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Beta carboxylation pathway of photosynthesis Berry, Joseph Andrew

Abstract

The occurrence of the ϐ-carboxylatlon pathway of photosynthesis Is correlated with the presence of two chlorophyllous tissues, within a leaf, which function co-operatively. It is postulated that: (a) Carbon dioxide fixation by ϐ-carboxylation is confined to the leaf mesophyll tissue. (b) The carboxyl transfer and the subsequent reactions leading to the synthesis of starch are confined to the bundle sheath tissue. (c) The carboxyl transfer reaction proceeds by decarboxylation of a C₄-acid coupled with RuDP carboxylase catalyzed synthesis of 3-PGA. (d) Cooperation between the two chlorophyllous tissues is made possible by the transport of metabolites between the tissues. Two plants Gomphrena globosa and Zea mays were used to test these postulates. These plants were found to form labelled aspartate and malate as early products of photosynthesis. Studies of the redistribution of ¹⁴C in pulse and chase type feedings of ¹⁴C0₂ and ¹²C0₂ indicate that nearly all labelled phosphorylated compounds may be formed from the C₄-aclds by a carboxyl transfer reaction. The location of some photosynthetic enzymes was studied by differential removal of enzymes from the two tissues. P-enolpyruvate carboxylase which is probably responsible for CO₂ fixation was found to occur predominantly in the mesophyll cells. "Malic" enzyme which will decarboxylate malic acid and RuDP carboxylase were found predominantly in the cells of the bundle sheath. Microradioautographs of thin sections of a Gomphrena globosa leaf indicate that ¹⁴C labelled C₄-acids rapidly appear in the cells of the bundle sheath. This apparently precedes the carboxyl transfer step. Studies of the location of labelled compounds within leaf cells by nonaqueous fractionation indicate that a portion of the C₄-acids are present in the cytoplasm and could be transported by cytoplasmic bridges. P-enolpyruvate carboxylase, "malic” enzyme, and RuDP carboxylase are present in extracts of these leaves in activities which approach or exceed the in vivo photosynthetic rate. These data suggest that the reactions of ϐ-carboxylation photosynthesis proceed as postulated. This mechanism involves CO₂ as a free intermediate as well as a substrate. An unusual C0₂ evolving process was detected at the onset of photosynthesis in Zea leaves after a short dark period. Feedings of ¹⁴CO₂ suggest that the C0₂2 evolved in this burst may be the result of a light stimulated decarboxylation of a C₄-acid. Studies of the kinetic properties of "malic" enzyme indicate that it would not be activated by light induced changes in the pH and Mg²⁺ concentrations, or in the adenylate pool.

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