01:694:301 Lecture Notes - Lecture 16: Malate Dehydrogenase, Pyruvate Carboxylase, Acetyl-Coa

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Chapter 16‘Glycolysis and Gluconeogenesis PART II
First, notice that all of the structures in Fig. 16.24 are exactly the same as the structures in
16.2 except for oxaloacetate (OAA), which is new
o Then notice that the enzymes shown in blue are all the same.
o The red enzymes replace the irreversible steps in Fig. 16.2
SO in a real way, despite what every textbook says,
gluconeogenesis can b thought of as a
reversal of glycolysis, with some useful changes
o Understanding this makes the pathway much easier to learn, especially if you already
know glycolysis
Gluconeogenesis is the synthesis of glucose from noncarbohydrate precursors; helps
maintain level of glucose in blood so brain and muscle can use to satisfy metabolic demand
o The gluconeogenic pathway converts pyruvate (PYR) to glucose
o Noncarb precursors are converted to PYR or enter pathway as intermediates (OAA and
DHAP)
! Majority of precursors " aa, lactate, glycerol
Lactate converted to PYR by lactate DH
AA derives from breakdown proteins
Glycerol/fatty acid produced by hydrolysis of triacylglycerols
o Animals cannot convert fatty acids to glucose
o Glycerol enters pathway as DHAP
o Major site is in the liver (minor kidney)
The THREE irreversible steps in glycolysis are bypassed in gluconeogenesis in new steps:
o 1)
PEP is formed from pyruvate by way of OAA
o 2)
Fructose 6- phosphate is formed from F-1,6-BP by hydrolysis of phosphate
ester at C-1
o 3)
Glucose is formed by hydrolysis of glucose-6-phosphate
Making phosphoenolpyruvate (PEP) is difficult, and in eukaryotic cells requires a visit
to the mitochondrial matrix
o Pyruvate carboxylase uses biotin, ATP, and activating acetyl CoA to form OAA
! Biotin is covalently attached prosthetic group which serves as carrier of O2;
attached to pyr carboxylase by long, flexible chain
Biotin is not carboxylated unless Acetyl CoA is bound to enzyme
o OAA converted to malate and moves back to cytoplasm, where it is re-oxidized to OAA
! OAA reduced malate inside mitochondria by
NADH-linked malate dehydrogenase
! After malate is back in cytoplasm, it gets oxidized to OAA by
NAD+-linked malate DH
! Also provides NADH for future use
o OAA simultaneously gets decarboxylated and phosphorylated by PEPCJ " PEP
! Phosphoryl donor is GTP
! Both carboxylation and decarboxylation are used to form PEP because…
Biotin
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