Gluconeogenesis
Gluconeogenesis and starch/glycogen synthesis
• In animals, lactate formed anaerobically in muscles
is converted to glucose in liver and kidney and
stored as glycogen or released as blood glucose.
• In plants, G3P product of photosynthesis is
converted to starch and stored in chloroplasts or
converted to glucose and sucrose and exported to
other tissues for starch storage.
• In some plant seeds, stored fats are converted to
glucose and sucrose upon germination and used to
make cell wall cellulose.
Gluconeogenesis is the synthesis of glucose.
Gluco
neo
genesis
Standard
free
energies
Go’
glycolysis neogenesis
-16.7
-17.7
+1.7
-1.7
-14.2
-16.7
+23.8
+7.6
-23.8
-7.6
+6.3
-6.3
-18.9
+18.9
+4.4
-4.4
+1.8
-1.8
-31.4
-22.6
________________
-74
-37.7
Gluco
neo
genesis
Working
free
energies
G’
glycolysis neogenesis
-33.9
-5.1
-2.9
-18.9
-8.6
-0.2
+2.4
-1.3
+0.1
+0.8
+1.1
-22.6
-23.0
-75.8
-36.3
Gluconeogenesis starts in the mitochondrion
..and finishes up in the endoplasmic reticulum (to release glucose).
+
+
+
-
Why control?
Note possible futile
cycle:
-
F1,6BP + H2O ->
F6P + Pi
F6P + ATP ->
F1,6BP + ADP
Net:
ATP + H2O ->
ADP + Pi
+
While animals can use many amino acids to make
glucose, they cannot use acetyl-CoA from fatty
acid metabolism.
Why? Removing malate from the TCA cycle to
make carbohydrates means that there will be a lack
of OAA to form citrate.
But plants and bacteria can convert acetyl-CoA to
glucose using the glyoxylate cycle.
A modification of the TCA cycle (glyoxylate cycle) lets plants and bacteria
make carbohydrates from fats.
to glucose
isocitric lyase
malate synthase
Starch/glycogen
synthesis
glucose + ATP --> G-6-P + ADP
hexokinase
G-6-P --> G-1-P
P-glucomutase
UTP + G-1-P --> UDPG + PPi
PPi + H2O --> 2 Pi
U D PG pyrophosphorylase (plants: ATP, ADPG)
PPP:
Pentose
Phosphate
Pathway
PPP:
Pentose
Phosphate
Pathway
P-pentose
isomerase
epimerase
TK
TA
TK
Using the PPP
to make ribose
Using the PPP
to make NADPH
Summary
•Synthesis of glucose from pyruvate involves the glycolytic pathway
plus 4 new enzymes.
•Glycolysis and gluconeogenesis show reciprocal controls to
prevent futile cycles.
•Plants can direct acetyl-CoA to gluconeogenesis with the glyoxylate
cycle.
•Starch/glycogen synthesis involves UDPG (ADPG).
•Pentose phosphate pathway provides a method for synthesizing
NADPH and ribose, among other compounds.