L3 - Cell metabolism and metabolic control

Question 1

Control of enzyme

Answer 1

Amount (“coarse control”) and activity (“fine control”)

Question 2

Substrate equations

Answer 2

E + S ⇌ ES -> E + P

MM: Vₘₐₓ[s]/Kₘ[S]

Question 3

MM graph: what is km equal to and what are products often used for?

Answer 3

kₘ = 1/2vₘₐₓ

Products are often used in other reactions

Question 4

Irreversible inhibition

Answer 4

Covalent modification, often at the active site

Toxic

Question 5

DFP: what is it, what is its chemical formula, and what does it do?

Answer 5

Diisopropyl fluorophosphate - Prototype for nerve gas sarin

C₆H₁₄FO₃P

Modifies serine in acetylcholinesterase by forming a covalent bond with the active bond

Question 6

Reversible inhibition: what types are there and what do they do?

Answer 6

Competitive - resembles enzyme substrate and binds active site but does not react, large excess of substrate overpowers inhibitors, though

Non-competitive - binds to inhibitor site, still allowing substrate binding but using allostery to prevent enzyme activity

Question 7

Succinate dehydrogenase

Answer 7

Important role in TCA - converts succinate (COO⁻-CH₂-CH₂-COO⁻) into fumarate (COO⁻-CH=CH-COO⁻)

Inhibited by malonate (COO⁻-CH₂-COO⁻)

Question 8

Competitive inhibition: effects on Vₘₐₓ and Kₘ

Answer 8

Vₘₐₓ unchanged - only binding is affected, RoR is unaffected
Kₘ increased - E-S binding is affected

Question 9

F1-6BP: what is it, what does it do, and what is it inhibited by?

Answer 9

Fructose 1,6-bisphosphatase

Important in gluconeogenesis - converts fructose 1,6 bisphosphate into fructose-6-phosphate

Inhibited by AMP - high AMP cell needs energy, glucose synthesis is inhibited so ATP can be produced

Question 10

Non-competitive inhibition

Answer 10

Reduces Vₘₐₓ - enzyme altered so RoR is reduced
Kₘ remains unchanged - E-S binding is unaffected

Question 11

Typical feedback regulation

Answer 11

The final end-product inhibits the first reaction, preventing intermediate build-up

Question 12

E.coli: end-product feedback

Answer 12

In the conversion of Threonine into Isoleucine, isoleucine acts as an allosteric inhibitor of threonine deaminase, the first enzyme in the pathway

Question 13

Sequential feedback inhibition

Answer 13

Occurs in branched pathways, and follows the same sort of process as typical feedback regulation

Question 14

DAHP synthase: what does it do and what is it inhibited by?

Answer 14

Catalyses Phosphoenolpyruvate and Erythrose-4-phosphate joining

End-product tryptophan does not inhibit DAHP synthase, the first intermediate that branches towards it does (chorismate)

End-product phenylalanine/tyrosine do not inhibit DAHP synthase, the first intermediate that branches towards them does (prephenate)

Question 15

Nested feedback inhibition

Answer 15

Single enzymes:
* Regulatory enzymes - binding sites for several inhibitors
* Multi-product inhibition

Question 16

Purine biosynthesis: what are the final products, what is the first enzyme in the reaction, and what inhibits this process?

Answer 16

AMP/GMP

Amidophosphoribosyl transferase

AMP/GMP separately inhibit but when both are present inhibition is much higher - this is advantageous so roughly equal amounts of AMP/GMP are produced

Question 16

Isoenzymes: what are they, what are the advantages of having them, and how are they produced?

Answer 16

Different molecular forms of the enzyme catalysing the same reaction

• Regulatory properties
• Cofactor requirements
• Localisation
• Genetic
• Post-translational modification
• Multi-subunit proteins

Question 17

Can two different inhibition pathways exist in the same process?

Answer 17

Yes, nested and sequential may exist for example