Lecture 7: Enzymes are Essential for Life (Bioc 192)

Flashcards covering enzyme roles, activation energy, cofactors, enzyme classes, reaction coupling, and how metabolism maintains non-equilibrium conditions.

What is the general role of enzymes in biology?

To catalyse biochemical reactions by lowering activation energy, thereby increasing reaction rates without changing the overall Gibbs free energy (ΔG) or the equilibrium position.

Do enzymes alter the equilibrium constant or ΔG of a reaction?

No. They accelerate the forward and reverse reactions equally and do not change the reaction's equilibrium position or ΔG.

What is the activation energy and how do enzymes affect it?

Activation energy is the energy barrier to reach the transition state; enzymes lower this barrier to speed up the reaction.

What does it mean that life is not at equilibrium but can be at a steady state?

Biological systems maintain ongoing fluxes and dissipate energy, staying far from equilibrium but in a steady state that enables work.

What is reaction coupling in biochemistry?

Attaching a non-spontaneous (endergonic) step to a spontaneous (exergonic) one so the overall process has ΔG < 0; often uses energy from ATP hydrolysis to drive the reaction.

Provide an example of enzymatic reaction coupling involving glucose phosphorylation.

Hexokinase couples glucose phosphorylation to ATP hydrolysis, converting glucose plus ATP to glucose-6-phosphate plus ADP.

Name the six major classes of enzymes.

Oxidoreductases; Transferases; Hydrolases; Lyases; Isomerases; Ligases.

What are cofactors and what are the two main classes?

Cofactors are non-protein helpers required for catalysis; the two main classes are metal ions and coenzymes.

What roles do metal ions play in enzyme catalysis?

They act as Lewis acids to participate in acid–base catalysis and help position substrates by forming precise coordination geometries (e.g., Mg2+, Zn2+, Fe2+/Fe3+).

What are coenzymes?

Small organic molecules that function as cofactors, often derived from vitamins, and act as carriers for electrons, atoms, or functional groups.

Give examples of common coenzymes and their roles.

NAD+/NADP+, FAD, CoA, biotin, PLP, THF, thiamine pyrophosphate, etc.—they carry electrons/functional groups in enzymatic reactions.

What is PLP and how is it involved in glycogen phosphorylase activity?

Pyridoxal phosphate (PLP) is a cofactor that facilitates glycogen phosphorylase activity; it is covalently linked to a lysine in the enzyme and participates in proton transfer during the reaction.

What is reaction coupling and how do enzymes mediate it (conceptually)?

Enzymes couple an exergonic reaction to an endergonic one so the overall reaction is favorable; energy released from the exergonic step drives the endergonic step.

How can the activation energy be lowered according to the notes?

Via ground state destabilisation and transition state stabilisation, often achieved by an active site that complements the transition state.

What are the two general strategies to lower the transition state's energy?

Ground state destabilisation and transition state stabilisation.

Why are cofactors often required in your diet?

Many cofactors (e.g., vitamins that synthesize coenzymes) must be obtained from the diet because organisms cannot synthesize them all.

What does 'spontaneous' mean in the context of Gibbs free energy?

Spontaneous refers to ΔG < 0 (energy release); it does not imply a fast reaction rate.

What is the role of the metabolic network in maintaining non-equilibrium conditions?

It consists of enzyme-catalyzed steps that keep most individual reactions away from equilibrium, enabling useful work and a net negative ΔG overall.

Do enzymes typically act in isolation or within networks?

They act as part of networks and pathways; individual enzymes rarely work alone.