Chapter 3: Enzymes

VCE Unit 3 Biology

enzyme

organic catalysts, usually a protein, that bind to a substrate in its active site which is complementary in shape to the substrate

substrate

molecule (reactant) undergoing an enzyme facilitated reaction forming a product/s

active site

The part of an enzyme where the chemical reaction occurs.

enzyme substrate complex

A temporary complex formed when an enzyme binds to its substrate molecule(s). (conformational change takes place)

features of an enzyme

reusable, specific, reversible, speeds up reactions, active site, proteins, subset of catalysts, act on entire biochemical pathways

reusable

enzymes are not consumed in reactions and can be used in multiple reactions

specific

most enzymes catalyse only one specific reaction by binding to a specific substrate

reversible

these reactions can often proceed in both direction (anabolic/catabolic)

anabolic

two/more smaller molecules combine to form a larger one; energy is stored

catabolic

larger molecules break down into two/more smaller molecules; energy is released

subset of catalysts

all enzymes are catalysts, but not all catalysts are enzymes

biochemical pathways (enzymes)

enzymes often catalyse multiple steps in a biochemical pathway

things to remember about enzymes

always end in -ase and are displayed above the reaction arrow

activation energy

the minimum amount of energy required for a reaction to occur.

enzymes' role in activation energy

presence of an enzyme lowers the activation energy by bringing reactants closer to the state they need to be to react, ultimately, increasing the rate of reaction.

factors that affect enzymes

temperature, pH, substrate and enzyme concentration, competitive and non-competitive inhibition and co-enzymes

too cold temperature

• enzyme activity decreases as molecules move slower and collide less frequently

• little to no activity occurs, but freezing is reversible

optimal temperature

• increased temp boosts kinetic energy, leading to more frequent collisions and the formation of enzyme-substrate complexes

too hot temperature

• enzymes denature as high temps break hydrogen bonds

• active site changes shape, preventing substrate binding

• denaturation is irreversible and the enzyme loses function permanently

pH

• when pH gets too acidic or basic for an enzyme, it can denature

optimal pH

optimal pH ranges of different enzymes vary greatly depending on where the enzymes are located

enzyme concentration

• higher concentration of enzymes compared to substrate leads to a faster reaction

• more enzymes = more active sites for substrates to bind to

what happens when enzymes exceed available substrates?

the reaction rate plateaus even with more enzymes

substrate concentration

• increasing substrate concentration increases reaction rate

• more collisions occur between enzyme and substrate, forming more enzyme-substrate complexes

• eventually, all active sites become saturated (full) and reactions plateaus

competitive inhibition

• occurs when an inhibitor binds to an enzyme's active site, blocking the substrate, preventing substrate binding meaning no reaction takes place

• must have a similar shape to the substrate to fit into the active site

non-competitive inhibition

• aka allosteric inhibition

• when an inhibitor binds to an enzyme at a site other than the active site (allosteric site), causing conformational change in the active site of the enzyme, preventing the substrate from binding to it, meaning the reaction does not occur

cofactors

• non protein molecules that assist enzymes which can be inorganic/organic

coenzymes

• organic, non-protein molecules that are a type of cofactors that carry electrons, energy or chemical groups during reactions

• they bind to the enzyme's active site, donate energy or molecules, and cannot be reused immediately