The role of chlorophyll b in photosynthesis: hypothesis.

BACKGROUND: The physico-chemical properties of chlorophylls b and c have been known for decades. Yet the mechanisms by which these secondary chlorophylls support assembly and accumulation of light-harvesting complexes in vivo have not been resolved. PRESENTATION: Biosynthetic modifications that introduce electronegative groups on the periphery of the chlorophyll molecule withdraw electrons from the pyrrole nitrogens and thus reduce their basicity. Consequently, the tendency of the central Mg to form coordination bonds with electron pairs in exogenous ligands, a reflection of its Lewis acid properties, is increased. Our hypothesis states that the stronger coordination bonds between the Mg atom in chlorophyll b and chlorophyll c and amino acid sidechain ligands in chlorophyll a/b- and a/c-binding apoproteins, respectively, enhance their import into the chloroplast and assembly of light-harvesting complexes. TESTING: Several apoproteins of light-harvesting complexes, in particular, the major protein Lhcb1, are not detectable in leaves of chlorophyll b-less plants. A direct test of the hypothesis--with positive selection--is expression, in mutant plants that synthesize only chlorophyll a, of forms of Lhcb1 in which weak ligands are replaced with stronger Lewis bases. IMPLICATIONS: The mechanistic explanation for the effects of deficiencies in chlorophyll b or c points to the need for further research on manipulation of coordination bonds between these chlorophylls and chlorophyll-binding proteins. Understanding these interactions will possibly lead to engineering plants to expand their light-harvesting antenna and ultimately their productivity.

A potential role of chlorophylls b and c in assembly of light-harvesting complexes.

Chlorophyll (Chl)-containing light-harvesting complexes (LHCs) in chloroplasts of plant and algal cells usually include an oxidized Chl (Chl b or c) in addition to Chl a. Oxidation of peripheral groups on the tetrapyrrole structure increases the Lewis acid strength of the central Mg atom. We propose that the resulting stronger coordination bonds between oxidized Chls and ligands in LHC apoproteins (LHCPs) stabilize the initial intermediates and thus promote assembly of LHCs within the chloroplast envelope.

Vacuolar granules in Chlamydomonas reinhardtii: polyphosphate and a 70-kDa polypeptide as major components.

The alga Chlamydomonas reinhardtii contains cytoplasmic vacuoles that are often filled with a dense granule that is released from the cell by exocytosis. Purified granules contained polyphosphate, complexed with calcium and magnesium, as the predominant inorganic components. Antiserum was raised against the major 70-kDa protein in granules purified from wall-deficient (cw15) mutants, which reacted on immunoblots with larger glycoprotein complexes in purified cell wall fractions from wild-type cells. Confocal fluorescence microscopy detected binding of these antibodies predominantly at the periphery of wall-containing C. reinhardtii y1 cells but primarily to loci in the interior of cells of the cw15 strain. Immunoelectron microscopy demonstrated that the 70-kDa protein was localized in vacuolar granules and the trans-Golgi network in sections of cw15 cells but not in the cytosol or chloroplast. Treatment of cells with a dye, fluorescent in its protonated form, indicated that the pH within vacuoles was lower than that in the cytosol, which suggested that the vacuoles are similar to lysosomes. Thus, the vacuoles may serve a dual function to provide an environment for degradation within the cell and also serve as a vehicle for secretion of specific proteins.

Chlorophyll binding to peptide maquettes containing a retention motif.

The motif Glu-X-X-His/Asn-X-Arg is conserved in the first and third membrane-spanning domains of all light-harvesting chlorophyll a/b- and a/c-binding proteins in chloroplasts. Molecular modeling of synthetic peptides containing the sequence Glu-Ile-Val-His-Ser-Arg, a motif found in the apoprotein of the major light-harvesting complex in plants, generated a loop structure formed by intrapeptide, electrostatic attraction between Glu and Arg. His, Asn, and charge-compensated Glu-Arg pairs are known ligands of the magnesium atom in chlorophyll. The prediction that this structure should bind two molecules of chlorophyll was confirmed experimentally with an assay based on fluorescence resonance energy transfer between peptides and chlorophyll a. Motifs with both potential ligands bound approximately two times the amount of chlorophyll as one in which His was replaced by Ala. These results support the conclusion that formation of this intermediate, within membranes of the envelope, is a crucial step in assembly of light-harvesting complexes and a mechanism that regulates import of the apoproteins into the chloroplast.

Prevalence of malnutrition in nonsurgical hospitalized patients and its association with disease complications.

The prevalence of malnutrition and its predictive value for the incidence of complications were determined in 155 patients hospitalized for internal or gastrointestinal diseases. At admission, 45% of the patients were malnourished according to the Subjective Global Assessment (physical examination plus questionnaire), 57% according to the Nutritional Risk Index [(1.5 x albumin) + (41.7 x present/usual weight)], and 62% according to the Maastricht Index [(20.68 - (0.24 x albumin) - (19.21 x transthyretin (prealbumin) - (1.86 x lymphocytes) - (0 04 x ideal weight)]. Crude odds ratios for the incidence of any complication in malnourished compared with well-nourished patients during hospitalization were 2.7 (95% CI: 1.4, 5.3) for the Subjective Global Assessment, 2.8 (1.5, 5.5) for the Nutritional Risk Index, and 3.1 (1.5, 6.4) for the Maastricht Index. Odds ratios were reduced to 1.7 (0.8, 3.6), 1.6 (0.7, 3.3), and 2.4 (1.1, 5.4), respectively, after a multivariate analysis that included disease category and disease severity. Because the confounding factors adjusted for are not only a measure of the severity of the disease but may also be influenced by malnutrition itself, the actual risk for complications due to malnutrition could be higher than the adjusted odds ratios. In conclusion, malnutrition was frequent in patients with gastrointestinal disease and other internal diseases at the time of admission. The severity of malnutrition in the patients predicted the occurrence of complications during their hospital stay and this association was not completely explained by confounding factors.