ΔG, Redox, Acid–Base, Enzyme Kinetics, Michaelis–Menten, and Inhibition.

Gibbs Free Energy (ΔG)

Energy that determines whether a reaction is spontaneous at constant temperature and pressure

ΔG equation

ΔG = ΔH − TΔS

Meaning of ΔG < 0

Reaction is spontaneous

Meaning of ΔG > 0

Reaction is non-spontaneous

Meaning of ΔG = 0

System is at equilibrium

Standard Gibbs Free Energy (ΔG°′)

ΔG measured under standard biochemical conditions (pH 7, 1 M, 25°C)

Relationship between ΔG and ΔG°′

ΔG = ΔG°′ + RT ln Q

Reaction quotient (Q)

Ratio of products to reactants at any given time

Effect of concentrations on ΔG

High product concentration increases ΔG, high reactant concentration lowers ΔG

Coupled reactions

Endergonic reactions proceed when coupled to exergonic reactions like ATP hydrolysis

ATP hydrolysis ΔG

Highly negative ΔG, drives unfavorable biochemical reactions

Redox reaction

Reaction involving transfer of electrons

Oxidation

Loss of electrons

Reduction

Gain of electrons

OIL RIG mnemonic

Oxidation Is Loss, Reduction Is Gain

Reducing agent

Donates electrons and gets oxidized

Oxidizing agent

Accepts electrons and gets reduced

NAD+ role

Electron acceptor, becomes NADH

NADH role

Electron donor in metabolism

FAD role

Electron acceptor, becomes FADH2

FADH2 role

Transfers electrons to ETC

Redox in TCA cycle

Isocitrate and α-ketoglutarate reactions produce NADH

Acid

Base that donates a proton (H+)

Base

Substance that accepts a proton (H+)

pKa

pH at which 50% of a group is protonated

pH > pKa

Group is deprotonated

pH < pKa

Group is protonated

Henderson-Hasselbalch equation

pH = pKa + log([A−]/[HA])

Physiological pH

Approximately 7.4

Principal species

Most abundant protonation state at a given pH

Aspartic acid at pH 7.4

Deprotonated and negatively charged

Lysine at pH 7.4

Protonated and positively charged

Histidine side chain pKa

Approximately 6.0, can be protonated or deprotonated at physiological pH

Enzyme kinetics

Study of rates of enzyme-catalyzed reactions

Michaelis-Menten equation

v = (Vmax[S]) / (Km + [S])

Vmax

Maximum reaction velocity at saturating substrate

Km

Substrate concentration at 1/2 Vmax

Meaning of low Km

High substrate affinity

Meaning of high Km

Low substrate affinity

When [S] = Km

Reaction velocity equals 1/2 Vmax

When [S] >> Km

Reaction velocity approaches Vmax

Effect of enzyme concentration on Vmax

Increasing enzyme concentration increases Vmax

Effect of enzyme concentration on Km

Km does not change

Competitive inhibition

Inhibitor competes with substrate for active site

Effect of competitive inhibition on Km

Km increases

Effect of competitive inhibition on Vmax

Vmax unchanged

How to overcome competitive inhibition

Increase substrate concentration

Noncompetitive inhibition

Inhibitor binds allosteric site

Effect of noncompetitive inhibition on Vmax

Vmax decreases

Effect of noncompetitive inhibition on Km

Km unchanged

Uncompetitive inhibition

Inhibitor binds enzyme-substrate complex

Effect of uncompetitive inhibition on Vmax

Vmax decreases

Effect of uncompetitive inhibition on Km

Km decreases

Most common inhibition tested

Competitive inhibition

Graph effect of competitive inhibitor

Right shift of curve, same Vmax

Graph effect of noncompetitive inhibitor

Lower Vmax, same Km

High-yield enzyme exam strategy

Check whether Km or Vmax changes to identify inhibitor type