Purpose, reactions, and regulation of gluconeogenesis

1. Gluconeogenesis
R. C. Gupta
Professor and Head
Department of Biochemistry
National Institute of Medical Sciences
Jaipur, India

2. Gluconeogenesis is synthesis of glucose
from non-carbohydrate substrates
Gluconeogenesis occurs when the
availability of carbohydrates is low

3. In diabetes mellitus, carbohydrates are
available but cannot be metabolized
During fasting, dietary carbohydrates are
not available
Availability of carbohydrates is low
during fasting and in diabetes mellitus

4. Though the energy requirements can be
met by lipids, provision of some amount of
carbohydrate is essential
Certain tissues, e.g. brain and erythro-
cytes, are dependent almost exclusively on
glucose as a source of energy
Adipose tissue requires glucose as a
source of glycerol-3-phosphate for esteri-
fication of fatty acids

5. Muscles require glucose as a source of
energy under anaerobic conditions
Glucose is also required for the synthesis
of lactose during lactation
Therefore, glucose requirement is met by
converting non-carbohydrates into glucose

6. EMB-RCG
The main substrates for gluconeogenesis
are:
• Lactate
• Glycerol
• Amino acids (except leucine & lysine)
• Intermediates of glycolysis
• Intermediates of citric acid cycle
The principal sites of gluconeogenesis are
liver and kidneys

7. Gluconeogenesis was believed in the past
to be a reversal of glycolysis
It was shown later that free energy is
released in some reactions of glycolysis
These functionally irreversible reactions are
energy barriers for gluconeogenesis

8. The energy barriers can be circumvented
by using different enzymes for catalysing
the reactions in backward direction
The reversible reactions are catalysed by
enzymes of the glycolytic pathway

9. EMB-RCG
The following reactions of glycolytic
pathway constitute energy barriers:
Conversion of glucose into glucose-
6-phosphate
Conversion of fructose-6-phosphate
into fructose-1,6-biphosphate
Conversion of phosphoenol pyruvate
into pyruvate
Energy barriers

10. EMB-RCG
In gluconeogenic pathway, the irreversible
glycolytic reactions are catalysed by:
Glucose-6-phosphatase
Fructose-1,6-biphosphatase
Pyruvate carboxylase
Phosphoenol pyruvate carboxykinase
Bypassing the energy barriers

13. Conversion of oxaloacetate into
phosphoenol pyruvate
Conversion of pyruvate into phosphoenol
pyruvate requires two reactions:
Conversion of pyruvate into oxalo-
acetate
EMB-RCG

14. Pyruvate enters mitochondria and is converted
into oxaloacetate by pyruvate carboxylase
Oxaloacetate comes out of mitochondria with
the help of malate shuttle
It is converted into phosphoenol pyruvate by
phosphoenol pyruvate carboxykinase
The phosphate group is provided by GTP

16. Thus, gluconeogenic enzymes (or key
enzymes of gluconeogenesis) are:
Glucose-6-phosphatase
Fructose-1,6-biphosphatase
Pyruvate carboxylase
Phosphoenol pyruvate carboxykinase

17. For converting pyruvate into glucose:
Some of the reactions are catalysed
by glycolytic enzymes
Some reactions are catalysed by
gluconeogenic enzymes

20. The substrates for gluconeogenesis enter
the pathway at different stages
Lactate is a major substrate for gluco-
neogenesis
It is formed in muscles during anaerobic
conditions

21. Lactate cannot be utilized in muscles as
gluconeogenic enzymes are not present
there
It is used for synthesis of glucose in liver
via Cori cycle

22. Cori cycle
This cycle transfers glucose from liver to
muscles
Glucose is converted into lactate by
glycolysis as the conditions are usually
anaerobic in muscles
Lactate is transferred to liver for
gluconeogenesis

24. Glucose-alanine cycle
This cycle transfers glucose from liver to
muscles and alanine from muscles to liver
Alanine carries carbon atoms for glucose
synthesis and amino group for urea
synthesis from muscles to liver

25. Pyruvate formed in muscles by aerobic
glycolysis is transaminated to alanine
Alanine goes to liver via circulation
This transports carbon atoms of pyruvate and
amino groups of amino acids to liver

26. Alanine transfers its amino group to a-keto-
glutarate in liver
Amino group of glutamate is used to synthesize
urea
Pyruvate is converted into glucose by gluco-
neogenesis which goes back to liver via blood
Alanine is converted into pyruvate, and a-keto-
glutarate into glutamate

28. Glycerol is released from triglycerides and
glycerophospholipids
It can be converted into dihydroxyacetone
phosphate via glycerol-3-phosphate
Dihydroxyacetone phosphate, a glycolytic inter-
mediate, can enter gluconeogenic pathway

30. Gluconeogenic amino acids are converted into
intermediates of glycolysis or citric acid cycle
Intermediates of glycolysis enter the pathway
directly at the stages where they are formed
Intermediates of citric acid cycle are converted
into oxaloacetate
Oxaloacetate is converted into PEP which is an
intermediate of gluconeogenesis

32. Regulation of gluconeogenesis is long-term
as well as short-term
Long-term regulation occurs through induction
and repression
Synthesis of gluconeogenic enzymes is
induced by glucocorticoids
The synthesis is repressed by insulin
EMB-RCG
Regulation

33. The immediate regulator is
fructose-2,6-biphosphate
The ultimate regulator is glucagon
Short-term regulation occurs by:
Covalent modification
Allosteric mechanism

34. Fructose-2,6-biphosphate is the allosteric
modifier of two enzymes
It activates phosphofructokinase-1 which
increases glycolysis
It inhibits fructose-1,6-biphosphatase which
decreases gluconeogenesis
EMB-RCG

35. Fructose-2,6-biphosphate is formed from
fructose-6-phosphate
EMB-RCG
The phosphate is removed from carbon 2
by fructose-2,6-biphosphatase
Phosphate is added to carbon 2 of fructose-
6-phosphate by phosphofructokinase-2

36. EMB-RCG
Phosphofructokinase-2 and fructose-2,6-
biphosphatase activities are present in a
common bifunctional enzyme
The bifunctional enzyme is subject to
covalent modification

37. When it is phosphorylated, phosphofructo-
kinase-2 is inactive and fructose-2,6-
biphosphatase is active
EMB-RCG
When the enzyme is dephosphorylated,
phosphofructokinase-2 is active and
fructose-2,6-biphosphatase is inactive

38. EMB-RCG
In the absence of glucagon (as after a
meal), the enzyme is dephosphorylated
Phosphofructokinase-2 is active and
fructose-2,6-biphosphatase is inactive
Concentration of fructose-2,6-biphosphate
rises

39. Raised concentration of
fructose-2,6-biphosphate:
Increases glycolysis by activating
phosphofructokinase-1
Decreases gluconeogenesis by
inhibiting fructose-1,6-biphosphatase

40. The reverse occurs after secretion of
glucagon
Glycolysis is decreased and gluco-
neogenesis is increased
Short-term regulation of gluconeogenesis
and glycolysis is reciprocal
EMB-RCG