por el dolor físico y emocional que pueden causar estas enfermedades. Este folleto le dará los hechos básicos acerca de su enfermedad muscular metabólica . in the producido-principalmente en el hígado liverandskeletal muscles. y los glicogénica del hígado glucogenolisis glycogenolysis (gli ́ ́kuo-jue-nol.
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Gluconeogenesis is the biosynthesis of new glucose, i. This process is frequently referred to as endogenous glucose production EGP.
The production of glucose from other carbon skeletons is necessary since the testes, erythrocytes and kidney medulla exclusively utilize glucose for ATP production. The brain also utilizes large amounts of the daily glucose consumed or produced via gluconeogenesis. However, in addition to glucose, the brain can derive energy from ketone bodies which are converted to acetyl-CoA and shunted into the TCA cycle.
The primary carbon skeletons used for gluconeogenesis are derived from pyruvate, lactate, glycerol, and the amino acids alanine and glutamine.
The liver is the major site of gluconeogenesis, however, as discussed below, the kidney and the small intestine also have important roles to play in this pathway. Synthesis of glucose from three and four carbon precursors is essentially a reversal of glycolysis.
The relevant features of the pathway of gluconeogenesis are diagrammed below:. Gluconeogenesis from two moles of pyruvate to two moles of 1,3-bisphosphoglycerate consumes six moles of ATP. This makes the process of gluconeogenesis very costly from an energy standpoint considering that glucose oxidation to two moles of pyruvate yields two moles of ATP.
The major hepatic substrates for gluconeogenesis glycerol, lactate, alanine, and pyruvate are enclosed in red boxes for highlighting. Pyruvate from the cytosol is transported across the inner mitochondrial membrane by the pyruvate transporter.
Transport of pyruvate across the plasma membrane is catalyzed by the SLC16A1 protein also called the monocarboxylic acid transporter 1, MCT1 and transport across the outer mitochondrial membrane involves a voltage-dependent porin transporter. Transport across the inner mitochondrial membrane requires a heterotetrameric transport complex mitochondrial pyruvate carrier consisting of the MPC1 gene and MPC2 gene encoded proteins.
When lactate is the gluconeogenic substrate the NADH is supplied by the lactate dehydrogenase LDH reaction indicated by the dashes linesand it is supplied by the malate dehydrogenase reaction when pyruvate and alanine are the substrates.
Secondly, one mole of glyceraldehydephosphate must be isomerized to DHAP and then a mole of DHAP can be condensed to a mole of glyceraldehydephosphate to form 1 mole of fructose-1,6-bisphosphate in a reversal of the aldolase reaction.
In hepatocytes the glucosephosphatase G6Pase reaction allows the liver to supply the blood with free glucose. Remember that due to the high K m of liver glucokinase most of the glucose will not be phosphorylated and will flow down its concentration gradient out of hepatocytes and into the blood. The three reactions of glycolysis that proceed with a large negative free energy change are bypassed during gluconeogenesis by using different enzymes.
In the liver, intestine, or kidney cortex, the glucosephosphate G6P produced by gluconeogenesis can be incorporated into glycogen. In this case the third bypass occurs at the glycogen phosphorylase catalyzed reaction.
Since skeletal muscle lacks glucosephosphatase it cannot deliver free glucose to the blood and undergoes gluconeogenesis exclusively as a mechanism to generate glucose for storage as glycogen. Conversion of pyruvate to PEP requires the action of two enzymes: The first reaction of bypass 1 utilizes the ATP muzcular biotin-requiring enzyme pyruvate carboxylase, PC.
As the name of the enzyme implies, pyruvate is carboxylated to form oxaloacetate OAA. PC is a somewhat gljcogenolisis enzyme in that it is one of only two metabolically important enzymes musscular requires an obligate activator. In the absence of its obligate activator, acetyl-CoA, PC is glucoegnolisis inactive. The primary source of the acetyl-CoA required by PC comes from glucogenolsiis oxidation of fatty acids which are being delivered to the liver after release from adipose mscular in response to fasting or stress.
Another critical enzyme that functions only in the presence of an obligate activator is carbamoylphosphate synthetase I CPS I of the urea cycle. Although the major function of PC is to drive precursor carbon atoms from pyruvate, lactate, and alanine into the generation of endogenous glucose, the production of oxaloacetate is also an important anaplerotic reaction since it can be used to fill-up the TCA cycle. Indeed, within the brain the primary function of PC is to ensure that glial cells have sufficient oxaloacetate to drive the TCA cycle.
Like the other biotin-dependent carboxylating enzymes in mammals, PC is mjscular and contains three distinct enzymatic domains: The human PC gene is located on chromosome 11q The reaction catalyzed by PC occurs in a two-step process.
During this initial stage of the reaction, biotin is moved to interact with the BC domain forming carboxybiotin. The carboxybiotin is brought into contact with the carboxyltransferase domain resulting in the formation of carboxylated biotin. This biotin carboxylase reaction involves a carboxyphosphate intermediate formed directly from ATP and bicarbonate. During the second step of the overall PC reaction, carboxybiotin is decarboxylated and pyruvate is concurrently glucigenolisis forming oxaloacetate.
PCK1 is located on chromosome 20q PCK2 is located on chromosome 14q The PCK2 gene is primarily expressed in the liver, kidney, and intestine as would be expected for a major gluconeogenic enzyme.
When glucagon binds its receptor the result is activation of adenylate cyclase with resultant increases in cAMP production. However, no transport mechanism exist for its direct transfer and OAA will glucogenloisis freely diffuse.
GLUCOGENOLISIS by Romina Rios on Prezi
Mitochondrial OAA can become cytosolic via three pathways: The transport of malate gluucogenolisis the cytosol is carried out by the transporter encoded by the SLC25A11 gene. The transport of aspartate to the cytosol is carried out by either of two transporters, one is encoded by the SLC25A12 gene and the other is encoded by the SLC25A13 gene. In the context of the transamination of OAA to aspartate and glucogenlisis reduction of OAA to malate, there is a need for adequate levels of the other intermediates of the malate-aspartate shuttle to ensure these latter two reactions can continue.
This shuttle is the principal mechanism for the movement of reducing equivalents in the form of NADH; highlighted in the red boxes from the cytoplasm to the mitochondria. The glycolytic pathway is a primary source of NADH. The electrons are “carried” into the mitochondria in the form of malate. Malate then enters the mitochondria where the reverse reaction is carried out by mitochondrial MDH.
Gluconeogenesis: Synthesis of New Glucose
Movement of mitochondrial OAA to the cytoplasm to maintain this cycle requires it be transaminated to aspartate Asp, D with the amino group being donated by glutamate Glu, E. The Asp then leaves the mitochondria and enters the cytoplasm. All the participants in the cycle are present in the proper cellular compartment for the shuttle to function due musculqr concentration dependent movement. GAPDH is glyceraldehydephosphate dehydrogenase.
AST is aspartate transaminase. Transamination of OAA to aspartate allows the aspartate to be transported to the cytosol where the reverse transamination occurs yielding cytosolic OAA. Therefore, this process is limited by the availability of these other substrates.
Either of these latter two reactions will predominate when the substrate for gluconeogenesis is lactate. Whether mitochondrial decarboxylation or transamination occurs is a function of the availability of PEPCK or transamination intermediates. The increased energy charge will allow cells to carry out the ATP costly process of gluconeogenesis. The NADH produced during the cytosolic oxidation of malate to OAA is utilized during the glucogeenolisis dehydrogenase reaction of gluconeogenesis.
The coupling of these two oxidation-reduction reactions is required to keep gluconeogenesis functional when pyruvate is the principal source of carbon atoms. The conversion of OAA to malate predominates when pyruvate derived from glycolysis or amino acid catabolism is the source of carbon atoms for gluconeogenesis. Hormonal glucogemolisis control the level of PEPCK protein as a means to regulate the flux through gluconeogenesis see below. Fructose-1,6-bisphosphate F1,6BP conversion to fructosephosphate F6P is the reverse of the rate limiting step of glycolysis.
Principles of Biochemistry/Gluconeogenesis and Glycogenesis
The reaction, a simple hydrolysis, is catalyzed by fructose-1,6-bisphosphatase F1,6BPase. The existence of two distinct forms of F1,6BPase was recognized by comparison of the kinetic and regulatory properties of the purified liver and muscle enzymes.
In addition, in patients with an inborn error in the gene encoding the liver F1,6BPase isoform, there is no reduction in skeletal muscle F1,6BPase activity. This led to the characterization of two F1,6BPase genes in the human genome. One expresses a liver version of the enzyme gene symbol: FBP1 and the other a muscle version of the enzyme gene symbol: The FBP1 gene is located on chromosome 9q The FBP2 gene is located at the same chromosomal location as the FBP1 gene but is composed of 7 exons that encode a protein of amino acids.
Like the regulation of glycolysis occurring at the PFK-1 reaction, the F1,6BPase reaction is a major point of control of gluconeogenesis see below. Glucosephosphate is converted to glucose through the action of enzymes of the glucosephosphatase G6Pase family.
Since the brain and skeletal muscle, as well as most non-hepatic tissues, lack G6Pase activity, any gluconeogenesis that might occur in these tissues is not utilized for blood glucose supply. In the kidney, muscle and especially the liver, G6P be shunted toward glycogen if blood glucose levels are adequate. The reactions necessary for glycogen synthesis are an alternate bypass series of reactions.
The glucosephosphatase activitites are membrane-associated multi-subunit complexes associated with the membranes of the endoplasmic reticulum, ER.
The complexes are composed of a catalytic subunit and transporter proteins for the transport of glucosephosphate, inorganic phosphate, and glucose across the membranes of the ER. The catalytic activity of G6Pases resides in a domain of the enzyme that is within the lumen of the ER, thus glucosephosphate must first be transported into the ER for the phosphate to be removed. The G6PC gene encodes the predominantly expressed functional phosphatase form of the glucosephosphatase.
The G6PC gene is located on chromosome 17q Only three human tissues express the G6PC gene, liver, kidney, and small intestine. Likewise, these are the only tissues that can contribute to endogenous glucose production. Defects in the G6PC gene are associated with the glycogen storage disease known as von Gierke disease glycogen storage disease type Ia.
The SLC17A3 gene generates two alternatively spliced mRNAs with one mRNA encoding a amino acid transporter that is localized to the apical membrane of epithelial cells of the proximal tubule of the kidney. The transport of free glucose, from the lumen of the ER to the cytosol, most likely occurs through the actions of plasma membrane localized GLUT transporters most likely GLUT2 in the liver as they are transiting the ER on their way to the plasma membrane.
The G6PC2 gene is expressed in pancreatic islets but the encoded protein does not possess glucosephosphatase activity. The G6PC2 gene is located on chromosome 2q The G6PC3 gene is located on chromosome 17q The G6PC3 gene encoded protein is not involved in endogenous free glucose production but is believed to have a function in neutrophil activities.
Although the G6PC3 encoded protein can hydrolyze phosphate from glucosephosphate in vitrothe enzyme has a preference for other substrates in vivo. Phosphorolysis of glycogen is carried out by glycogen phosphorylase, whereas, glycogen synthesis is catalyzed by glycogen synthase.
The G6P produced from gluconeogenesis can be used as a substrate for the synthesis of glycogen. Lactate is a predominate source of carbon atoms for glucose synthesis by gluconeogenesis. During anaerobic glycolysis in skeletal muscle, pyruvate is reduced to lactate by lactate dehydrogenase LDH.