Download Gluconeogenesis

GOALS FOR LECTURE 10:
Describe the biological purpose of gluconeogenesis. Summarize the physiological states
where gluconeogenesis would occur.
Identify the biochemical reactions that are identical between glycolysis and
gluconeogenesis, and those that are distinct. Rationalize why the distinct reactions must be
different between the two pathways.
Describe the involvement of the mitochondria and the endoplasmic reticulum in
gluconeogenesis.
Describe the allosteric effectors that regulate key enzymes in gluconeogenesis. Contrast
the allosteric regulation of gluconeogenesis with the allosteric regulation of enzymes in
glycolysis. Rationalize the roles of these effectors based on cellular energy needs.
Recommended reading:
TOPIC
STRYER, 4th edition
DEVLIN, 5th edition
Gluconeogenesis
Ch. 16, 450-460
Ch. 14, 629-643
Gluconeogenesis
The energy requirement of the brain is derived almost entirely from glucose. Since
nerve cells store very little glycogen, the brain and certain other tissues including cornea and
red blood cells depend on a constant supply of glucose in the blood. One of the important
functions of the liver is to maintain the blood glucose level. Degradation of liver glycogen is
the primary source of blood glucose in the early fasting state. However, when glycogen
stores are depleted, the liver is able to synthesize glucose from lactate, via
gluconeogenesis. In the Cori cycle, depicted below, gluconeogenesis in the liver supports
anaerobic glycolysis in red blood cells.
10.1
Gluconeogenesis uses many of the same reactions as glycolysis, but running in
reverse. Among the glycolytic reactions, recall that all are readily reversed with the
exception of the reactions catalyzed by hexokinase/glucokinase, phosphofructokinase, and
pyruvate kinase. Previously, we have seen that these reactions are regulated. In
gluconeogenesis, they are bypassed.
Pyruvate kinase is bypassed in two steps
Conversion of pyruvate to phosphoenolpyruvate occurs by a combination of two
reactions, and requires hydrolysis of two ATP to ADP. In the first reaction, pyruvate is
converted to oxaloacetate by pyruvate carboxlyase, which uses biotin as a cofactor.
This reaction takes place in the mitochondrial matrix and is also used to generate
oxaloacetate as an intermediate in the TCA cycle, as will be discussed later. This reaction
depends on an ATP-dependent carboxylation of biotin.
10.2
To be used for gluconeogenesis, the oxaloacetate must be transferred back into the
cytoplasm. However, mitochondria lack an efficient transporter for oxaloacetate. Therefore,
oxaloacetate is reduced to malate, by malate dehydrogenase which converts one molecule
of NADH to NAD+. Malate is then transported out and reoxidized to oxaloacetate,
regenerating NADH from NAD+ in the cytoplasm.
Phosphoenolpyruvate carboxykinase simultaneously decarboxylates and
phosphorylates oxaloacetate to generate phosphoenolpyruvate. GTP is used as the
phosphoryl donor. Decarboxylation drives this reaction, which would otherwise be
endergonic.
10.3
Glucokinase and phosphofructokinase are bypassed by phosphatases
Phosphoenolpyruvate is converted to fructose-1,6-bisphosphate by a simple
reversal of glycolytic steps 9, 8, 7, 6, 5 and 4, which all operate near equilibrium. Fructose1, 6-bisphosphate is dephosphorylated by fructose-1,6-bisphosphatase, releasing free
phosphate, and bypassing phosphofructokinase. Fructose-6-phosphate is converted to
glucose-6-phosphate by a phosphoglucose isomerase. Glucose-6-phosphate is
converted to glucose by glucose-6-phosphatase, bypassing glucokinase.
Glucose-6-phosphatase is found in the lumen of the endoplasmic reticulum rather
than in the cytoplasm. Thus, for the final step of gluconeogenesis, G6P must be
transported into the ER, the phosphate is cleaved off, and then glucose and phosphate are
transported back out. Deficiencies in either glucose-6-phosphatase or any of the three
transporters result in von Gierke’s disease, with symptoms of hypoglycemia, lacticacidemia
and ketoacidosis after mild fasting.
Note that these two phosphatase reactions do not reverse the reciprocal kinase
reactions, because ATP is not regenerated.
The stoichiometry for gluconeogenesis from pyruvate is:
2 pyruvate + 4 ATP + 2 GTP + 2 NADH + 6 H2 O Õ
glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+ + 2 H +
By contrast, the stoichiometry for conversion of glucose to pyruvate by glycolysis is:
glucose + 2 ADP + 2 P i + 2NAD + Õ 2 pyruvate + 2 ATP + 2 NADH + 2 H2 O
In cycling from glucose to pyruvate to glucose in the Cori cycle, four high-energy
phosphate bonds are hydrolyzed. This expenditure of energy is required to turn an
energetically unfaovable process (the reversal of glycolysis, ∆G o ’ = +20 kcal/mol) into a
favorable one (gluconeogenesis, ∆G o ’ = -9 kcal/mol). The extra cost of nucleotide
hydrolysis is borne by the liver, another example of its altruism toward other tissues.
10.4
Summary of gluconeogenesis, contrasted with glycolysis
10.5
Alternate substrates for gluconeogenesis
Any precursor which can be converted to pyruvate can ultimately be converted to
glucose by gluconeogenesis. These precursors include all amino acids except for leucine
and lysine and propionate.
Ethanol consumption and gluconeogenesis
Ethanol is metabolized primarily in the liver, by alcohol dehydrogenase.
CH 3 CH 2 OH + NAD +
CH 3 CHO + NADH + H +
This reaction elevates the ratio between NADH and NAD+ in liver cytosol. High
NADH levels block the conversion of cytoplasmic malate to oxaloacetate, preventing
gluconeogenesis. Similarly, glyceraldehyde-3-phosphate dehydrogenase is forced to run
backward by high [NADH], so glycolysis is also inhibited. Conversion of lactate to
pyruvate by lactate dehydrogenase is also inhibited by high [NADH]. Consequently,
consumption of large amounts of ethanol can result in hypoglycemia and mild lactic acidosis.
Severe hypoglycemia can cause irreversible damage to the central nervous system, which
in its early stages might be mistaken for simple intoxication.
10.6
Regulation of gluconeogenesis
The enzymes of glycolysis and gluconeogenesis in the liver are reciprocally
regulated so that either glucose is converted to pyruvate or pyruvate is converted to
glucose. Fructose-2,6-bisphosphate, which we have already seen serves to activate
phosphofructokinase, is an inhibitor of fructose-1,6-bisphosphatase. Because of this
reciprocal effect, only one of the two enzymes is active at any given time.
The liver also contains glucokinase inhibitor protein, which is activated by fructose-6phosphate. When bound to F6P, glucokinase inhibitor protein sequesters and inactivates
glucokinase, shutting down the first step in glycolysis. There is no equivalent inhibitory
protein for hexokinase, so accumulation of F6P shuts down glycolysis and enables
activation of gluconeogenesis only in the liver. Allosteric regulation of gluconeogenesis and
glycolysis is summarized below:
10.7
Hormonal control of gluconeogenesis
As with glycolysis, glucagon-dependent protein phosphorylation also regulates
enzyme activities in gluconeogenesis. Recall that F-2,6-BP levels are regulated by
glucagon, with high glucagon (low blood sugar) favoring conversion of F-2,6-BP back into
F6P
In addition, glucagon activates lipases is adipose tissue, promoting release of fatty
acids into the bloodstream. These fatty acids are broken down in the mitochondria of liver,
resulting in high concentrations of acetyl CoA. Acetyl CoA acts as an allosteric activator of
pyruvate carboxylase.
Both insulin and glucagon regulate the transcription of bypass enzymes: insulin
inhibits transcription of phosphoenolpyruvate carboxykinase, and glucagon activates its
transcription.
10.8