protein function and enzymes

Monomer

Single polypeptide chain

Homodimer

Two identical protein subunits

Heterodimer

Two different protein subunits

Tetramer

Protein complex of four subunits

Quaternary structure

Spatial arrangement of multiple polypeptide chains

Secondary structure

Alpha helices and beta sheets stabilized by hydrogen bonds

Myoglobin function

Oxygen storage in muscle

Haemoglobin function

Oxygen transport in blood

Myoglobin structure

Monomer

Haemoglobin structure

Heterotetramer (2 alpha, 2 beta chains)

Oxygen affinity (MvsH)

Myoglobin > Haemoglobin

Cooperative binding

Increased oxygen affinity after initial binding

Protein showing cooperativity

Haemoglobin

Allosteric protein

Haemoglobin

T state

Low oxygen affinity state

R state

High oxygen affinity state

Oxygen-binding curve of haemoglobin

Sigmoidal

Oxygen-binding curve of myoglobin

Hyperbolic

Bohr effect

Decreased oxygen affinity at low pH / high CO2

Physiological role of Bohr effect

Promotes oxygen release in tissues

Haem group

Protoporphyrin IX with Fe2+

Effect of O2 binding on Fe2+

Fe2+ moves into haem plane

Enzyme

Biological catalyst that lowers activation energy

Activation energy (ΔG‡)

Energy barrier to reach transition state

Effect of enzymes on ΔG

No change

Transition state

Highest-energy unstable intermediate

Active site

Region where substrate binds and catalysis occurs

Enzyme specificity

Due to shape and chemistry of active site

Michaelis–Menten kinetics

Relationship between reaction rate and substrate concentration

Vmax

Maximum reaction rate at enzyme saturation

Km

Substrate concentration at Vmax/2

Km and affinity

Low Km = high affinity

Saturation

All enzyme active sites occupied

Shape of V vs [S] plot

Hyperbolic

📉 Lineweaver–Burk Plot (EXAM FAVOURITE)

Lineweaver–Burk plot

Double reciprocal plot (1/V vs 1/[S])

Y-intercept

1/Vmax

X-intercept

−1/Km

Competitive inhibition effect on Km

Increases Km

Competitive inhibition effect on Vmax

No change

Effect on Lineweaver–Burk slope

Increases

Competitive inhibition

Inhibitor competes with substrate for active site

Non-competitive inhibition

Inhibitor binds away from active site

Allosteric regulation

Regulator binding causes conformational change

Covalent modification

Regulation by phosphorylation or acetylation

Example of covalent drug action

Aspirin acetylates cyclooxygenase

Cofactor

Non-protein molecule required for enzyme activity

Coenzyme

Loosely bound organic cofactor

Inorganic cofactors

Metal ions (e.g. Mg2+, Zn2+)

Apoenzyme

Enzyme without cofactor

Holoenzyme

Enzyme with cofactor bound

Why enzymes are good drug targets

Small activity change gives large product change

Statins

Competitive inhibitors of cholesterol synthesis enzymes

NSAIDs

Inhibit cyclooxygenase

Penicillin

Inhibits bacterial cell wall synthesis enzymes