Pentose Phosphate Pathway

Pentose Phosphate Pathway (PPP)

A metabolic pathway parallel to glycolysis that generates NADPH and pentoses.

Two phases of PPP

Oxidative phase (produces NADPH) and nonoxidative phase (interconverts sugars).

Key enzyme of oxidative phase

Glucose 6-phosphate dehydrogenase (G6PDH).

Products of oxidative phase

2 NADPH, ribulose 5-phosphate, and CO₂.

Function of NADPH

Provides reducing power for biosynthesis and protects against oxidative stress.

Rate-limiting step of PPP

Dehydrogenation of glucose 6-phosphate by G6PDH.

Mode 1 of PPP operation

Ribose 5-phosphate > NADPH.

Mode 2 of PPP operation

Balanced NADPH and ribose 5-phosphate.

Mode 3 of PPP operation

NADPH > ribose 5-phosphate.

Mode 4 of PPP operation

NADPH and ATP both required.

Role of glutathione (GSH)

Neutralizes reactive oxygen species (ROS).

Effect of G6PDH deficiency

Leads to oxidative damage in RBCs and Heinz body formation.

Triggers for RBC oxidation in G6PDH deficiency

Fava beans and anti-malaria drugs.

Protective effect of G6PDH deficiency

Provides resistance to malaria by limiting parasite growth.

Essential amino acids

9 amino acids that must be obtained from the diet.

Transamination

Transfer of an amino group catalyzed by aminotransferases.

Vitamin required for transamination

Vitamin B6.

Glutamate dehydrogenase

Catalyzes reversible conversion between glutamate and α-ketoglutarate.

Alanine's role in gluconeogenesis

Converted to pyruvate via alanine aminotransferase (ALT).

Ketogenic amino acids

Leucine and lysine.

Cumulative feedback inhibition

Multiple products inhibit a committed step independently.

Amino acid derivatives

Include histamine, serotonin, epinephrine, nitric oxide, glutathione.

Histidine derivative

Histamine.

Tryptophan derivative

Serotonin.

Tyrosine derivatives

Epinephrine and norepinephrine.

Arginine derivative

Nitric oxide.

Glutathione composition

Tripeptide made of glutamate, cysteine, and glycine.

Function of ALT

Converts alanine to pyruvate for gluconeogenesis.

PPP in RBCs

Critical for maintaining reduced glutathione and preventing oxidative damage.

Nonoxidative phase enzymes

Transketolase and transaldolase.

Transketolase cofactor

Thiamine (Vitamin B1).