Topic A8

Question 1

Pentose phosphate pathway

Steps of oxidative and non-oxidative phase

Answer 1

• Under most circumstances:
  
  • The pentose phoshate pathway ends at ribose-5-P
  • Its overall equation:
    
    • Glucose-6-P + 2NADP⁺ → ribose-5-P + CO₂ + 2 NADPH + H⁺
  • The net result is:
    
    • Production of NADP-H for reductive biosynthetic reaction
    • Production of ribose as a precrusor for nucleotide synthesis
• Under other cirumstances:
  
  • When cells do not require ribose, the pentose-phosphate pathway continues further
  • Ribsoe-5-P undergoes other transformations by two additional enzymes:
    
    • Transketolase
    • Transaldolase
  • Result: complete oxidation of glucose-6-P to CO₂ by a complex sequence of reactions
  • Its overall equation:
    
    • ​Glucose-6-P + 12 NADP⁺ → 6 CO₂ + 12 NADPH + 12 H⁺ + Pⁱⁿ

Question 2

Pentose phosphate pathway

Location

Answer 2

• Each cell → cytoplasm
  
  • Liver
  • Adipose tissue
  • Adrenal cortex
  • Teats
  • Lactating mammary glands
  • Red blood cells have especially high PPP activity
• The pathway is especially intensive in tissues that acctively carry out the reductive synthesis of fatty acids
• Skeletal muscles, which is not active in synthesizing fatty acids, is lacking this pathway

Question 3

Pentose phosphate pathway

Regulation

Answer 3

• Key enzyme in the regulation: glucose-6-P dehydrogenase
• ​​The most important regulatory factor for glucose-6-P dehydrogenase is the ratio of NADP⁺/NADP-H:
  • ​Activated by NADP⁺
  • Inhibited by NADP-H

Question 4

Pentose phosphate pathway

Importance

Answer 4

1. Generation of NADP-H ​for the biosynthesis of fatty acids and steroids
2. Formation of ribose for the synthesis of nucleic acids
3. Contribution in the interconvension of hexones and pentoses
4. Participation in the formation of glucose from CO₂ in the reaction of photosynthesis

• Carbohydrate degradation (in addition to the citric acid cycle)
• Its primary purpose in most cells is to generate reducing power in the form of NADP-H
• A second function is to build up ribose
  
  • ​Ribose is used in the synthesis of nucleic acids