2.4 - Enzymes Flashcards
What is an enzyme?
Biological catalyst. Speeds up chemical reactions by lowering activation energy. They are not used up in the reaction/remain unchanged. Intracellular - act within the cell. Extracellular - act outside the cell. Specific to one substrate(s)/reaction.
Give an example and function of an intracellular enzyme.
Catalase - breaks down hydrogen peroxide, formed during metabolic processes, into water and oxygen.
Four polypeptide chains and a haem group.
Held in vesicles in white blood cells, breaks down ingested pathogens.
Give an example and function of an extracellular enzyme.
Amylase - produced in salivary glands, hydrolysis amylose to maltose.
Trypsin - made in pancreas, released into lumen of small intestine; hydrolysis peptide bonds.
What is a cofactor?
A substance that must be present to ensure enzyme reaction takes places. Prosthetic group or coenzyme.
What is a prosthetic group?
Molecule permanently bound to enzyme molecule via covalent bond. Carbonic anhydrase has zinc ion bound to active site.
Define a cofactor.
Ion or organic coenzyme not permanently bound to active site. Temporary bonds form between enzyme or substrate to ease formation of enzyme - substrate complex. Co-substrates bind to substrate to enable substrate to form correct complement to active site. Cofactor may change distribution of charges on surface of substrate.
What is a coenzyme?
Small organic, non-protein molecule. Binds temporarily to active site of enzyme. Coenzyme altered by reaction and is recycled to original state.
What is the active site?
Cleft or pocket on surface of enzyme. Complementary to shape of substrate.
What is an enzyme substrate complex?
Enzyme with substrate(s) held in active site. Joined by non-covalent forces.
What is an enzyme product complex?
Enzyme with product(s) held in active site. Joined by non-covalent forces.
Outline the lock and key hypothesis.
Tertiary structure of enzyme gives active site a shape that is exactly complementary to specific substrate. Substrate fits exactly into active site. Temporary hydrogen bonds form ESC. Substrate broken into product, products leaves active site.
Note: if more than one substrate enters active site to form larger product, an enzyme product complex forms as an intermediary stage before product is released.
Outline the induced fit hypothesis.
Active site has shape that complements the substrate but is not an exact fit when substrate not bound. On binding of substrate, interactions between R groups lining active site and surface of substrate cause conformational change. Enzyme active site molds around substrate. ESC forms. Hydrogen bonds, ionic interactions and hydrophobic interactions bind substrate to active site. Substrate converted to product, product remains in active site, enzyme product forms. Product has different conformation to substrate, active site relaxes. Product released.
Describe the relationship between heat and kinetic energy and enzyme catalysed reactions.
An increase in heat leads to an increase in kinetic energy. Enzyme and substrate gain kinetic energy. An increase in successful collisions leads to an increase in ESC. More product is released. Until an optimum temperature is reached.
Describe what happens to the rate of reaction as the temperature rises above the optimum.
Molecules vibrate as kinetic energy increases. Hydrogen bonds break and tertiary structure of enzyme active site alters. Substrate no longer fits into active site and rate of reaction decreases. As temperature rises further ionic interactions and disulphide bonds break and enzyme active site is irreversibly altered. The enzyme is denatured. Heat does not break the peptide binds that hold the primary structure.
What is the temperature coefficient?
For most enzyme catalysed reactions - between 0°C and 40°C the rate of reaction will double for every 10°C increase. Above the optimum temperature the coefficient drops as the enzyme active site denatures.
Q10 = rate of reaction at (x + 10) °C divided by rate of reaction at x °C.