A kinetic study of a ternary cycle between adenine nucleotides

Abstract

In the present paper, a kinetic study is made of the behavior of a moiety‐conserved ternary cycle between the adenine nucleotides. The system contains the enzymes S‐acetyl coenzyme A synthetase, adenylate kinase and pyruvate kinase, and converts ATP into AMP, then into ADP and finally back to ATP. l‐Lactate dehydrogenase is added to the system to enable continuous monitoring of the progress of the reaction. The cycle cannot work when the only recycling substrate in the reaction medium is AMP. A mathematical model is proposed whose kinetic behavior has been analyzed both numerically by integration of the nonlinear differential equations describing the kinetics of the reactions involved, and analytically under steady‐state conditions, with good agreement with the experimental results being obtained. The data obtained showed that there is a threshold value of the S‐acetyl coenzyme A synthetase/adenylate kinase ratio, above which the cycle stops because all the recycling substrate has been accumulated as AMP, never reaching the steady state. In addition, the concept of adenylate energy charge has been applied to the system, obtaining the enabled values of the rate constants for a fixed adenylate energy charge value and vice versa.

On the Dynamics of the Adenylate Energy System: Homeorhesis vs Homeostasis

“…In fact, it is not clear yet what methodologies are the most appropriate to monitor the values of adenine nucleotides [192] . Although bioluminescence assays and high-performance liquid chromatography are the ones most commonly used for most of the studies [151] , [192] , these procedures are discontinuous and do not allow to observe real-time variations at short temporal periods. Moreover, adenosine nucleoside levels are critically dependent on sample manipulation and extraction by traditional methods.…”

“…Thus, for ATP synthase we have assumed Michaelis–Menten kinetics with competitive inhibition by the product [150] . An iso-random Bi Bi mechanism has been reported for adenylate kinase kinetics [151] – [152] . We have also considered that a fraction of the adenylate kinases exhibit the balance shifted to the left and simultaneously the rest of the adenylate kinase macromolecules present a balance shifted to the right, depending their catalytic activities on the system demand.…”

Effects of airway surface liquid height on the kinetics of extracellular nucleotides in airway epithelia

“…Since the ecto-enzymes are attached to the apical membrane of epithelial cells, the reactions occur on the cell surface, which means that [S] is the concentration at z ¼0. It was assumed that the reversible reaction ATP þ AMP⇄2ADP catalyzed by the enzyme ecto-AK follows a Bi Bi mechanism (Valero et al, 2006;Voet and Voet, 2010) …”

Modeling the ascorbate-glutathione cycle in chloroplasts under light/dark conditions

“…One important feature of the pathway under study is the existence of three moiety-conserved cycles (metabolic structures interconverting different forms of a chemical moiety, while the sum of these forms remains constant [ 15 – 17 ]), which are the sums (NADPH + NADP + ), (GSH + 2GSSG) and (ASC + DHA + MDA) (Fig. 1a ).…”