User talk:Menka Saha

Menka Saha (talk) 15:51, 14 January 2014 (UTC)

Glycolysis is Central Metabolic Process.
''' Glycolysis is an anaerobic process through which ATP is synthesized during the conversion of the six-carbon sugar glucose to two molecules of the three-carbon compound pyruvate. It has two phases: an energy investment phase, where ATP is consumed, and an energy generation phase, where ATP is produced. Glycolysis - One of the central metabolic pathways is glycolysis, shown schematically in relation to other pathways in Figure 12.2. It is classified as a stage 2 pathway for degradation of carbohydrates, in either aerobic or anaerobic cells. Another schematic view of the pathway is shown in Figure 12.3. Pyruvate, the product of glycolysis, is handled differently by anaerobic (fermentation) and aerobic pathways. Anaerobic pathways lead to a variety of products, including lactate and ethanol, while aerobic pathways lead to acetyl-CoA and ultimately to carbon dioxide in the citric acid cycle (Figure 12.4). Introduction Glucose is metabolised in the cell through glycolysis, also know as the Embden-Meyerhoff Pathway "Glycolysis is the primary pathway for anaerobic degradation of D-glucopyranoses and other D-hexopyranoses. It is probably universal among organisms: certainly the enzymes which catalyze the pathway's reactions are among the most conserved (and therefore presumably most ancient) among proteins." (Quoted from ref. 3 below). Under anaerobic conditions, glycolysis is a self-contained process leading to the production of fermentation products which vary from organism to organism. In mammalian cells, the primary product is lactate; in yeasts, ethanol and CO2. The process can be split into several stages: •	"Activation" of glucose o	Glucose is first phosphorylated to form glucose-6-Pi, then isomerised into fructose-6-Pi. The phosphorylation reactions requires ATP. This series of reactions serves two main purposes: 	the glucose is "activated" so as to be able to enter the pathway; 	the glucose is removed from solution in the cytoplasm, thus lowering the concentration and favoring the transport gradient into the cell. •	Formation of triose phosphates o	The fructose-6-Pi is phosphorylated again to give fructose-1,6-bisphosphate, using another ATP, and then split into two triose-Pi molecules, dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-Pi, using the enzyme aldolase. The triose phosphates are interconvertible through triose phosphate isomerase. •	Substrate level phosphorylation through oxidation of glyceraldehyde-3-phosphate o	Glyceraldehyde-3-Pi is oxidized in a reaction in which phosphate is bound, and NAD+ is reduced to NADH. The free-energy of the oxidation reaction is used to form a phosphate bond with a high negative free energy of hydrolysis (a "high-energy" bond). The 1,3-bisphosphoglycerate is then converted to 3-phosphoglycerate by a kinase reaction in which the "high-energy" phosphate on the carboxylate end is transfered to ADP to form ATP. •	ATP synthesis linked to conversion of phosphoenolpyruvate to pyruvate o	3-phosphoglycerate is converted to 2-phosphoglycerate by phosphoglycerate mutase, and then dehydrated to give phosphoenolpyruvate, using the enzyme enolase. This converts the phosphate bond at the 2-position to a "high-energy" bond. o	The phosphoenolpyruvate reacts with ADP to form ATP and pyruvate, using pyruvate kinase •	Reduction of pyruvate to regenerate NAD+ o	In order to restore the NAD+ used up in glyceraldehyde-3-Pi oxidation, the pyruvate is reduced to lactate using NADH The net yield of anaerobic glycolysis is 2ATP / glucose, with an overall reaction: glucose + 2 ADP + 2 phosphate <==> 2 lactate + 2 ATP Central Role of ATP in energy metabolism The conversion between ATP, and ADP and phosphate, plays a central role in the energy metabolism of the cell. The poise of the reaction in a metabolic compartment plays a determining role in the direction of metabolism, either directly through thermodynamics, or indirectly through the activating (or inhibitory) effects on enzymes. Compartmentalization of the eukaryote cell •	Cytoplasmic metabolism probably reflects an archeal origin •	Mitochondrial structure and eubacterial origin o	The Virtual Cell •	Distribution of metabolic activities between cytoplasmic and mitochondrial compartments

Figure 12.2: Overview of metabolism.

Figure 12.3: The initial phase of carbohydrate catabolism: glycolysis.

Metabolic pathway In biochemistry, metabolic pathways are series of chemical reactions occurring within a cell. In each pathway, a principal chemical is modified by a series of chemical reactions. Enzymes catalyze these reactions, and often require dietary minerals, vitamins, and other cofactors in order to function properly. Because of the many chemicals (a.k.a. "metabolites") that may be involved, metabolic pathways can be quite elaborate. In addition, numerous distinct pathways co-exist within a cell. This collection of pathways is called the metabolic network. Pathways are important to the maintenance of homeostasis within an organism. Catabolic (break-down) and Anabolic (synthesis) pathways often work interdependently to create new biomolecules as the final end-products. A metabolic pathway involves the step-by-step modification of an initial molecule to form another product. The resulting product can be used in one of three ways: •	To be used immediately, •	To initiate another metabolic pathway, called a flux generating step •	To be stored by the cell A molecule called a substrate enters a metabolic pathway depending on the needs of the cell and the availability of the substrate. An increase in concentration of anabolic and catabolic intermediates and/or end-products may influence the metabolic rate for that particular pathway. Overview Each metabolic pathway consists of a series of biochemical reactions that are connected by their intermediates: the products of one reaction are the substrates for subsequent reactions, and so on. Metabolic pathways are often considered to flow in one direction. Although all chemical reactions are technically reversible, conditions in the cell are often such that it is thermodynamically more favorable for flux to flow in one direction of a reaction. For example, one pathway may be responsible for the synthesis of a particular amino acid, but the breakdown of that amino acid may occur via a separate and distinct pathway. One example of an exception to this "rule" is the metabolism of glucose. Glycolysis results in the breakdown of glucose, but several reactions in the glycolysis pathway are reversible and participate in the re-synthesis of glucose (gluconeogenesis). •	Glycolysis was the first metabolic pathway discovered: 1.	As glucose enters a cell, it is immediately phosphorylated by ATP to glucose 6-phosphate in the irreversible first step. 2.	In times of excess lipid or protein energy sources, certain reactions in the glycolysis pathway may run in reverse in order to produce glucose 6-phosphate which is then used for storage as glycogen or starch. •	Metabolic pathways are often regulated by feedback inhibition. •	Some metabolic pathways flow in a 'cycle' wherein each component of the cycle is a substrate for the subsequent reaction in the cycle, such as in the Krebs Cycle (see below). •	Anabolic and catabolic pathways in eukaryotes often occur independently of each other, separated either physically by compartmentalization within organelles or separated biochemically by the requirement of different enzymes and co-factors. Major metabolic pathways

Cellular respiration Main article: Cellular respiration Several distinct but linked metabolic pathways are used by cells to transfer the energy released by breakdown of fuel molecules into ATP and other small molecules used for energy (e.g. GTP, NADPH, FADH). These pathways occur within all living organisms in some form: 1.	Glycolysis 2.	Aerobic respiration and/or Anaerobic respiration 3.	Citric acid cycle / Krebs cycle (not in most obligate anaerobic organisms) 4.	Oxidative phosphorylation (not in obligate anaerobic organisms) Synthesis of energetic compounds from non-living matter: •	Photosynthesis (plants, algae, cyanobacteria) •	Glycolysis is an anaerobic metabolic pathway, found in the cytosol of all cells, which forms adenosine triphosphate ( ATP ) by degrading glucose. It also serves as a source of precursors for other pathways, and as a recipient of products of various pathways for use as metabolic fuels. Its universal and central role in metabolism suggests that glycolysis evolved early in the history of life.