The Adenylate System

The pH within cells appears to be tightly controlled although small variations are sometimes observed. Red blood cells, thymocytes, liver, skeletal muscles, and intact hearts all maintain a pH in the range 7.0 - 7.3.b/h/1 However, the pH can fall to 6.2 within 13 minutes of oxygen deprivation (ischemia) and to 6.1 after exhaustive exercise.13 0 The 31P NMR technique permits the monitoring of pH as well as the state of the adenylate system (Section D) in human limbs suffering from circulatory insufficiency.0... 

Of central importance to the energy metabolism of all cells is the adenylate system which consists of adenosine 5 -triphosphate (ATP), adenosine 5-diphosphate (ADP), and adenosine 5 -monophosphate (AMP) together with inorganic phosphate (P ), pyrophosphate (PP ), and magnesium ions. Remember that P refers to the mixture of ionic forms of phosphoric acid present under experimental conditions. Between pH 4 and pH 10 this will be mainly LLPO,... 

Various measures of the phosphorylating potential of the adenylate system within cells have been proposed. One measure is the product [ATP] / [ADP][P ], which will be called the phosphorylation state ratio... 

Another way in which the phosphorylation state of the adenylate system can regulate the cycle depends upon the need for GDP in step/of the cycle (Fig. 17-4). Within mitochondria, GTP is used largely to reconvert AMP to ADP. Consequently, formation of GDP is promoted by AMP, a compound that arises in mitochondria from the utilization of ATP for activation of fatty acids (Eq. 13-44) and activation of amino acids for protein synthesis (Eq. 17-36). 

From these data Krebs and Veech concluded that the oxidation state of the NAD system is determined largely by the phosphorylation state ratio of the adenylate system.169 If the ATP level is high the equilibrium in Eq. 17-42a will be reached at a higher [NAD+]/[NADH] ratio and lactate may be oxidized to pyruvate to adjust the [lactate]/[pyruvate] ratio. 

An interesting experiment is to allow oxidative phosphorylation to proceed until the mitochondria reach state 4 and to measure the phosphorylation state ratio Rp, which equals the value of [ATP] / [ADP][PJ that is attained. This mass action ratio, which has also been called the "phosphorylation ratio" or "phosphorylation potential" (see Chapter 6 and Eq. 6-29), often reaches values greater than 104-105 M 1 in the cytosol.164 An extrapolated value for a zero rate of ATP hydrolysis of log Rf) = 6.9 was estimated. This corresponds (Eq. 6-29) to an increase in group transfer potential (AG of hydrolysis of ATP) of 39 kj/mol. It follows that the overall value of AG for oxidation of NADH in the coupled electron transport chain is less negative than is AG. If synthesis of three molecules of ATP is coupled to electron transport, the system should reach an equilibrium when Rp = 106 4 at 25°C, the difference in AG and AG being 3RT In Rp = 3 x 5.708 x 6.4 = 110 kj mol-1. This value of Rp is, within experimental error, the same as the maximum value observed.165 There apparently is an almost true equilibrium among NADH, 02 and the adenylate system if the P/O ratio is 3. 

However, if the electron transport between 3-hydroxybutyrate and cytochrome b562 is tightly coupled to the synthesis of one molecule of ATP, the observed potential of the carrier will be determined not only by the imposed potential E of the equilibrating system but also by the phosphorylation state ratio of the adenylate system (Eq. 18-7). Here AG atp is the group transfer potential (-AG of hydrolysis) of ATP at pH 7 (Table 6-6), and n is the number of electrons passing through the chain required to synthesize one ATP. In the upper part of the equation n is the number of electrons required to reduce the carrier, namely one in the case of cytochrome b562. 

Jakobs KH, Aktories K, Schultz G (1984) Mechanism of pertussis toxin action on the adenylate system inhibition of the turn-on reaction of the inhibitory regulatory site. In Eur. J. Biochem. 140 177-181... 

The coupling of reaction sequences through the adenylate system is entirely different from the two types of stoichiometric relationships discussed above. The stoichiometry of each reaction in which ATP is regenerated or used is, of course, fixed by the nature of the reaction, but the number and types of reactions in which ATP is involved have been determined by evolutionary processes, rather than by simple chemical necessity. Indeed the use of ATP as a coupling agent and the evolutionary adjustments of stoichiometric relationships for maximal metabolic advantage are at the very center of biological function. This fact is underscored by the participation of ATP in every extended metabolic sequence. 

The amount of metabolically-available energy which is momentarily stored in the adenylate system of a living cell is linearly related to the mole fraction of ATP plus half of the mole fraction of ADP. Expressed in terms of concentration of the nucleotides, this available energy is called the energy charge [26], which can be calculated from the following formula ... 

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