Enzymes

Question 1

How do enzymes work?

Answer 1

• They work by lowering the activation energy required for the molecular reaction to take place
• To do this, the molecule must interact specifically with the substrate molecule or one or a small number of closerly related substrates
• The molecule (or substrate) fits within a particular cavity on the enzyme (active site) and is attracted to particular points of charge within the cavity

Question 2

Co-factors

Answer 2

• The catalytical activity of many enzymes depends on the presence of components called co-factors
• They assist biochemical transformations
• They can either be small molecules (organic) or metals (non organic)
• Loosely-bound cofactors are called coenzymes
• Tightly-bound cofactors are called prosthetic groups

Question 3

Examples of co-enzymes and prosthetic groups

Answer 3

• Prosthetic - small inorganic ions, mostly metal ions, act as activators and/or inhibitors of activity
• Co-enzymes - small non-protein molecules that catalyse reactions, transfer electrons, form or break a covalent bond, transfer a group, NAD+/NADH, CoA, vitamins

Question 4

Binding site specificity

|Two models

Answer 4

• ‘Lock-and-key’ model: 3D shape recognizes substrate
• ‘Induced-fit’ model: enzyme conformational change after substrate binds

Question 5

Substrate affects enzyme activity

Answer 5

1. At low concentration of substrate = steep increase in the RoR with increasing substrate concentration
2. As the concentration of substrate increase = the enzymes become saturated with substrate and the RoR stays the same, adding more substrate wont affect the RoR after this point

Question 6

Michaelis-Menten kinetics

Rate of reaction equation

Answer 6

V = (Vmax[S])/([S]+KM)
Where:
* V = mols of product per second (rate of reaction)
* Vmax = maximum observed rate
* [S] = substrate conc
* KM = k2+k3/k1 units mol dm-3 –> k1, k2, k3 are rate constants

Question 7

Michaelis-Menten kinetics

Answer 7

• If [S] < KM then the rate is proportional to [S]
• If [S] = KM then the rate is half Vmax
• If [S] > KM then the rate is Vmax

Question 8

Enzyme inhibition

Answer 8

1. Competitive inhibition = Inhibitor competes reversibly with substrate for the active site
2. Uncompetitive inhibition = Inhibitor binds only to the ES complex, leading to EIS intermediate (very rare)
3. Non-competitive inhibition = Inhibitor binds non-covalently to sites other than the active site (allosteric site), changes conformation of the active site by changing the whole shape of the protein
4. Irreversible inhibition = Irreversible inhibitors form covalent or very tight bonds with functional groups in the active site

Question 9

Michaelis-Menten kinetics

Fraction of filled active sites equation

Answer 9

Fes = V/Vmax = [S]/[S] + KM