Metabolism interconnected reactions

Metabolism is the sum of many interconnected reaction sequences that interconvert cellular metabolites. Each sequence is regulated so as to provide what the cell needs at a given time and to expend energy only when necessary. 

The glycolytic pathway resembles a series of lakes (metabolic pools) connected by short rivers (the reactions between the pools). This pattern is reflected in the ways that functional metabolic relationships have evolved. Reactions involving ATP and ADP occur in the interconnecting reactions, or rivers. Clearly this is where they are expected, because an ATP-linked reaction within a metabolic pool makes no more sense than a hydroelectric power plant in the middle of a lake. 

Metabolism Is Composed of Many Coupled, Interconnecting Reactions... 

METABOLISM IS COMPOSED OF MANY COUPLED, INTERCONNECTING REACTIONS... 

Thousands of reactions mediated by an equal number of enzymes are occurring at any given instant within the cell. Metabolism has many branch points, cycles, and interconnections, as a glance at a metabolic pathway map reveals... 

Although the importance of a systemic perspective on metabolism has only recently attained widespread attention, a formal frameworks for systemic analysis has already been developed since the late 1960s. Biochemical Systems Theory (BST), put forward by Savageau and others [142, 144 147], seeks to provide a unified framework for the analysis of cellular reaction networks. Predating Metabolic Control Analysis, BST emphasizes three main aspects in the analysis of metabolism [319] (i) the importance of the interconnections, rather than the components, for cellular function (ii) the nonlinearity of biochemical rate equations (iii) the need for a unified mathematical treatment. Similar to MCA, the achievements associated with BST would warrant a more elaborate treatment, here we will focus on BST solely as a tool for the approximation and numerical simulation of complex biochemical reaction networks. 

As one can perhaps glean from figure 16.1, metabolism can be organized into a series of interconnected metabolic pathways. For example, the linear set of reactions shown in boldface running down the figure is the glycolytic pathway to which I turn in... 

The metabolism of a cell or organism is a complex and carefully controlled set of interconnected chemical reactions, each under the control of a specific enzyme. These are organized into pathways. 

Although we have described metabolic transformations in plant cells in terms of individual pathways, these pathways interconnect so completely that we should instead consider pools of metabolic intermediates shared among these pathways and connected by readily reversible reactions (Fig. 20-37). One such metabolite pool includes the hexose phosphates glucose 1-phosphate, glucose 6-phosphate, and fructose 6-phosphate a second includes the 5-phosphates of the pentoses ri-bose, ribulose, and xylulose a third includes the triose phosphates dihydroxyacetone phosphate and glycer-aldehyde 3-phosphate. Metabolite fluxes through these... 

The first section of this book has dealt with the basic structures and with many of the complexities of the materials formed by living cells. In the next major section we will look at the chemical reactions that build and maintain a cell and that permit it to grow and to be responsive to external stimuli. These reactions are organized into metabolic sequences or pathways that form a complex, branched, and interconnected network. It would be pointless to try to memorize all of them. However, at this point it will be worthwhile to consider the significance of a few of the major sequences, which describe central pathways of metabolism. 

Once inside the cell, folates participate in a number of interconnected metabolic pathways involving (1) thymidine and purine biosynthesis necessary for DNA synthesis, (2) methionine synthesis via homocysteine remethylation, (3) methylation reactions involving S-adenosylmethionine (AdoMet), (4) serine and glycine interconversion, and (5) metabolism of histidine and formate (see Figure 8). Via these pathways. 

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