Influence of Adenosine Phosphates and Magnesium on Photosynthesis in Chloroplasts from Peas, Sedum, and Spinach

Abstract

PGA-supported 02 evolution by pea chloroplasts was not inhibited hnmediately by ADP; the rate of 02 evolution slowed as time passed, corresponding to the effect of ADP on CO2 assimilation, and indicating that glycerate 3-phosphate kinase was a site of inhibition. Likewise, upon the addition of AMP, inhdbition of PGA-dependent 02 evolution became more severe with time. This did not mirror CO2 assimilation, which was inhibited immediately by AMP. In Sedwn chloroplasts, PGA-dependent 02 evolution was not inhibited by ADP and AMP. In chloroplasts from peas and Sedun, the magnitude of MgADP and MgATP stimulation of PGAdependent 02 evolution was not much larger than that given by ATP, and it was much smaDler than MgATP stimulation ofCO2 assimilation. Analysis of stromal metabolite levels by anion exchange chromatography indicated that ribulose 1,5-bisp te carboxylase was inhibited by ADP and stimulated by MgADP in Sedn chloroplasts. (22), although penetration through the chloroplast membrane has been shown to occur via an adenosine-P translocator (14). In contrast, pea (25, 31) and wheat (9) chloroplasts from very young plants are sensitive to adenosine-P. In all of these studies, scant attention was paid to the effect of Mg2+, even though MgATP is required for the kinases of the reductive pentose-P cycle (26), and MgADP is used by coupling factor (4). In addition, transport across the chloroplast membrane of externally added adenosine-P (without Mg2+) could result in chelation of chloroplastic Mg2+, affecting the activity of the Mg2"-requiring enzymes, fructose and sedoheptulose bisphosphatase (6, 24) and RuBP3 carboxylase (2). In this paper, the dependence of CO2 assimilation upon the addition of adenosine-P with and without Mg2' has been investigated in isolated chloroplasts from two C3 plants, peas and spinach, and Sedum, a CAM species. An attempt was made to pinpoint reaction steps of the reductive pentose-P czcle or other factors which are responsive to adenosine-P and Mg +, using chloroplasts from peas and Sedum.In studying the interaction of Mg2+ with adenosine-P, it is essential that the real concentration of the Mg2+ complex (as opposed to free adenosine-P) be taken into consideration. Such an analysis begins by calculation of the apparent metal-ligand stability constant at the medium pH. For any metal binding ligand L which has two different relevant states of ionization, both of which can bind a metal ion M, the following equilibria apply, where Ka is the acid dissociation constant of the ligand, and K1 and K2 are the metal stability constants for the more protonated and less protonated forms of the ligand, respectively.

Localization of the Enzymes Involved in the Photoevolution of H2 from Acetate in Chlamydomonas reinhardtii

“…The most convenient means to affect the proper balance would be to enroll a rapid ATP/ADP exchange system. Adenine nucleotides, complexed to Mg2+, were shown to undergo an exchange with [U-'4C]ADP in Sedum chloroplasts (22).…”

Development and Use of Chlorotetracycline Fluorescence as a Measurement Assay of Chloroplast Envelope-Bound Mg2+

“…9B, data points with no Mg24 added to the reaction medium) find no substantial effect of added K4 on photosynthesis. Also, the effect of exogenous Mg2e on photosynthesis has been reported to be quite variable, with some researchers reporting differing sensitivity of photosynthesis to exogenous Mg24 in the same publication (8,19). S. C. Huber (personal communication) has speculated that growth conditions may greatly affect the level of endogenous Mg2' bound to the envelope of isolated chloroplasts, and that this variation may be responsible for the varying sensitivity of photosynthesis to K4 (as shown in Fig.…”

Fermentative Metabolism of Chlamydomonas reinhardii

“…The routes of acetate metabolism involving both reductive and oxidative carbon pathways (Fig. 3) would be understandable if ATP could traverse the Chlamydomonas chloroplast either complexed to Mg2` (30) or by an adenylate transporter (31). In the latter mechanism, adenine nucleotides are exchanged for PPi, a product of the acetate thiokinase reaction.…”