Help explaining glycolysis?

Question

what factors affect the rate of glycolysis

how does lactic acid build up affect glycolysis

what role do negative and positive feedback play in the regulation

where does glycolysis regulation take place?

what is the pasteur effect?

Answer 1

Glycolysis is the sequence of reactions that converts glucose into pyruvate with the concomitant production of a relatively small amount of adenosine triphosphate (ATP). The word is derived from Greek γλυκύς (sweet) and λύσις (letting loose).

It is the initial process of most carbohydrate catabolism, and it serves three principal functions:

Generation of high-energy molecules (ATP and NADH) as cellular energy sources as part of aerobic respiration and anaerobic respiration; that is, in the former process, oxygen is present, and, in the latter, oxygen is not present

Production of pyruvate for the citric acid cycle as part of aerobic respiration

Production of a variety of six- and three-carbon intermediate compounds, which may be removed at various steps in the process for other cellular purposes.

As the foundation of both aerobic and anaerobic respiration, glycolysis is the archetype of universal metabolic processes known and occurring (with variations) in many types of cells in nearly all organisms. Glycolysis, through anaerobic respiration, is the main energy source in many prokaryotes, eukaryotic cells devoid of mitochondria (e.g., mature erythrocytes) and eukaryotic cells under low-oxygen conditions (e.g., heavily-exercising muscle or fermenting yeast).

In eukaryotes and prokaryotes, glycolysis takes place within the cytosol of the cell. In plant cells, some of the glycolytic reactions are also found in the Calvin-Benson cycle, which functions inside the chloroplasts. The wide conservation includes the most phylogenetically deep-rooted extant organisms, and thus it is considered to be one of the most ancient metabolic pathways.[1]

The most common and well-known type of glycolysis is the Embden-Meyerhof pathway, initially explained by Gustav Embden and Otto Meyerhof. The term can be taken to include alternative pathways, such as the Entner-Doudoroff Pathway. However, glycolysis will be used here as a synonym for the Embden-Meyerhof pathway.

regulation of glycolysis

The reactions catalyzed by hexokinase, PFK-1 and PK all proceed with a relatively large free energy decrease. These nonequilibrium reactions of glycolysis would be ideal candidates for regulation of the flux through glycolysis. Indeed, in vitro studies have shown all three enzymes to be allosterically controlled.

Regulation of hexokinase, however, is not the major control point in glycolysis. This is due to the fact that large amounts of G6P are derived from the breakdown of glycogen (the predominant mechanism of carbohydrate entry into glycolysis in skeletal muscle) and, therefore, the hexokinase reaction is not necessary. Regulation of PK is important for reversing glycolysis when ATP is high in order to activate gluconeogenesis. As such this enzyme catalyzed reaction is not a major control point in glycolysis. The rate limiting step in glycolysis is the reaction catalyzed by PFK-1.

PFK-1 is a tetrameric enzyme that exist in two conformational states termed R and T that are in equilibrium. ATP is both a substrate and an allosteric inhibitor of PFK-1. Each subunit has two ATP binding sites, a substrate site and an inhibitor site. The substrate site binds ATP equally well when the tetramer is in either conformation. The inhibitor site binds ATP essentially only when the enzyme is in the T state. F6P is the other substrate for PFK-1 and it also binds preferentially to the R state enzyme. At high concentrations of ATP, the inhibitor site becomes occupied and shifting the equilibrium of PFK-1 comformation to that of the T state decreasing PFK-1's ability to bind F6P. The inhibition of PFK-1 by ATP is overcome by AMP which binds to the R state of the enzyme and, therefore, stabilizes the conformation of the enzyme capable of binding F6P. The most important allosteric regulator of both glycolysis and gluconeogenesis is fructose-2,6-bisphosphate, F-2,6-BP, which is not an intermediate in glycolysis or in gluconeogenesis.

Glycolysis

Digestion of Dietary Carbohydrates

The Energy Derived form Glycolysis

Reactions of Glycolysis

Anaerobic Glycolysis

Regulation of Glycolysis

Metabolic Fates of Pyruvate

Lactate Metabolism

Ethanol Metabolism

Entry of Non-Glucose Carbons into Glycolysis

Glycogen Metabolism

Regulation of Blood  Glucose Levels

Digestion of Dietary Carbohydrates  

Dietary carbohydrate from which humans gain energy enter the body in complex forms, such as disaccharides and the polymers starch (amylose and amylopectin) and glycogen. The polymer cellulose is also consumed but not digested. The first step in the metabolism of digestible carbohydrate is the conversion of the higher polymers to simpler, soluble forms that can be transported across the intestinal wall and delivered to the tissues. The breakdown of polymeric sugars begins in the mouth. Saliva has a slightly acidic pH of 6.8 and contains lingual amylase that begins the digestion of carbohydrates. The action of lingual amylase is limited to the area of the mouth and the esophagus; it is virtually inactivated by the much stronger acid pH of the stomach. Once the food has arrived in the stomach, acid hydrolysis contributes to its degradation; specific gastric proteases and lipases aid this process for proteins and fats, respectively. The mixture of gastric secretions, saliva, and food, known collectively as chyme, moves to the small intestine.

The main polymeric-carbohydrate digesting enzyme of the small intestine is a-amylase. This enzyme is secreted by the pancreas and has the same activity as salivary amylase, producing disaccharides and trisaccharides. The latter are converted to monosaccharides by intestinal saccharidases, including maltases that hydrolyze di- and trisaccharides, and the more specific disaccharidases, sucrase, lactase, and trehalase ( the enzyme responsible for the degradation of the disaccharide alpha,alpha-trehalose  yielding  two  glucose subunits.  It is an enzyme  found  in  a  wide variety of organisms and whose sequence has been highly conserved throughout evolution). The net result is the almost complete conversion of digestible carbohydrate to its constituent monosaccharides.

The resultant glucose and other simple carbohydrates are transported across the intestinal wall to the hepatic portal vein and then to liver parenchymal cells and other tissues. There they are converted to fatty acids, amino acids, and glycogen, or else oxidized by the various catabolic pathways of cells.

Glucose cross the plasma membrane of the intestinal cells using a Na+/glucose transporter which allows sodium ions and glucose to enter the cell together ( that is, both molecules are passing trough the membrane in the same direction: symporter).The sodium ions flow down their concentration gradient while the glucose molecules are pumped up theirs. The sodium ions are pumped back out of the cell by the Na+/K+ ATPase in order to maintain their concentration gradient. Glucose is then  released into the bloodstream by the actionod a glucose transporters ( GLUT 5) present in the basal membrane of the endothelial cells .  

  



Copyright 2001 S.Marchesini

Five glucose transporters ( GLUT1,2,3,4,5) are present in mammals, having different kinetic characteristics both all consisting of a single polypeptide with characteristic 12 transmembrane - spanning domains.

  



Copyright 2001 S.Marchesini

GLUT 1,3 are present in nearly all mammalian cells and have a km for glucose of 1mM.  Since normal serum glucose levels are in the range 4-8mM, these proteins transport glucose at a constant rate.  Thus, they represent a basal glucose uptake. GLUT 2 is present in  hepatocytes and pancreatic B cells.  Its very high km for glucose (15-20mM) assures that glucose will rapidly enter hepatocytes for storage only in time of abundance. The  pancreas sense glucose   levels through GLUT 2 and adjust insulin levels accordingly.  

GLUT 4 is present in insulin-sensitive tissues  (muscle and adipocytes).  It has a km value of 5mM.   This transporter remains docked on vesicles inside the cell and reaches the plasma membrane when insulin is present.

Oxidation of glucose is known as glycolysis. Glucose is oxidized to either lactate or pyruvate. Under aerobic conditions, the dominant product in most tissues is pyruvate and the pathway is known as aerobic glycolysis. When oxygen is depleted, as for instance during prolonged vigorous exercise, the dominant glycolytic product in many tissues is lactate and the process is known as anaerobic glycolysis.



  The Energy Derived from Glucose Oxidation

Aerobic glycolysis of glucose to pyruvate, requires two equivalents of ATP to activate the process, with the subsequent production of four equivalents of ATP and two equivalents of NADH. Thus, conversion of one mole of glucose to two moles of pyruvate is accompanied by the net production of two moles each of ATP and NADH.

Glucose + 2 ADP + 2 NAD+ + 2 Pi --------> 2 Pyruvate + 2 ATP + 2 NADH + 2 H+

The NADH generated during glycolysis is used to fuel mitochondrial ATP synthesis via oxidative phosphorylation, producing either two or three equivalents of ATP depending upon whether the glycerol phosphate shuttle or the malate-aspartate shuttle is used to transport the electrons from cytoplasmic NADH into the mitochondria. The net yield from the oxidation of 1 mole of glucose to 2 moles of pyruvate is, therefore, either 6 or 8 moles of ATP. Complete oxidation of the 2 moles of pyruvate, through the TCA cycle, yeilds an additional 30 moles of ATP; the total yield, therefore being either 36 or 38 moles of ATP from the complete oxidation of 1 mole of glucose to CO2 and H2O.



The Individual Reactions of Glycolysis  

The pathway of glycolysis can be seen as consisting of 2 separate phases. The first is the chemical priming phase requiring energy in the form of ATP, and the second is considered the energy-yielding phase. In