ATP The Primary Energy Carrier

Acid dissociation constants and dissociation constants of complex ions determine the concentrations of species that are present in a solution at equilibrium under specified conditions. Ionic dissociation reactions occur rapidly and tend to remain at equilibrium during an enzyme-catalyzed reaction. Since ATP (see Fig. 1.1) is the primary carrier of energy in biochemical systems and since a good deal is known about its binding properties, these properties are considered here in some detail. 

It IS intriguing to note that, although all of the nucleotide triphosphates are energetically equivalent, ATP is nonetheless the primary cellular energy carrier. In addition, two important electron carriers, NAD and FAD, are derivatives of ATP. The role oj ATP in energy metabolism is paramowH. 

At Sheffield Krebs embarked upon the work that would elucidate some of the complex reactions of cell metabolism (the processes that extract energy from food). This extraction of energy is achieved via a series of chemical transformations that remove energy-rich electrons fiom molecules obtained from food. These electrons pass along a chain of molecular carriers in a way that ultimately gives rise to water and adenosine triphosphate (ATP), which is the primary source of chemical energy that powers cellular activity. 

Primary active transport occurs when the transport of a substrate is coupled to an energy-yielding metabolic reaction. The energy required may come from several different sources (a) the high-energy compound ATP used by a specific ATPase (ATPase pump) (b) energy from the electron transport system released as electrons that flow down the cytochrome chain (redox-pump) and (c) the electric field produced by free radicals. Implicit in these three theories is the participation of ions and ion transport. Secondary active transport is a term often used to denote the transport of one substrate linked to the flow of a second substrate. Wilbrandt (1975) refers to this as flow-coupled active transport it may be this form of transport that is most often involved in the active uptake of sugars and amino acids. A review of some models of carrier-mediated active transport transport has recently been presented by Crane (1977). 

It is clearly impossible to give a comprehensive overview of this rapidly expanding field. I have chosen a few experts in their field to discuss one (class of) transport protein(s) in detail. In the first five chapters pumps involved in primary active transport are discussed. These proteins use direct chemical energy, mostly ATP, to drive transport. The next three chapters describe carriers which either transport metabolites passively or by secondary active transport. In the last three chapters channels are described which allow selective passive transport of particular ions. The progress in the latter field would be unthinkable without the development of the patch clamp technique. The combination of this technique with molecular biological approaches has yielded very detailed information of the structure-function relationship of these channels. 

Primary transport the active transport of a solute against the electrochemical gradient by carrier proteins which derive energy for transport directly from the hydrolysis of ATP. 

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