25: GENERATION OF REDUCING EQUIVALENTS

Question 1

NAD+ and NADP+ e carriers

Answer 1

• only difference, P on 2’OH of ribose; P tag enables enzyme to distinguish between the two as they’re involved in different reactions
• enzymes are v. specific for NAD/NADP
• both have nicotinamide ring
• both derived from vit.B3 (niacin)
• both carry 2e in same way: ring accepts 2e; 2H+ come in from oxidation of substrate, 1H+ is retained on ring, other is released into solution

Question 2

NAD+ and NADP+ roles

Answer 2

-NAD+ is reduced in ox. of fuel molecules (e.g. glycolysis, TCA cycle)l; e eventually passed to O2

• NADPH acts as e donor
• NADP+ acts in reductive biosynthesis; need high potential e because precursors are more oxidised than products
• eg. FA synthesis (liver and adipose tissue); cholesterol and steroid hormone synthesis (liver, adrenal glands, ovaries, testes) so pentose pathway active in these tissues
• prevents oxidative damage, specifically in red blood cells as they’re directly exposed to O2

-using different e carriers in ox and red. biosynthesis allows independent regulation

• [NAD+] high for catabolism (ox); accepts e
• [NADPH] high for biosynthesis; donates e

-[NAD+]/[NADPH] ratio is high; opposite is low in cytoplasm

Question 3

Pentose phosphate pathway oxidative phase

Answer 3

-glu-6-P converted to ribulose-5-P (isomerisation to ribose-5-P) and 2xNADPH

1. ox of glu-6-P by DH (makes 1NADPH)
2. hydrolysis of 6-phosphogluconolactone to 6-phosphogluconate
3. oxidation and decarboxylation to give NADPH and ribulose-5-P (pentose sugar) by DH enzyme

-ribose-5-P needed to make nucleotides for DNA/RNA in rapidly dividing tissues (e.g. skin, bone marrow, intestinal mucosa)

Question 4

glutathione

Answer 4

-glutathione (antioxidant in cytoplasm); reduced reactive O2 species e.g. peroxides; keeps protein SH groups in reduced form in cytoplasm

• tripeptide (Gly + Cys + y-amide bond Glu)
• important part is Cys (SH)
• oxidised form; diS bond with another glutathione as it reduces reactive O2 species
• NADPH maintains levels of reduced glutathione by reducing ox form back to red form

Question 5

pentose phosphate pathway non-oxidative phase

Answer 5

• if nucleotides not needed, not isomerized to ribose-5-P
• ribulose-5-P converted back into intermediates of glycolysis or gluconeogenesis
• 2x fructose-6-P and 1x glyceraldehyde-3-P suing transketolase and transaldolase enzymes

-occurs in red blood cells as they don’t need to divide/make DNA