inhibitors, prosthetic groups

what are reversible inhibitors?

• enzyme’s activity is reduced/stopped temporarily

• competitive inhibitors: similar shape to substrate molecules and compete with substrate for active site; reduces rate of reaction for given substrate concentration

• non competitive inhibitors: bind to enzyme at allosteric site which alters active site’s shape and prevents substrate binding; increasing substrate/enzyme concentration has no effect but increasing inhibitor concentration decreases rate of reaction

what is end product inhibition?

• enzyme inhibition that occurs when reaction’s product acts as inhibitor to enzyme that produces it

• metabolic reactions are controlled using end product as non competitive, reversible inhibitor

• as enzyme converts substrate into product, process slows down as end product binds to alternative site and prevents formation of enzyme substrate complexes

• end product detaches from enzyme, allowing active site to reform and enzyme returns to active state; enzyme catalyzes reaction in continuous feedback loop

what’s an example of end product inhibition?

• respiration is metabolic pathway resulting in ATP production

• first, addition of two phosphate groups to glucose molecule and then addition of second phosphate group which results in breakdown of glucose catalyzed by PFK - competitively inhibited by ATP

• when ATP levels are high, more ATP binds to allosteric site on PFK, preventing addition of second phosphate group to glucose; not broken down and ATP isn’t produced at same rate

• as ATP is used up, less binds to PFK and enzyme catalyzes addition of second phosphate group to glucose; respiration resumes, leading to more ATP production

what are non reversible inhibitors?

• inhibitors which form covalent bonds with enzymes, inhibiting them permanently; results in complete inactivation of enzyme

• dangerous as biological reactions in organisms can be completely stopped; to avoid, organism should produce more enzyme that’s being inhibited (occurs by transcribing/translating genes)

• some non reversible inhibitors are considered as metabolic poisons e.g. cyanide as it stops metabolic reactions

what are cofactors?

• components which contain non protein substance which changes tertiary structure

• require inorganic ions to function - inorganic cofactors; stabilizes enzyme’s structure or take part in reaction at active site

what are coenzymes?

• larger organic cofactors where they’re permanently/temporarily bound to enzyme; carry electrons or chemical groups between enzymes

• link different enzyme catalyzed reactions into sequence during metabolic processes e.g. vitamins

what are prosthetic groups?

• cofactors which contain permanent part of enzyme’s structure they assist; essential to enzyme proper function as helps form final 3D shape e.g. zinc ion in carbonic anhydrase

• inactive precursor enzymes where these enzymes cause damage within cells; often undergo change in shape by adding cofactor

• before cofactor is added, precursor protein is called apoenzyme and when added and activated, it’s called holoenzyme

• changes in conditions e.g. pH results in tertiary structure change called proenzymes