C1.1 Properties of Enzymes

Enzymes are globular proteins that act as biological catalysts, speeding up biochemical reactions by lowering activation energy without being consumed.

he enzyme’s** tertiary structure** determines the shape and chemical properties of its active site, allowing binding to a specific substrate.

Each enzyme only catalyzes one reaction or acts on a specific substrate — the “lock-and-key” principle ensures metabolic control.

Enzymes control metabolic rate by catalyzing reactions in metabolic pathways — both catabolic (breakdown) and anabolic (synthesis) reactions.

The minimum energy required for a reaction to occur. Enzymes lower activation energy by stabilizing the transition state.

They orient reactants correctly, destabilize bonds, and form enzyme–substrate complexes that make reactions more likely.

For a reaction to occur, substrates must collide with correct orientation and sufficient energy at the enzyme’s active site.

Some are free in the cytoplasm; others are fixed in membranes (e.g. ATP synthase in mitochondria and chloroplasts).

It is a transmembrane enzyme that synthesizes ATP from ADP + Pi using a proton gradient during respiration or photosynthesis.

Exergonic reactions that break down complex molecules into simpler ones, releasing energy. Example: cellular respiration, digestion.

Endergonic reactions that build complex molecules from simpler ones, requiring energy. Example: photosynthesis, protein synthesis.

Together, they form metabolism — catabolism provides ATP energy used in anabolism.

Anabolism → condensation reactions (release water) Catabolism → hydrolysis reactions (use water)

When a substrate binds, the enzyme undergoes a conformational change, tightening the fit and lowering activation energy.

A temporary structure formed when the substrate binds to the active site, positioning reactants to form products efficiently.

Intermediate formed after catalysis; products are released and the enzyme returns to its original shape.

X-ray crystallography and inhibitor studies show enzymes are flexible and alter shape when interacting with substrates.

RuBisCo (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes carbon fixation in the Calvin cycle, attaching CO₂ to RuBP.

Its active site is not perfectly specific — it can bind both CO₂ and O₂, leading to photorespiration.

A loss of tertiary structure and active site shape, caused by extreme temperature or pH, leading to loss of function.

Hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions stabilize the tertiary structure.

The active site changes shape so the substrate no longer fits — the reaction stops.

The specific temperature and pH where an enzyme’s activity is highest.

When a moderately denatured enzyme regains its structure and activity after returning to normal pH or temperature.