Adenosine diphosphate glycolysis

The importance of phosphates in intermediary metabolism became evident with the discovery of the role of ATP, adenosine diphosphate (ADP), and inorganic phosphate (Pj) in glycolysis (Chapter 17). 

Figure 17-2. The pathway of glycolysis. ( ,—P, HOPOj " .inhibition.) At asterisk Carbon atoms 1-3 of fructose bisphosphateform dihydroxyacetone phosphate, whereas carbons 4-6 form glyceraldehyde 3-phosphate. The term "bis-," as in bisphosphate, indicates that the phosphate groups are separated, whereas diphosphate, as in adenosine diphosphate, indicates that they are joined.

In the past decade, a large number of studies emphasized the heterogeneous scale-free degree distribution of metabolic networks Most substrates participate in only a few reactions, whereas a small number of metabolites ( hubs ) participate in a very large number of reactions [19,45,52]. Not surprisingly, the list of highly connected metabolites is headed by the ubiquitous cofactors, such as adenosine triphosphate (ATP), adenosine diphosphate (ADP), and nicotinamide adenine dinucleotide (NAD) in its various forms, as well as by intermediates of glycolysis and the tricarboxylic acid (TCA) cycle. 

ATP plays a central role in cellular maintenance both as a chemical for biosynthesis of macromolecules and as the major soirrce of energy for all cellular metabolism. ATP is utilized in numerous biochemical reactions including the eitric acid cycle, fatty acid oxidation, gluconeogenesis, glycolysis, and pyruvate dehydrogenase. ATP also drives ion transporters sueh as Ca -ATPase in the endoplasmic reticulum and plasma membranes, H+-ATPase in the lysosomal membrane, and Na+/K+-ATPase in the plasma membrane. Chemieal energy (30.5 kJ/mol) is released by the hydrolysis of ATP to adenosine diphosphate (ADP). 

Enzymes are highly specific and usually catalyze only one type of reaction. Some enzymes show absolute specificity. For example, pyruvate kinase catalyzes the transfer of a phosphate group only from phosphoenolpyruvate to adenosine diphosphate during glycolysis (Chapter 13). Examples of enzymes that show less specificity are ... 

The reaction is essentially irreversible under physiological conditions and is a major regulatory step of glycolysis. PFK-1 is an inducible, highly regulated, allosteric enzyme. In its active form, muscle PFK-1 is a homotetramer (M.W. 320,000) that requires K+ or NH4, the latter of which lowers Km for both substrates. When adenosine triphosphate (ATP) levels are low during very active muscle contraction, PFK activity is modulated positively despite low concentration of fructose-6-phosphate. Allosteric activators of muscle PFK-1 include adenosine monophosphate (AMP), adenosine diphosphate (ADP), fructose-6-phosphate, and inorganic phosphate (Pi) inactivators are citrate, fatty acids, and ATP. 

Phosphoenolpyruvate is produced in the next-to-last step in the metabolic pathway called glycolysis, which is the first stage of carbohydrate breakdown. In the final reaction of glycolysis the phosphoryl group from phosphoenolpyruvate is transferred to adenosine diphosphate (ADP). The reaction produces ATP, the major energy currency of the cell. 

This is the first reaction in the biochemical pathway called glycolysis. A phosphoryl group is transferred from a donor molecule, adenosine triphosphate, to the recipient molecule, glucose. The products are glucose-6-phosphate and adenosine diphosphate. This enz)rme, called hexokinase, is an example of a transferase. 

First one glucose molecule breaks down into two pyruvate molecules in a process known as glycolysis. The energy from this exothermic reaction is used to bind inorganic phosphate (Pj) to ADP (adenosine diphosphate) forming ATP (adenosine triphosphate) and convert NAD+ (nicotinamide adenine dinucleotide) to NADFI (reduced nicotinamide adenine dinucleotide) ... 

Glycolysis is a nearly universal pathway in biological systems. It includes a key step in which adenosine triphosphate (ATP) loses its terminal phosphate group to a glucose molecule and becomes adenosine diphosphate (ADP) (Figure 9.19). 

Figure 6.1 Pathways involved in glucose oxidation by plant cells (a) glycolysis, (b) Krebs cycle, (c) mitochondrial cytochrome chain. Under anoxic conditions. Reactions 1, 2 and 3 of glycolysis are catalysed by lactate dehydrogenase, pyruvate decarboxylase and alcohol dehydrogenase, respectively. ATP and ADP, adenosine tri- and diphosphate NAD and NADHa, oxidized and reduced forms of nicotinamide adenine dinucleotide PGA, phosphoglyceraldehyde PEP, phosphoenolpyruvate Acetyl-CoA, acetyl coenzyme A FP, flavoprotein cyt, cytochrome e, electron. (Modified from Fitter and Hay, 2002). Reprinted with permission from Elsevier...

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