Enzymes Flashcards
Are all cofactors required for enzyme activity?
All apart from co-enzymes
What are the three types of cofactors?
Inorganic cofactors, co-enzymes, prosthetic groups
Where are inorganic cofactors obtained from?
via the diet as minerals usually ions
Do inorganic cofactors bind temporarily or permanently?
may be either as may bind temporarily or may be a permanent part of an enzyme
Where are co-enzymes derived from?
vitamins, organic molecules
Do co-enzymes bind temporarily or permanently to an enzyme?
temporarily
What is the role of a co-enzyme?
transfer substances between molecules to increase the rate of reaction/enzyme activity
Do prosthetic groups bind permanently or temporarily to an enzyme?
permanently bound to an enzyme to form a permanent feature of an enzyme
How may inorganic cofactors and prosthetic groups increase an enzyme’s rate of activity?
- they change the shape/tertiary structure of the active site
- they may affect the charges of the active site
- they could bind to or interact with the substrate
An enzyme’s active site is a specific shape created by which level of protein structure?
tertiary
Is catalase an intracellular or extracellular enzyme?
intracellular
Is trypsin an intracellular or extracellular enzyme?
extracellular
What is the cofactor for carbonic anhydrase?
Zn2+
What are enzymes?
Proteins that act as biological catalysts for intra and extracellular reactions to determine structure and function. Therefore affect metabolism of cells and whole organism.
Specific tertiary structure determines shape of active site, complementary to a specific substrate.
Formation of enzyme-substrate complexes lowers activation energy of metabolic reactions.
Give an example of an enzyme that catalyses intracellular reactions.
Catalase: catalyses decomposition of hydrogen peroxide (which causes oxidative stress) into water and oxygen
Give two examples of enzymes that catalyse extracellular reactions
Amylase: carbohydrase catalyses 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 ES 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 so is only complementary to 1 substrate. Formation of ES complex lowers activation energy.
Bonds in enzyme-product complex are weak, so product desorbs.
Name 5 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 ES 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 ES 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 H bonds in tertiary structure break = active site no longer complementary to substrate (denaturation).
What is the temperature coefficient?
Q10 measures the change in rate of reaction per 10 degrees C temperature increase.
Q10 = R2/R1 (where R represents rate)
How does pH affect rate of reaction?
Enzymes have a narrow optimum pH range.
Outside range, H+/OH- ions interact with H-bonds and ionic bonds in tertiary structure = denaturation
How do competitive inhibitors work?
Bind to active site since they have similar shape to substrate. Temporarily prevent ES 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 on 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 formation of ES complexes. Heavy metal ions e.g. mercury, silver cause disulphide bonds in tertiary structure to break. Bind to enzymes by strong (covalent) bonds e.g. cyanide binds to cytochrome C
What are reversible inhibitors?
May be competitive or non-competitive. Bind to enzyme temporarily e.g. by H-bonds or a few ionic bonds. ES complexes can form after the inhibitor is released.
Define metabolic poison.
Substance that damages cells by interfering with metabolic reactions. Usually an inhibitor.
Give some examples of metabolic poisons
Respiratory inhibitors include:
- cyanide: non-competitive, irreversible, inhibits cytochrome c oxidase
- 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 e.g. proteases.
One part of the precursor acts as an inhibitor. ES 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 transport molecules or electrons between enzymes. Frequently derived from water-soluble vitamins e.g. the hydrogen acceptor NAD is derived from niacin
What are inorganic cofactors? Give an example
Facilitate temporary binding between substrate and enzyme. Often metal ions e.g. Cl- is the cofactor for amylase
What are prosthetic groups? Give an example
Tightly-bound cofactors act as a permanent part of enzyme’s binding site e.g. Zn2+ for carbonic anhydrase
Suggest how a student could produce a desired concentration of solution from a stock solution.
volume of stock solution = required concentration x final volume needed / concentration of stock solution
volume of distilled water = final volume needed - volume of stock solution
