EnergyProducing and EnergyUtilizing Systems

Page 218
6.1— Energy­Producing and Energy­Utilizing Systems
Living cells are composed of a complex, intricately regulated system of energy­producing and energy­utilizing chemical reactions. Metabolic reactions involved in energy generation break down ingested or stored fuels such as carbohydrate, lipid, or protein in what are termed catabolic pathways. These reactions usually result in the conversion of large complex molecules to smaller molecules (ultimately CO2 and H2O), with production of storable or conservable energy, and often require the consumption of oxygen during this process. Such reactions are accelerated during periods of fuel deprivation or stress to an organism.
Energy­utilizing reactions perform various necessary, and in many instances tissue­specific, cellular functions, for example, nerve impulse conduction, muscle contraction, growth, and cell division. Metabolic pathways involved in the biosynthesis of large, complex molecules from smaller precursors are termed anabolic pathways and require the expenditure of energy. Such reactions are accelerated when energy is readily available, when precursor molecules are in abundance, or during periods of growth or regeneration of cellular material.
ATP Links Energy­Producing and Energy­Utilizing Systems
The relationship between energy­producing and energy­utilizing functions of the cell is illustrated in Figure 6.1. Energy may be derived from oxidation of metabolic fuels presented to the organism usually in the form of carbohydrate, lipid, and protein. The proportion of each fuel utilized as an energy source depends on the tissue and the dietary and hormonal state of the organism. For example, mature erythrocytes and adult brain in the fed state use only carbohydrate as a source of energy, whereas the liver of a diabetic or fasted individual metabolizes primarily lipid to meet the energy demands. Energy may be consumed during performance of various energy­linked (work) functions, some of which are indicated in Figure 6.1. Note that the liver and the pancreas are primarily involved in biosynthetic and secretory work functions, whereas the primary function of cardiac and skeletal muscle involves converting metabolic energy into mechanical energy during muscle contraction.
The essential link between energy­producing and energy­utilizing pathways is the nucleoside triphosphate, adenosine 5¢­triphosphate (ATP) (Figure 6.2). The ATP molecule is a purine (adenine) nucleotide in which the adenine is attached in a glycosidic linkage to D­ribose. Three phosphoryl groups are esterified to the 5 position of the ribose moiety in phosphoanhydride bonds. The
Figure 6.1 Relationship between energy production and energy utilization.
Page 219
Figure 6.2 Structure of ATP and ADP complexed with Mg2+.
two terminal phosphoryl groups (i.e., b and g) are involved in the phosphoric acid anhydride bonding and are designated as energy­rich or high­energy bonds. Synthesizing ATP as a result of a catabolic process or consuming ATP in an energy­linked cellular function involves formation and either hydrolysis or transfer of the terminal phosphate group of ATP. The physiological form of this nucleotide is chelated with a divalent metal cation such as magnesium. Adenosine diphosphate also chelates magnesium, but the affinity of the metal cation for ADP is considerably less than for ATP. Although adenine nucleotides are mainly involved in energy generation or conservation, various nucleoside triphosphates, including ATP, are involved in transferring energy during biosynthetic processes. As indicated in Figure 6.3, the guanine nucleotide GTP serves as the source of energy in gluconeogenesis and protein synthesis, whereas
Figure 6.3 Structures of purine and pyrimidine bases involved in various biosynthetic pathways.
Figure 6.4 Nucleoside diphosphate kinase and nucleoside monophosphate kinase reactions. N represents any purine or pyrimidine base; (d) indicates a deoxyribonucleotide.