Pentose Phosphate Pathway

Starting substrate of PPP

Glucose 6-phosphate

Where does PPP take place?

In the cytosol of lipogenic tissues

Oxidative phase of PPP

All steps are irreversible

Step 1 of OxPPP

Glucose 6-phosphate → 6-phosphogluconolactone

Enzyme used in step 1 of OxPPP

glucose 6-phosphate dehydrogenase

Step 2 of OxPPP

6-phosphogluconolactone → 6-phosphogluconate

Enzyme used in Step 2 of OxPPP

6-phosphogluconolactonase

Step 3 of OxPPP

6-phosphogluconate → ribulose 5-phosphate

Enzyme used in Step 3 of OxPPP

6-phosphogluconate dehydrogenase

Step 4 of OxPPP

Ribulose 5-phosphate → xylulose 5-phosphate and ribose 5-phosphate

Which steps of OxPPP produce NADPH?

Step 1 and 3

Which step of OxPPP produce 1CO2

Step 3

Non-oxidative phase of PPP

All steps are reversible

First conversion of nonOxPPP

Transfer of 2-C carbon from xylulose 5-phosphate to ribose 5-phosphate

Second conversion of nonOxPPP

Transfer of 3-C fragment from another xylulose 5-phosphate to glyceraldehyde 3-phosphate

Products of second conversion of nonOxPPP

erythrose 4-phosphate and fructose 6-phosphate

Third conversion of nonOxPPP

Transfer of 2-C fragment from another xylulose 5-phosphate to erythrose 4-phosphate

Products of third converison of nonOxPPP

glyceraldehyde 3-phosphate and fructose 6-phosphate

Transketolase

catalyzes transfer of 2-C reactions

Transaldolase

catalyzes transfer of 3-C fragment

Activity of PPP

Depends on NAPDH/ribose 5-phosphate demand and glucose 6-phosphate provision

Rate-limiting step of OxPPP

Step 1

What activates glucose 6-phosphate dehydrogenase?

NADP+

What inhibits glucose 6-phosphate dehydrogenase?

NADPH

What controls direction of PPP?

Levels of pentose phosphates and NADPH

Pentose-insufficiency mode (ribose needs unmet)

Net non-oxPPP towards ribose 5-phosphate synthesis

Pentose Overflow Mode (NADPH demand > ribose demand)

Glucose directed towards oxPPP to produce NADPH, nonOxPPP consumes ribose 5-phosphate to produce glycolytic intermediates

Pentose Cycling Mode (NADPH demand >>> ribose demand)

Glycolytic intermediates redirected up glycolysis pathway → regenerate glucose 6-phosphate → reused by OxPPP to regenerate NADPH