01:694:301 Lecture Notes - Lecture 20: Light-Independent Reactions, Transketolase, Ribulose
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14 Nov 2018
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Chapter 20 – ‘The Calvin Cycle and the Pentose Phosphate Pathway’
• The pentose phosphate pathway (PPP) provides a mean by which glucose can be oxidized to
generate NADPH
(the currency of readily available reducing power in the cell)
o Note the difference between NADH vs. NADPH
! NADH is
oxidized
by respiratory chain to generate ATP
! NADPH serves as reductant in biosynthesis
o PPP can also be used for catabolizing pentose sugars, synthesis of pentose sugars for
nucleotide biosynthesis, and catabolism/synthesis of less common 4 and 7 sugars
• You should be able to write the PPP (oxidative and non-oxidative branches) with all structures,
enzymes, and cofactors
o The PPP consists of two phases: (all take place cyto)
! (1) Oxidative generation of NADPH
• NADPH is generated when glucose-6-phosphate is oxidized to ribulose 5-
phosphate " ribose 5-phosphate
! (2) Nonoxidative interconversion of sugars (‘carbon shuffling’)
• The pathway catalyzes the interconversion of 3,4,5, 6, 7—C sugars in series
nonoxy rxns
• PPP – OXIDATIVE Phase
o The rxn yields 2 molecules of NADPH and 1 molecule of ribulose 5-phosphate for per G6P
• PPP – NONOXIDATIVE Phase
o The PPP and glycolysis are linked by the enzymes
transketolase
and
transaldolase
! Ribose 5-phosphate can be converted " G3P and F,6-P by these
enzymes
o These enzymes create a
reversible
link b/w PPP and glycolysis by
catalyzing 3 successive rxns:
! The net result of these rxns is the formation of 2 hexoses and
1 triose from 3 pentoses
o (1) Formation of GA3P/sedoheptulose 7-phosphate from 2 pentose (5)
! *Note that a ketose is substrate of
transketolase
ONLY if –OH group at C-3 has
xylulose configuration (not ribulose)