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.

Go’ glycolysis neogenesis -16.7 -17 .7 +1.7 -1 .7 -14.2 -16 .7 +23.8 -23.8 +7.6 -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

Gluconeogenesis

Standardfree 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 - 2 2 . 6 -23.0 -75.8 -36.3

Gluconeogenesis

Working freeenergies

Gluconeogenesis starts in the mitochondrion

..and finishes up in the endoplasmic reticulum (to release glucose).

Why control?Note possible futilecycle:

F1,6BP + H2O -> F6P + Pi

F6P + ATP -> F1,6BP + ADP

Net:ATP + H2O -> ADP + Pi

-

++--

+----

--

+

While animals can use many amino acids to makeglucose, they cannot use acetyl-CoA from fattyacid metabolism.

Why? Removing malate from the TCA cycle tomake carbohydrates means that there will be a lack of OAA to form citrate.

But plants and bacteria can convert acetyl-CoA toglucose using the glyoxylate cycle.

A modification of the TCA cycle (glyoxylate cycle) lets plants and bacteriamake carbohydrates from fats.

isocitric lyase

malate synthase

to glucose

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)

Starch/glycogensynthesis

PPP:PentosePhosphatePathway

PPP:PentosePhosphatePathway

P-pentoseisomerase

epimeraseTK

TKTA

Using the PPPto make ribose

Using the PPPto 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.