hydrolysing peptide bonds – made in the pancreas, but used in the small intestine.
What is a turnover number?
The number of reactions per second an enzyme catalyses.
What is anabolic reaction?
Anabolic reactions – energy is used to synthesise larger molecules from smaller ones.
What is a catabolic reaction?
Catabolic reactions – metabolites are broken down into smaller molecules to release energy.
Describe the mechanism of enzymes.
Each enzyme has an active site with a specific shape, due to the specific tertiary structure, and therefore the shape of substrate must be complementary to the specific shape of the enzyme’s active site to bind to the active site.
Describe the lock-and-key model.
The shape of the active site will only allow one shape of molecule to fit it, like a key fitting into a lock. The shape of the substrate is complementary to the shape of the active site.
Describe the induced-fit model.
Substrate collides with the enzyme active site and the enzyme molecule changes its shape slightly. The active site will now fit more closely around the substrate and an ESC forms. The changing shape destabilises the substrate molecule and the reaction occurs more easily, forming products and an EPC. The products are a different shape from the substrate and so, no longer fit the active site so the products leave the active site.
Why do enzymes lower the activation energies of reactions?
Enzymes lower the activation energy of a reaction so less energy is needed for the reaction to take place.
How does pH affect enzyme activity?
Different enzymes can have a different optimum pH to each other (see graph on the right) – i.e. pepsin: pH 2, amylase: pH 7, trypsin: pH 9. Reducing/increasing the pH away from the optimum pH, will reduce the rate of reaction. This is because the concentration of the H+/OH- ions in the solution affects the tertiary structure of the enzyme, by breaking bonds between the amino acids. The substrate can no longer fit into the active site of the enzyme and therefore, ESCs will not be able to form. Extreme of pH can and will lead to denaturation of the enzyme.
How does temperature affect enzyme activity?
As temperature rises, the enzyme and substrate molecules move faster due to the increased kinetic energy. Successful collisions between the substrate and the enzyme’s active site occur more often and as a result, more ESCs form. The successful collisions also have a sufficient activation energy to react. But if the temperature is too high, the activity of the enzyme will be reduced. The structure of the enzyme molecule vibrates energetically, so the bonds between R-groups (especially hydrogen bonds or ionic bonds), holding the enzyme in its specific shape will break. The enzyme will lose its tertiary shape and is irreversibly, denatured – the substrate is no longer complementary to the active site.
How does the substrate concentration affect enzyme activity?
At first, the substrate concentration is the limiting factor. Collisions between the substrate and enzymes occur more frequently, more ESCs and products formed, the rate of reaction increases. Then, all the enzyme molecules are forming ESCs, as fast as possible but all the active sites are occupied.
Therefore, an increased rate of substrate concentration has no further effect on the rate of reaction. The enzyme concentration is now the limiting factor.
How does enzyme concentration, with excess substrate concentration, affect enzyme activity?
If there is excess substrate or substrate is continually added, there are more enzyme molecules that can react with more substrates. The rate of reaction will increase. The limiting factor will be the enzyme concentration.
What is the optimum temperature for enzymes?
An optimum temperature for enzymes is the temperature at which an enzyme catalyses at the maximum rate. It is 40C in human – but some enzymes work best in cool temperatures (i.e. psychrophilic bacteria) and some work best in high temperatures (i.e. thermophilic bacteria – found in hot springs/volcanoes).
How does enzyme concentration, with a fixed substrate concentration, affect enzyme activity?
If there is a fixed concentration of the substrate, at first there are more active sites available, so there are more successful collisions occurring between the substrate and enzyme, forming more ESCs. The limiting factor, at first, will be the enzyme concentration but the rate of reaction is increasing. When the plateau begins, all the substrate molecules have been converted into product so increased enzyme concentration will have no effect. The limiting factor now is the substrate concentration but there is no further effect on the rate of reaction.
What is a temperature coefficient (Q10)?
This refers to the increase in the rate of a process when the temperature is increased by 10C. For every increasing in 10C, the rate of reaction is approximately doubled, from 0-40C but above the optimum temperature, the Q10 will drop.
What is a cofactor?
A cofactor is a non-protein substance that are sometimes required to make an enzyme-controlled reaction take place at an appropriate rate. They alter the shape of the enzyme’s active site so that the substrate can fit more easily.
What is a coenzyme?
Vitamins are a source of coenzymes – i.e. vitamin B3.
What is a prosthetic group?
A cofactor that is a permanent part of an enzyme molecule. Contribute to the final 3D shape and charges. Zinc ions (Zn2+) are a prosthetics group for carbonic anhydrase.
What is an inorganic ion cofactor?
The presence of certain ions can increase the rate of reaction. The ions combine with the enzyme/substrate and ESCs form more easily – as the cofactor (ions) will affect the charge distribution and shape of the complex. Chlorine ions (Cl-) acts as a cofactor for amylase.