Ning G. Pon

Correlation of structure with function in Spinacea oleracea chloroplasts

Spinach chloroplasts, isolated in isotonic sucrose medium, were sonically ruptured and separated by differential centrifugation into three main fractions: a green precipitate, a colourless supernatant and a yellow, low density lipid-containing layer. These fractions were analyzed for their physical, chemical and biochemical properties. Comparison of a thin section of an isolated osmium-stained chloroplast with a thin section of osmium-stained green precipitate, by electron microscopy, shows that the particles in the green precipitate correspond both in thickness and lateral dimensions to the lamellar structure of the isolated chloroplast. Heavy metal shadowing of both air-dried and lyophilized green precipitate demonstrates the presence of lamellar structures from 10,000 to 20,000 A in diameter. The lamellar structure, totaling 160 A in thickness, is composed of two layers; each layer is made up of granular subunits 100 A thick. Chlorophyll to nitrogen weight ratios are fairly constant for green lamellar structures from 800 to 20,000 A in diameter, indicating that chlorophyll is uniformly distributed throughout the lamellar structure of the chloroplast. The similarity of relative photosynthetic pigment concentrations over the visible region, of the Hill reaction activity and of the carbon dioxide fixation capacities for the various sized particles in the green precipitate suggests that the smallest lamellar fragments used in these experiments are large in comparison with the smallest fragment which converts electromagnetic to chemical energy. A model for chloroplast lamellar structure is proposed on the basis of the results obtained. The supernatant, by electron microscopy, is seen to consist principally of oblately spherical particles, 100 A thick and 200 A in diameter. These particles are indistinguishable from the main water-soluble protein of the chloroplast with sedimentation coefficient 16 (Fraction I protein). About 90% of the total carboxydismutase activity is associated with the supernatant proteins. In addition, these proteins are able to convert fructose-6-phosphate into other photosynthetic carbon cycle intermediates. This supernatant must be added to the green precipitate in order to obtain maximum carbon dioxide fixation rates. Electron micrographs of unfractionated osmium-fixed chloroplast sonicate show large osmium-stained spherical objects corresponding in size and staining properties to the osmiophyllic granules observed in thin sections of osmiumstained chloroplast in the intact leaf. Similar granules, 500 to 1500 A in diameter, were obtained when the yellow, low density lipid layer was fixed with osmium and then shadowed.

X-Ray Structure of Rhodobacter Capsulatus Cytochrome bc (1): Comparison with its Mitochondrial and Chloroplast Counterparts.

Ubihydroquinone: cytochrome (cyt)c oxidoreductase, or cyt bc1, is a widespread, membrane integral enzyme that plays a crucial role during photosynthesis and respiration. It is one of the major contributors of the electrochemical proton gradient, which is subsequently used for ATP synthesis. The simplest form of the cyt bc1 is found in bacteria, and it contains only the three ubiquitously conserved catalytic subunits: the Fe–S protein, cyt b and cyt c1. Here we present a preliminary X-ray structure of Rhodobacter capsulatus cyt bc1 at 3.8 Å and compare it to the available structures of its homologues from mitochondria and chloroplast. Using the bacterial enzyme structure, we highlight the structural similarities and differences that are found among the three catalytic subunits between the members of this family of enzymes. In addition, we discuss the locations of currently known critical mutations, and their implications in terms of the cyt bc1 catalysis.

Chemical composition and the substructure of lamellae isolated from Spinacea oleracea chloroplasts

Purified chlorophyll-containing lamellae isolated from Spinacea oleracea chloroplasts were active in the Hill reaction. Spectrochemical and chemical analyses established the ratios of protein, lipid, organic phosphorus, iron, manganese, copper and chlorophyll in the lamellae. A minimum molecular weight for a lamellar subunit on the basis of manganese content is 960,000. Electron microscopy reveals particles which correspond to a unit of this size within the lamellae. The particulate nature of the lamellae is more pronounced after lipid removal. This observation supports the view that the lamellae consist of proteins embedded in a lipid matrix.

PHOTO SPIN RESONANCE IN CHLOROPHYLL-CONTAINING PLANT MATERIAL

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D-Threose 2,4-diphosphate inhibition of 3-phosphoglyceric acid photoreduction by a sonically ruptured spinach chloroplast system.

Abstract 1. 1. [I- 14 C]PGA is readily reduced by a sonically ruptured chloroplast system in light. This reduction is inhibited by threose 2,4-diphosphate. 2. 2. PGA accumulates when 14 CO 2 is incubated in light with the sonically ruptured chloroplast system in the presence of threose 2,4-diphosphate. This indicates that PGA is in the major pathway of photosynthetic CO 2 fixation in the system. 3. 3. Threose diphosphate inhibition of the total 14 CO 2 fixation by the carbon cycle of sonically ruptured spinach chloroplasts is not due to inhibition of carboxydismutase, but due primarily to inhibition of triose phosphate dehydrogenase which in turn limits the formation rate of the photosynthetic carbon cycle CO 2 acceptor, RuDP.

The metabolism of [14C]ribulose diphosphate by spinach chloroplasts

Abstract Our study of the metabolism of [14C]ribulose diphosphate by sonically ruptured spinach chloroplasts shows that the products of the primary car☐ylation reaction of this system in light are not phosphoglyceric acid and triose phosphate, but only phosphoglyceric acid.