Gluconeogenesis

What is the net equation of the pentose phosphate pathway? What happens in this pathway? Where does it occur?

glucose-6-phosphate is oxidized into pentose and NADPH (electron carrier)

occurs entirely in the cytosol

glucose-6-phosphate → pentose (ribose) + NADPH

what is the net equation for gluconeogenesis? What can enter it? What is it the same as? Where does it begin? What cells do it?

glucose can be synthesized for 3- to 4- carbon precursors: pyruvate, lactate, glycerol, most amino acids (most often pyruvate)

is the reversal of glycolysis using the same enzymes except for 3 irreversible steps

begins in the mitochondria

2 pyruvate + 4 ATP + 2 GTP + 2 NADH → glucose + 4 ADP + 2 GDP + 6Pi + 2 NAD(+)

liver, some kidney and small intestine cells

how do anabolic and catabolic pathways differ?

anabolic and catabolic pathways can overlap extensively but key reactions will differ to allow regulation

What are the first and second irreversible, regulating steps in gluconeogenesis and how do they compare to glycolysis?

Gly-10 (pyruvate kinase) bypassed by two key, irreversible steps:

1. CO2 added to pyruvate via carboxylation forming oxaloacetate

   • in mitochondrial matrix

   • enzyme is pyruvate carboxylase and requires ATP

   • 2 CO2 + 2 pyruvate + 2 ATP → oxaloacetate + 2 ADP

2. carboxyl group removed in decarboxylation forming phosphoenolpyruvate

   • in cytosol

   • enzyme is phosphoenolpyruvate carboxykinase (PC/PEPCK) and requires GTP

   • key committed step into gluconeogenesis

   • oxaloacetate + 2 GTP → phosphoenolpyruvate (PEP) + 2 GDP + 2 CO2

How are the first two irreversible, regulated steps in glycolysis bypassed by gluconeogenesis?

• phosphofructokinase 1 (PFK-1) is bypassed by fructose-1,6-biphosphatase (F1,6BPase)

• hexokinase bypassed by glucose-6-phosphatase

  • glucose-6-phosphatase locates in endoplasmic reticulum (localized in ER)

    • specific transporters for glucose-6-phosphate, glucose, and Pi present in ER membrane

What are two general regulators of glycolysis and gluconeogenesis?

allosteric control (short term)

regulated gene expression (longer term)

What inhibits or activates the key regulated enzymes in glycolysis?

hexokinase:

• inhibited by glucose 6-phosphate (feedback inhibition)

phosphofructokinase-1:

• inhibited by ATP and citrate

• activated by AMP and fructose 2,6-biphosphate

pyruvate kinase:

• inhibited by acetyl-CoA and ATP

• activated by fructose-1,6-biphosphate

What inhibits or activates the key regulated enzymes in gluconeogensis?

pyruvate carboxylase:

• activated by acetyl-CoA

phosphoenolpyruvate carboxykinase (PEPCK):

• regulated at transcription level by insulin/glucagon

fructose-1,6-bisphosphatase:

• inhibited by AMP and fructose-2,6-bisphosphate

glucose-6-phosphatase

What regulators affect enzymes in both glycolysis and gluconeogenesis? How do they affect each pathway?

AMP, acetyl-CoA, and fructose-2,6-bisphosphate

• AMP formed by adenylate kinase; ADP + ADP → ATP + AMP

• high AMP: activates glycolysis, inhibits gluconeogenesis

• high acetyl-CoA: activates gluconeogenesis, inhibits glycolysis

• fructose-2,6-bisphosphate: activates glycolysis, inhibits gluconeogenesis

What is the most important regulator of glycolysis and gluconeogenesis? What is it controlled by? What is it catalyzed by?

fructose-2,6-bisphosphate (F2,6BP) is the most important regulator under hormonal control

• catalyzed by phosphofructokinase-2 (PFK-2)

What happens when glucagon and epinephrine levels are low? What does this accumulates and what does this affect?

• PFK-2 active when glucagon and epinephrine are low

• active PFK-2 accumulates fructose-2,6-bisphosphate

  • high F2,6BP: allosterically activates PFK-1, inhibits F1,6BPase

  • glycolysis increased, gluconeogenesis decreased

What happens when glucagon and epinephrine levels are high? What does this result in?

• high glucagon and epinephrine indirectly activate cAMP-activated kinase that phosphorylates and activates fructose-2,6-bisphosphatase (F2,6BPase)

• active F2,6BPase lowers fructose-2,6-bisphosphate levels

  • low F2,6BP: removes activation of PFK-1, removes inhibition of F11,6BPase

  • glycolysis decreases, gluconeogenesis increased

What are cAMP levels increased by and what does this affect?

cAMP level in cells increased by hormones glucagon and epinephrine (adrenaline) which signal body as a whole

• high cAMP:

  • activates protein kinase that blocks PFK-2 activity and increases F2, 6BPase activity

  • activates transcription factor that increases PEPCK

Why do the same steps that activate one pathway inhibit another?

to prevent a futile cycle of degrading and synthesizing glucose

net cost of futile cycle: 4 ATP + 2 GTP wasted

If you need energy, which pathway will be activated? What if you don’t need energy?

Need energy: glycolysis activated, gluconeogenesis inhibited

don’t need energy: gluconeogenesis activated, glycolysis inhibited

What does high AMP signify? What about high ATP and citrate? What about high acetyl-CoA?

high AMP: low energy

high ATP and citrate: abundant energy

high acetyl-CoA: biosynthetic fuel available

What does allosteric regulation meet the needs of? What about hormonal regulation?

allosteric regulation: adjusts pathways to meet needs of the cell

hormonal regulation: adjusts pathways to meet needs of the body

Is glucose the only sugar that can enter glycolysis? If no, name some of these molecules.

alternative sugars to glucose can enter glycolysis, most often at intermediate steps

• sucrose (glucose + fructose)

• fructose: bypasses insulin and PFK-1; bypasses key steps that regulate glucose catabolism

• lactose (glucose + galactose)

• galactose: undergoes conversion that can enter glycolysis

• glycerol: undergoes conversion that can enter glycolysis

• other polysaccharides (i.e.. storage polysaccharides of glucose) are cleaved by phosphorolysis into glucose-1-phosphate that can be converted into glucose-6-phosphate