cutting FBP by Aldolase

Question 1

aldolase has what kind of rxn?

Answer 1

retro-aldol (aldol in reverse direction for glycolysis)
This reaction is at equilibrium, can be used in gluconeo as an aldol

Question 1

Class I aldolase

Answer 1

Class I aldolase is a homotetramer that uses a schiff base, also known as a imine, for its intermediate. (schiff base is a protonated imine, it is called an iminium ion)
Lysine acts as an electron sink in this case, N is the actual e sink (N is less electronegative, so it can accomodate a + charge)

Question 2

Why did PGI occur?

Answer 2

PGI isomerization occurred because you need correctly positioned carbonyl with an adjacent OH to break into equal sized Pi compounds.

Cutting the sugar ring in half.

Question 3

lysine as physiological pH

Answer 3

Lys at physiological pH would expect to be 100% protonated because 7 (phys pH) is two orders of magnitude to the left (acidic) side of the pka of Lys.

But this does not happen. Why?
- if it was, no way lys Amino group can be a nucleophile.

Pka has been lowered because its in the active site (isolated, act like organic solvent)

Question 4

What does lys form in the “sugar cutting” mechanism

Answer 4

Lys forms iminium (protonated imine/schiff base), then an enamine

Why are iminium and enamine better than an enolate?
iminium better EWG than C=O in enolate due to positive charged on N.

Question 5

Why is an enamine more stable than an enolate?

Answer 5

• enamine is a neutral, not charged, species.

Question 6

How is aldolase stereospecific?

Answer 6

occurs after step 3.

Enamine has two faces due to double bond
one of those faces if protonated to give a new tetrahedral center. (pro-S)