Energetics and Metabolic Rate

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Adenosine Triphosphate (ATP) Functions as an "Energy Currency" in Metabolism

In the past few chapters, we have pointed out that carbohydrates, fats, and proteins can all be used by cells to synthesize large quantities of adenosine triphosphate (ATP), which can be used as an energy source for almost all other cellular functions. For this reason, ATP has been called an energy "currency" in cell metabolism. Indeed, the transfer of energy from foodstuffs to most functional systems of the cells can be done only through this medium of ATP (or the similar nucleotide guanosine triphosphate, GTP). Many of the attributes of ATP are presented in Chapter 2.

An attribute of ATP that makes it highly valuable as an energy currency is the large quantity of free energy (about 7300 calories, or 7.3 Calories [kilocalories], per mole under standard conditions, but as much as 12,000 calories under physiologic conditions) vested in each of its two high-energy phosphate bonds. The amount of energy in each bond, when liberated by decomposition of ATP, is enough to cause almost any step of any chemical reaction in the body to take place if appropriate energy transfer is achieved. Some chemical reactions that require ATP energy use only a few hundred of the available 12,000 calories, and the remainder of this energy is lost in the form of heat.

ATP Is Generated by Combustion of Carbohydrates, Fats, and Proteins. In previous chapters, we discussed the transfer of energy from various foods to ATP. To summarize, ATP is produced from

1. Combustion of carbohydrates—mainly glucose, but also smaller amounts of other sugars such as fructose; this occurs in the cytoplasm of the cell through the anaerobic process of glycolysis and in the cell mitochondria through the aerobic citric acid (Krebs) cycle.

2. Combustion of fatty acids in the cell mitochondria by beta-oxidation.

3. Combustion of proteins, which requires hydrolysis to their component amino acids and degradation of the amino acids to intermediate compounds of the citric acid cycle and then to acetyl coenzyme A and carbon dioxide.

ATP Energizes the Synthesis of Most Important Cellular Components. Among the most important intracellular processes that require ATP energy is the formation of peptide linkages between amino acids during the synthesis of proteins. The different peptide linkages, depending on which types of amino acids are linked, require from 500 to 5000 calories of energy per mole. It will be recalled from the discussion of protein synthesis in Chapter 3 that four high-energy phosphate bonds are expended during the cascade of reactions required to form each peptide linkage. This provides a total of 48,000 calories of energy, which is far more than the 500 to 5000 calories eventually stored in each of the peptide linkages.

ATP energy is also used in the synthesis of glucose from lactic acid and in the synthesis of fatty acids from acetyl coenzyme A. In addition, ATP energy is used for the synthesis of cholesterol, phospholipids, the hormones, and almost all other substances of the body. Even the urea excreted by the kidneys requires ATP to cause its formation from ammonia. One might wonder about the advisability of expending energy to form urea, which is simply discarded by the body. However, remembering the extreme toxicity of ammonia in the body fluids, one can see the value of this reaction, which keeps the ammonia concentration of the body fluids at a low level.

ATP Energizes Muscle Contraction. Muscle contraction will not occur without energy from ATP. Myosin, one of the important contractile proteins of the muscle fiber, acts as an enzyme to cause breakdown of ATP into adenosine diphosphate (ADP), thus releasing the energy required to cause contraction. Only a small amount of ATP is normally degraded in muscles when muscle contraction is not occurring, but this rate of ATP usage can rise to at least 150 times the resting level during short bursts of maximal contraction. The postulated mechanism by which ATP energy is used to cause muscle contraction is discussed in Chapter 6.

ATP Energizes Active Transport Across Membranes. In

Chapters 4, 27, and 65, active transport of electrolytes and various nutrients across cell membranes and from the renal tubules and gastrointestinal tract into the blood is discussed. In each instance, we noted that active transport of most electrolytes and substances such as glucose, amino acids, and acetoacetate can occur against an electrochemical gradient, even though the natural diffusion of the substances would be in the opposite direction. To oppose the electrochemical gradient requires energy, which is provided by ATP.

ATP Energizes Glandular Secretion. The same principles apply to glandular secretion as to the absorption of substances against concentration gradients, because energy is required to concentrate substances as they are secreted by the glandular cells. In addition, energy is required to synthesize the organic compounds to be secreted.

ATP Energizes Nerve Conduction. The energy used during propagation of a nerve impulse is derived from the potential energy stored in the form of concentration differences of ions across the membranes. That is, a high concentration of potassium inside the fiber and a low concentration outside the fiber constitute a type of energy storage. Likewise, a high concentration of sodium on the outside of the membrane and a low concentration on the inside represent another store of energy. The energy needed to pass each action potential along the fiber membrane is derived from this energy storage, with small amounts of potassium transferring out of the cell and sodium into the cell during each of the action potentials. However, active transport systems energized by ATP then retransport the ions back through the membrane to their former positions.

Phosphocreatine Functions as an Accessory Storage Depot for Energy and as an "ATP Buffer"

Despite the paramount importance of ATP as a coupling agent for energy transfer, this substance is not the most abundant store of high-energy phosphate bonds in the cells. Phosphocreatine, which also contains high-energy phosphate bonds, is three to eight times as abundant. Also, the high-energy bond (~) of phosphocreatine contains about 8500 calories per mole under standard conditions and as much as 13,000 calories per mole under conditions in the body (37°C and low concentrations of the reactants).This is slightly greater than the 12,000 calories per mole in each of the two high-energy phosphate bonds of ATP. The formula for crea-tinine phosphate is the following:

Unlike ATP, phosphocreatine cannot act as a direct coupling agent for energy transfer between the foods

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