The pentose phosphate pathway

Question 1

what are the three stages of the pathway?

Answer 1

1- production of NADPH
2- isomerization of pentose
3- recovery of glycolytic intermediates

Question 2

what are the end products of this pathway?

Answer 2

NADPH (for reduction biosynthesis) and ribose-5-phosphate (for nucleotide biosynthesis)

Question 3

where does this pathway take place?

Answer 3

in the cytoplasm of the liver, adipose tissue, mammary glands, adrenal cortex

Question 4

explain the first stage? (oxidations)

Answer 4

glucose-6-phosphate dehydrogenase catalyzes the transfer of a hydride ion from G6P to form 6-phospho-glucono-lactone (A). 6-phosphoglucono-lactonase increases the rate of hydrolysis of (A) into 6-phosphogluconate(B). 6-phosphogluconate dehydrogenase catalyzes the oxidative decarboxylation of (B) into ribulose-5-phosphate and CO2

Question 5

what is the function of phosphopentose isomerase?

Answer 5

it converts ketoses to aldoses

Question 6

what is the function of phosphopentose epimerase?

Answer 6

epimerizes at C3

Question 7

explain the steps of the second stage? (epimerization and isomerization)

Answer 7

ribulose-5-phosphate is turned into ribose-5-phosphate by the enzyme ribulose-5-phosphate isomerase.
Also ribulose-5-phosphate is turned into Xu5P by the enzyme ribulose-5-phosphate epimerase

Question 8

what is the function of transketolase?

Answer 8

the transfer of 2- carbon units. it is dependent on TPP

Question 9

what is the function of transaldolase?

Answer 9

it transfers a 3 carbon unit. This is a Schiff base mechanism

Question 10

explain the third stage?

Answer 10

It is the conversion of 3 C5 carbon sugars into 2 C6 carbon sugars. Transketolase transfers a C2 unit from Xu5P to R5P forming GAP and S7P.
Transaldolase catalyses the transfer of a C3 form S7P to GAP forming E4P and F6P.
There is now a second transketolase reaction where a C2 unit is transferred forma second molecule of Xu5P to E4P to form GAP and F6P

Question 11

what is the mechanism of transketolase?

Answer 11

1- TPP adds to the Xu5P carbonyl group,
2- C2-C3 bond cleavage yields GAP and a resonance stabilized carbanion of the C2 unit.
3- C2 carbanion adds to the aldehyde carbon of R5P
4- TPP is eliminated yielding S7P and regenerating TPP.E

Question 12

what is the mechanism of transaldolase?

Answer 12

Schiff base formation, aldol cleavage, carbanion addition to carbonyl, Schiff base hydrolysis

Question 13

the rate limiting reaction is

Answer 13

G6P to 6-phosphogluconolactone by G6PDH, it is inhibited by NADPH

Question 14

what substances are common between the PPP and glycolysis

Answer 14

G6P, F6P, GAP