These are detailed Revision Notes for Topic 2.1.4 of OCR A-Level Biology (Enzymes). They were written by me using a combination of the textbook and class notes. I will also be uploading the other topics and creating bundles.
Topics Included:
- Enzyme action
- Factors affecting enzyme activity
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Enzymes
4.1 Enzyme action
4.1 Enzyme Action
- Enzymes are biological catalysts. They are large globular 4.2 Factors affecting enzyme activity
proteins with a highly folded polypeptide chain that 4.3 Enzyme inhibitors
interact with substrate molecules causing them to react
at much faster rates. 4.4 Cofactors, coenzymes and prosthetic groups
- Catabolic enzymes are enzymes that break down
molecules such as in hydrolysis reactions. E.g., amylase in digestion
- Anabolic enzymes are enzymes that build molecules such as in condensation reactions. E.g., polymerase
in protein synthesis.
- Reactions usually happen as part of a multi-step pathway.
- All biological reactions require an unput of free energy to get them started, this is called activation
energy.
- Enzymes lower the activation energy and provide an alternate route. The reaction will be more likely to
occur at lower temperatures and much more often as more collisions of the substrate with the
enzymes active site will have enough energy to react.
- The shape of the substrate and the active site are complementary to each other, only a substrate
with a complementary shape will fit and bind to the active site. Different molecules have different
shapes so require different shaped active sites. This is what makes enzymes specific.
- Lock and key hypothesis ~ When the substrate is bound to the active site an enzyme-substrate
complex is formed. The substrate then reacts, and the products are then released., leaving the enzyme
unchanged and able to take part in subsequent reactions. The R-groups within the active site of the
enzyme will also interact with the substrate, forming temporary bonds. These put strain on the bonds
within the substrate, which also helps the reaction along.
- Induced-fit hypothesis ~ When the substrate collides with the enzyme it goes into the active site and
binds to it with ionic and hydrogen bonds making the enzyme substrate complex. The binding of the
substrate to the active site causes a change in the tertiary structure of the enzyme’s active site. This
means the active site and substrate may bond more effectively and stress is placed on the bonds in
the substrate or exposes certain parts of it so that the reaction is more likely to happen (lowering the
activation energy).
- Extracellular enzymes are secreted outside the cell, often to digest large molecules so they can then be
absorbed and used inside cells. E.g., amylase, maltase, trypsin and pepsin
- Intracellular enzymes act inside the cell, either in a functional role (e.g., needed for respiration) or
structural (e.g., synthesising polymers from monomers). E.g., catalase breaks down toxic hydrogen peroxide
which is made in many metabolic reactions inside cells.
- Digestion of Starch
o Starch polymers are broken down into maltose. This is done by amylase which is produced by
the salivary glands and the pancreas. It is released in the saliva and in pancreatic juice into the
small intestine.
o Maltose is then broken down into glucose. This is done by maltase which is present in the small
intestine.
- Digestion of Proteins
o Trypsin is a protease which catalyses the digestion of proteins into smaller peptides, which can
then be broken down into further amino acids by other proteases. Trypsin is produced in the
pancreas and released with the pancreatic juice into the small intestine, where it works on
proteins.
o The amino acids that are produced by the action of proteases are absorbed by the cells lining
the digestive system and then absorbed into the bloodstream.
4.2 Factors Affecting Enzyme Activity
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