Association with rubisco

A number of other enzymes deserve consideration because they are involved in pathways that are closely linked to nitrogen metabolism, such as Rubisco and other carboxylases that are critical to the coupling of C and N metabolism (Huppe and Turpin, 1994), or electron transport system (ETS) enzymes such as isocitrate dehydrogenase that are involved in respiratory metabolism (Roy and Packard, 2001). Others are relevant to N losses (e.g., esterases that are used as indices of cell lysis Agusti et al., 1998 or proteases associated with active cell death processes, e.g., Berman-Frank et al, 2004) (Table 32.1). 

The full range of carbon isotopic compositions seen in the Cheshire and Manjeri Fm stromatolites is thus collectively interpreted as evidence for abundant rubisco-based oxygenic photosynthetic cyanobacteria, possibly associated with anoxygenic photosynthesizers, and supporting a community of other microbial organisms (as in modern stromatolites Pierson 1994). 

Two important aspects of the carbon isotopic fractionation imposed by the C4 pathway are summarized graphically in Figures 9 and 10. The first schematically indicates the carbon-isotopic relationships between dissolved CO2, bicarbonate, and the carbon that is added to phosphoenolpyruvate in the reaction catalyzed by phosphoenolpyruvate carboxylase. It shows that, because the kinetic isotope effect associated with PEP carboxylase is smaller than the equilibrium isotope effect between dissolved CO2 and bicarbonate, the fixed carbon is enriched in relative to that in the dissolved CO2. As resi lt, the CO2 subsequently made available to rubisco in cam and C4 plants is enriched in relative to atmospheric CO2. If that p arbon were fixed with perfect efficiency, the biomass of the plant would be enriched in relative to CO2 from the atmosphere—Sp4 would be negative, indicating an inverse fractionation. 

SITE-DIRECTED MUTAGENESIS OF AMINO ACIDS ASSOCIATED WITH THE ACTIVE SITE OF RUBISCO FROM ANACYSTIS NIDULANS... 

The dissociation constant (pK) for this reaction has recently been corrected to 6.06 for the ionic strength of enzyme assay, indicating a Km value for dissolved carbon dioxide of about 8 micromolar. At the same time, the oxygenase activity of Rubisco associated with photorespiration in C3 plants is also regulated, changing as the pH rises and bicarbonate replaces carbon dioxide. 

The A/Cj results therefore show that thylakoid processes are less susceptible to O3 than the primary carboxylase. The sulphydryl groups associated with this enzyme would be expected to be particularly susceptible to oxidation. Previous investigations have found that both the amount (8) and activity (9) of RubisCO decrease following O3 treatments. 

Site-Directed Mutagenesis of Amino Acids Associated with the Active Site of Rubisco from Anacystis nidulans 351... 

Bovet L, Mtlller MO, Siegenthaler PA. The 26 and 14 kDa phosphoproteins associated with spinach chloroplast envelope membranes are distinct membrane-bound pools of LHCII and of Rubisco small subunit. Planta 1994 (in press)... 

The term "molecular chaperone" is not novel and was first introduced (7) to describe the transient association of nucleoplasmin with the histones during the formation of nucleosomes. Subsequently, a similar transient association was found in the chloroplast of higher plants between nascent Large subunits (LSU) of die enzyme Ribulose bisphosphate carboxylase (Rubisco) and a SdOkDa protein complex composed of 12-14 60kDa subunits (2). The nascent chloroplast-synthesized LSU was found to bind this complex in a non-covalent manner and in sub-stoichiometric amounts. 

An association between photorespiration and the metabolism in chloroplasts of ammonia from protein catabolism has been shown with the alga Chlamydomonas. Evidence that increased oxygen stimulated this ammonia utilisation and carbon dioxide alone or anaerobiosis abolished it was obtained. The results were consistent with a close link between Rubisco s oxygenase activity and protein catabolism releasing ammonia, which was then reassimilated in the chloroplast. Results suggesting a similar link when nitrate was utilised by the cells were also obtained. Whether this link also exists in higher plants was not investigated. 

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