Enzymes Flashcards
(25 cards)
What are enzymes?
Proteins that act as biological catalysts for intra and extracellular reactions.
They affect metabolism of cells and whole organism.
Specific tertiary structure determines shape of active site, complementary to specific substrate.
Formation of enzyme-substrate complexes lowers activation energy of metabolic reactions.
What is an example of an enzyme that catalyses intracellular reaction?
Catalase: catalyses decomposition of hydrogen peroxide (which causes oxidative stress) into water + oxygen.
Give 2 examples of enzymes that catalyse extracellular reactions.
Amylase: carbohydrase catalyse digestion of starch to maltose in saliva/ small intestine lumen.
Trypsin: pancreatic endopeptidase catalyses hydrolysis of peptide bonds in small intestine lumen.
Explain the induced fit model of enzyme action.
Shape of active site is not directly complementary to substrate and is flexible.
Conformational change enables enzyme-substrate complexes to form when substrate adsorbs.
This puts strain on substrate bonds, lowering activation energy. Bonds in enzyme-product complex are weak, so product desorbs.
Explain the lock and key model of enzyme action.
Suggests that active site has a rigid shape determined by tertiary structure to is only complementary to one substrate. Formation of enzyme-substrate complex lowers activation energy.
Bonds in enzyme-product complex are weak so product desorbs.
What are five factors that affect the rate of enzyme-controlled reactions?
Enzyme concentration.
Substrate concentration.
Concentration of inhibitors.
pH.
Temperature.
How does substrate concentration affect rate of reaction?
Given that enzyme concentration is fixed, rate increases proportionally to substrate concentration.
The rate levels off when maximum number of enzyme-substrate complexes form at any given time.
How does enzyme concentration affect rate of reaction?
Given that substrate is in excess, rate increases proportionally to enzyme concentration.
Rate levels off when maximum number of enzyme-substrate complexes form at any given time.
How does temperature affect the rate of enzyme-controlled reactions?
Rate increases as kinetic energy increases and peaks at optimum temperature.
Above optimum, ionic and hydrogen bonds in break= active site no longer complementary to substrate (denaturation).
What is the temperature coefficient?
Q₁₀ measures the changes in rate of reaction per 10⁰C temperature increases.
Q₁₀ = R₂/ R₁ (where R represents rate)
How does pH affect rate of reaction?
Enzymes have narrow optimum pH range.
Outside range, H⁺/ OH⁻ ions interact with H-bonds and ionic bonds in structure = denaturation.
How do competitive inhibitors work?
Bind to active site since they have similar shape to substrate. Temporarily prevent enzyme-substrate complexes from forming until released.
Increasing substrate concentration decreases their effect.
How do non-competitive inhibitors work?
Bind at allosteric binding site.
Trigger conformational change of active site.
Increasing substrate concentration has no impact of their effect.
What is end-product inhibition?
One of the products of a reaction acts as a competitive or non-competitive inhibitor for an enzyme involved in the pathway.
Prevents further formation of products.
What are irreversible inhibitors?
Permanently prevent formations of enzyme-substrate complexes.
Heavy metal ions (e.g. mercury, silver) cause disulphide bonds in tertiary structure to break.
Binds to enzymes by strong covalent bonds.
What are reversible inhibitors?
May be competitive or non-competitive.
Bind to enzyme temporarily by hydrogen bonds or few ionic bonds.
Enzyme-substrate complexes can form after the inhibitor is released.
Define metabolic poison.
Substance that damaged cells by interfering with metabolic reactions.
Usually an inhibitor.
What are three examples of metabolic poisons?
Respiratory inhibitors include:
Cyanide: non-competitive, irreversible.
Malonate: competitive, inhibits succinate dehydrogenase.
Arsenic: competitive, inhibits pyruvate dehydrogenase.
How do some medicinal drugs act as inhibitors?
Penicillin: non-competitive inhibitor of transpeptidase to prevent formation of peptidoglycan cross-links in bacterial cell wall.
Ritonavir: inhibits HIV protease to prevent assembly of new virions.
What are inactive precursors in metabolic pathways?
To prevent damage to cells, some enzymes in metabolic pathways are synthesised as inactive precursors.
One part of the precursor acts as an inhibitor.
Enzyme-substrate complexes form when it is removed.
What are cofactors?
Non-protein compounds required for enzyme activity: coenzymes, inorganic cofactors, prosthetic groups.
What are coenzymes?
Organic cofactors. Do not bind permanently. Often transports molecules or elections between enzymes.
What are inorganic cofactors?
Facilitate temporary binding between substrate and enzyme. Often metal ions.
What are prosthetic groups?
Tightly-bound cofactors that act as a permanent part of enzyme’s binding site.
