Gluconeogenesis

Some tissues, such as brain and erythrocytes, depend on a constant supply of glucose. If the amount of carbohydrate taken up in food is not sufficient, the blood sugar level can be maintained for a limited time by degradation of hepatic glycogen. If these reserves are also exhausted, de-novo synthesis of glucose(gluconeogenesis) begins. The liver is also mainly responsible for this, but the tubular cells of the kidney also show a high level of gluconeogenetic activity. The main precursors for gluconeogenesis are amino acids derived from muscle proteins. Another important precursor is lactate, which is formed in erythrocytes and muscle when there is oxygen deficiency. Glycerol produced from the degradation of fats can also be used for gluconeogenesis. However, the conversion of fatty acids into glucose is not possible in animal metabolism. The human organism can synthetize several hundred grams of glucose per day by gluconeogenesis.

A.Gluconeogenesis

Many of the reaction steps involved in gluconeogenesis are catalyzed by the same enzymes that are used in glycolysis. Other enzymes are specific to gluconeogenesis and are only synthesized, under the influence of cortisol and glucagon when needed. Glycolysis takes place exclusively when needed in the cytoplasm, but gluconeogenesis also involves the mitochondria and the endoplasmic reticulum. Gluconeogenesis consumes 4 ATP(3 ATP + 1 GTP) per glucose – i.e. , twice as many as glycolysis produces.

1)      Lactate is a precursor for gluconeogenesis is mainly derived from muscle and erythrocytes. LDH oxidizes lactate to pyruvate, with NADH+H⁺ formation.

2)      The first steps of actual gluconeogenesis take place in the mitochondria. The reason for this detour is the equilibrium state of pyruvate kinase reaction. Even coupling to ATP hydrolysis would not be sufficient to convert pyruvate directly into phosphenol pyruvate(PEP). Pyruvate derived from lactate or amino acids is therefore initially transported into the mitochondrial matrix, and-in a biotin-dependent reaction catalyzed by pyruvate carboxylase – is carboxylated there to oxaloacetate. Oxaloacetate is also an intermediate in the tricarboxylic acid cycle. Amino acids with break down products that enter the cycle or supply pyruvate can therefore be converted into glucose.

3)      The oxaloacetate formed in the mitochondrial matrix is initially reduced to malate, which can leave mitochondria via inner membrane transport systems.

4)      In the cytoplasm, oxaloacetate is reformed and then converted into phosphoenol pyruvate by a GTP-dependent PEP carboxykinase. The subsequent steps up to fructose 1,6-biphosphate represent the reverse of the corresponding actions involved in glycolysis. One additional ATP per C3 fragment is used for synthesis of 1,3 biphosphoglycerate. Two gluconeogenesis-specific phosphatases then successively cleave off the phosphate residues from fructose 1,6-biphosphate. In between these reactions lies the isomerization of fructose 6-phosphate to glucose 6-phosphate – another glycolitic reaction.

5)      The reaction catalized by fructose-1,6 biphosphatase is an important regulation point in gluconeogenesis.

6)      The last enzyme in the pathway, glucose 6-phosphatase, occurs in the liver, but not in muscle. It is located in the interior of the smooth endoplasmic reticulum. Specific transporters allow glucose 6-phosphate to enter the ER and allow the glucose formed there to run to the cytoplasm. From there, it is ultimately released into the blood. 

Glycerol initially undergoes phosphorylation at C-3. The glycerol 3-phosphate formed is then oxidized by an NAD⁺-dependent dehydrogenase to form glycerone 3-phosphate and thereby channeled into gluconeogenesis. An FAD-dependent mitochondrial enzyme is also able to catalyze this reaction.

           
“Coloured atlas of biochemistry”, second edition; J. Koolman, K.H. Roehm