Gluconeogenesis

Questions

1. Does gluconeogenesis continue during starvation and in diabetes?
2. What happens to gluconeogenesis during starvation? Is it reduced or maintained?

Answers

The 2 conditions above are mostly discussed in biochemistry - starvation and diabetes. Pay attention to them.

In trying to answer the questions above, I have stuck only to hepatic glucose production in humans. I have broken up the answers into many parts so you can see what thoughts must go into trying to answer the questions.

I have included that on animals at the end to avoid confusion.

I have excluded the glyoxylate cycle which occurs in plants and nematodes.


1. Means for Producing Glucose

Blood glucose must be maintained within normal range. There are 2 sources of hepatic glucose production when blood glucose becomes low (hypoglycaemia). One is gluconeogenesis, and the other is glycogenolysis (degradation or breakdown of glycogen).

2. How does gluconeogenesis operate?

Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as:

1. pyruvate, 
2. lactate, 
3. glycerol, 
4. glucogenic amino acids (14 of them), and 
5. odd-chain fatty acids.

Lactate is transported back to the liver where it is converted into pyruvate by the Cori cycle using the enzyme lactate dehydrogenase (LD). 

Pyruvate, the first designated substrate of the gluconeogenic pathway, can then be used to generate glucose. 

Transamination or deamination of amino acids facilitates entering of their carbon skeleton into the cycle directly (as pyruvate or oxaloacetate), or indirectly via the citric acid cycle (Kreb's cycle).

Gluconeogenesis is highly exergonic until ATP or GTP are utilized, effectively making the process endergonic. For example, the pathway leading from pyruvate to glucose-6-phosphate requires 4 molecules of ATP and 2 molecules of GTP.

Exergonic reactions release energy as ATP or GTP.
Endergonic reactions consume energy in the form of ATP or GTP.


3. Human Gluconeogenesis

In vertebrates (includes humans), gluconeogenesis takes place mainly in the liver and, to a lesser extent, in the cortex of kidneys.

In humans the main gluconeogenic precursors are lactate, glycerol (which is a part of the triacylglycerol molecule), alanine (Ala) and glutamine (Gln). Altogether, they account for over 90% of the overall gluconeogenesis. 

Other glucogenic amino acid as well as all citric acid cycle intermediates, the latter through conversion to oxaloacetate, can also function as substrates for gluconeogenesis.

Gluconeogenesis is often associated with ketosis; this indicates that gluconeogenesis goes on in starvation and diabetes.

Gluconeogenesis is also a target of therapy for type II diabetes, such as metformin, which inhibits glucose formation and stimulates glucose uptake by cells.

The existence of glyoxylate cycles in humans has not been established. It is widely held that fatty acids cannot be converted to glucose in humans directly. However, carbon-14 has been shown to end up in glucose when it is supplied in fatty acids. Despite these findings, it is considered unlikely that the 2-carbon acetyl-CoA derived from the oxidation of fatty acids would produce a net yield of glucose via the citric acid cycle. Put simply acetic acid (in the form of acetyl-CoA) is used to partially produce glucose; acetyl groups can only form part of the glucose molecules (not the 5th carbon atom) and require extra substrates (such as pyruvate) in order to form the rest of the glucose molecule.

In mammals, gluconeogenesis is restricted to the liver, the kidney (and possibly the intestine). However these organs use somewhat different gluconeogenic precursors. Liver uses primarily lactate and alanine while kidney uses lactate and glutamine.

In humans, PEP carboxykinase that converts oxaloacetate to PEP, can be found dispersed evenly between the mitochondria and the cytosol. 

4. Catabolism of Amino Acids

Amino acids are classified according to the abilities of their downstream products to enter gluconeogenesis or ketogenesis or both:

1. Glucogenic amino acids have the ability to enter gluconeogenesis and produce glucose. They are alanine (Ala), glycine (Gly), threonine (Thr), cysteine (Cys), serine (Ser), asparagine (Asn), aspartate (Asp), arginine (Arg), proline (Pro), histidine (His), glutamine (Gln), glutamate (Glu), valine (Val), and methionine (Met).
2. Ketogenic amino acids do not enter gluconeogenesis and do not produce glucose. Their products are used for ketogenesis or lipid synthesis. They are leucine (Leu) and lysine (Lys).
3. Some amino acids are catabolized into both glucogenic and ketogenic products. They are isoleucine (Ile), phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp).

5. Gluconeogenesis Pathway

Gluconeogenesis is a pathway consisting of a series of 11 enzyme-catalyzed reactions. The pathway may begin in the mitochondria or cytoplasm, this being dependent on the substrate being used. Many of the reactions are the reversible steps found in glycolysis (in cytosol).

What are the reactions in gluconeogenesis?

1. Gluconeogenesis begins in the mitochondria with the formation of oxaloacetate (OAA) by the carboxylation of pyruvate. This reaction also requires one molecule of ATP, and is catalyzed by pyruvate carboxylase. This enzyme is stimulated by high levels of acetyl-CoA (produced in β-oxidation in fat breakdown in the liver) and inhibited by high levels of ADP.
2. Oxaloacetate (OAA) is reduced to malate using NADH, a step required for its transportation out of the mitochondria. Refer OAA-malate cycle.
3. Malate is oxidized to oxaloacetate (OAA) using NAD+ in the cytosol, where the remaining steps of gluconeogenesis take place.
4. Oxaloacetate (OAA) is decarboxylated and then phosphorylated to form phosphoenolpyruvate (PEP) using the enzyme phosphoenolpyruvate carboxykinase (PEP carboxykinase). A molecule of GTP is hydrolyzed to GDP during this reaction.
5. The next steps in the reaction are the same as reversed glycolysis. However, fructose-1,6-bisphosphatase converts fructose-1,6-bisphosphate to fructose 6-phosphate, using one water molecule and releasing one phosphate. This is also the rate-limiting step of gluconeogenesis.
6. Glucose-6-phosphate (G6P) is formed from fructose 6-phosphate (F6P) by phosphoglucoisomerase. Glucose-6-phosphate (G6P) can be used in other metabolic pathways or dephosphorylated to free glucose. Whereas free glucose can easily diffuse in and out of the cell, the phosphorylated form (glucose-6-phosphate) is locked in the cell, a mechanism by which intracellular glucose levels are controlled by cells.
7. The final reaction of gluconeogenesis, the formation of glucose, occurs in the lumen of the endoplasmic reticulum, where glucose-6-phosphate (G6P) is hydrolyzed by glucose-6-phosphatase to produce glucose. 
8. Glucose is shuttled into the cytoplasm by glucose transporters located in the endoplasmic reticulum's membrane.

6. How is gluconeogenesis regulated?

1. Reciprocal control by 3 major enzymes

While most steps in gluconeogenesis are the reverse of those found in glycolysis, three regulated and strongly exergonic reactions are replaced with more kinetically favorable reactions. 

Hexokinase/glucokinase, phosphofructokinase, and pyruvate kinase enzymes of glycolysis are replaced with glucose-6-phosphatase, fructose-1,6-bisphosphatase, and PEP carboxykinase. 

This system of reciprocal control allow glycolysis and gluconeogenesis to inhibit each other and prevent the formation of a futile cycle.

The majority of the enzymes responsible for gluconeogenesis are found in the cytoplasm; the exceptions are mitochondrial pyruvate carboxylase and, in animals, phosphoenolpyruvate carboxykinase. The latter exists as an isozyme located in both the mitochondrion and the cytosol.

The rate of gluconeogenesis is ultimately controlled by the action of a key enzyme, fructose-1,6-bisphosphatase, which is also regulated through signal transduction by cAMP and its phosphorylation.

Most factors that regulate the activity of the gluconeogenesis pathway do so by inhibiting the activity or expression of key enzymes. However, both acetyl CoA and citrate activate gluconeogenesis enzymes (pyruvate carboxylase and fructose-1,6-bisphosphatase, respectively). Due to the reciprocal control of the cycle, acetyl-CoA and citrate also have inhibitory roles in the activity of pyruvate kinase.

2. Hormonal control and acid-base imbalance

Global control of gluconeogenesis is mediated by glucagon (released when blood glucose is low); it triggers phosphorylation of enzymes and regulatory proteins by protein kinase A (a cyclic AMP regulated kinase) resulting in inhibition of glycolysis and stimulation of gluconeogenesis. 

Recent studies have shown that the absence of hepatic glucose production has no major effect on the control of fasting plasma glucose (FPG) concentration. 

Compensatory induction of gluconeogenesis occurs in the kidneys and intestine, driven by glucagon, glucocorticoids, and acidosis.

To answer the questions then;

1. Does gluconeogenesis continue during starvation and in diabetes? Yes.

2. What happens to gluconeogenesis during starvation? Is it reduced or maintained?

It is not reduced but it is maintained so that blood glucose remains at a level that is compatible with life (sustains life), and coma is avoided (which happens when hypoglycaemia dips further). All possible sources for gluconeogenesis to proceed will be used, but lipid sources will prevail since lipids are the eventual source of energy in starvation.

Animal Gluconeogenesis

In ruminants (cows, goats, and sheep), gluconeogenesis tends to be a continuous process. In ruminants, because metabolizable dietary carbohydrates tend to be metabolized by rumen organisms, gluconeogenesis occurs regardless of fasting, low-carbohydrate diets, exercise, etc.

In ruminants, propionate is the principal gluconeogenic substrate.

In many other animals (that hibernate or hunt for food), gluconeogenesis occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise.

Whether even-chain fatty acids can be converted into glucose in animals has been a longstanding question in biochemistry. It is known that odd-chain fatty acids can be oxidized to yield propionyl CoA, a precursor for succinyl CoA, which can be converted to pyruvate and enter into gluconeogenesis.

Propionate is the principal substrate for gluconeogenesis in the ruminant liver. The ruminant liver may make increased use of gluconeogenic amino acids, e.g. alanine, when glucose demand is increased. The capacity of liver cells to use lactate for gluconeogenesis declines from the preruminant stage to the ruminant stage in calves and lambs. In sheep kidney tissue, very high rates of gluconeogenesis from propionate have been observed. The intestine uses mostly glutamine and glycerol.

In all species, the formation of oxaloacetate from pyruvate and TCA cycle intermediates is restricted to the mitochondria, and the enzymes that convert phosphoenolpyruvic acid (PEP) to glucose are found in the cytosol. The location of the enzyme that links these two parts of gluconeogenesis by converting oxaloacetate to PEP, PEP carboxykinase, is variable by species: it can be found entirely within the mitochondria, entirely within the cytosol, or dispersed evenly between the two, as it is in humans. Transport of PEP across the mitochondrial membrane is accomplished by dedicated transport proteins; however no such proteins exist for oxaloacetate. Therefore, in species that lack intra-mitochondrial PEP carboxykinase, oxaloacetate must be converted into malate or aspartate, exported from the mitochondria, and converted back into oxaloacetate in order to allow gluconeogenesis to continue.

Source: http://en.wikipedia.org/wiki/Gluconeogenesis