Formation Of Light-harvesting Complex Research Articles

Molecular assembly of Zn porphyrin complexes using synthetic light-harvesting model polypeptides

Synthetic single alpha-helix hydrophobic polypeptides, which have similar amino acid sequences to the hydrophobic core in the native light-harvesting 1-beta polypeptide from Rhodobacter sphaeroides, formed Zn porphyrin complexes on a gold electrode, as well as in n-octyl-beta-glucoside micelles: this process is dependent on the structure of the pigments and the polypeptides. Interestingly, an enhanced photoelectric current was observed when Zn mesoporphyrin monomer complexed with the synthetic light-harvesting model polypeptide in an alpha-helical configuration was assembled with a defined orientation onto the electrode. Analog of these light-harvesting model complexes are also useful in providing insights into the effect of polypeptide structure on the formation of light-harvesting complexes on and off electrodes.

The composition and spectral properties of three different forms of light-harvesting complex II

Different aggregates of LHC II play a very important role in regulating the light absorption and excitation energy transfer of plant. Trimeric LHC II was purified from spinach thylakoid membrane. In order to obtain the dimeric and monomeric LHC II, the trimer was treated with the mixture of 2% OGP and 10 microg/mL PLA(2), then loaded onto the sucrose density gradient in the presence of 0.06% triton X-100. The LHC II trimer, dimer and monomer isolated by sucrose density gradient all contained three polypeptides with molecular weight of 29, 28 and 26 kd respectively. The pigment composition showed much difference in the content of Chl b and xanthophyll among three forms of LHC II. To study the light capture and excitation energy transfer in different forms of LHC II, the absorption and fluorescence spectra were analyzed. The results clearly showed that the efficiency of energy absorption and transfer was different in the three kinds of LHC II, the highest for trimeric LHC II, intermediate for dimeric LHC II, and the lowest for monomeric LHC II. It was suggested that there might be a physiological homeostasis of different aggregates of LHC II in plants, which is significant for the plant self-regulating upon exposure to variable light environment.

Self-molecular assembly of light-harvesting complex isolated from photosynthetic bacteria on modified electrodes

Pigments play an important role on energy-transfer process in light-harvesting (LH) polypeptides complex of photosynthesis, where porphyrins such as bacteriochlorophylls (BChls) are assembled according to cooperative interactions between the LH polypeptides and BChls. It is interesting to note that the LH polypeptides organize the BChls so that an efficient energy transfer process between BChls may occur. In this paper, we examine the self-assemble of Zn-BChla using LH polypeptides separately isolated from photosynthetic bacteria to organize an artificial LH complex. The key to the molecular assembly is ready availability of the LH complex-formation on modified indium-tin-oxide (ITO) electrode. We selected the native LH polypeptides isolated from photosynthetic bacteria, because the native polypeptides formed a stable Zn-BChla complex in octylglucoside (OG) solution analogous to the LH 1-type complex. The ITO electrode was silanized to introduce amino groups and then, the LH polypeptides complex was assembled on the modified ITO by soaking the ITO electrode into the OG solution forming the LH polypeptides complex (phosphate buffer, pH = 7.0). The Qy band of Zn-BChla in the LH complex on the ITO was red-shifted analogous to the subunit-tipe complex of the LH 1 complex, indicating the self-molecular assembly of the LH 1-type complex on the electrode

Construction of artificial light-harvesting complex using light-harvesting polypeptide or its model synthetic polypeptides with zinc substituted bacteriochlorophyll a

Molecular assemblies of zinc-substituted bacteriochloropyll a (Zn-BChl a ) using light-harvesting ( LH ) polypeptide separately isolated from photosynthetic bacteria, R. rubrum and its model synthetic polypeptides were examined in lipid bilayers as well as in n-Octyl-b-D-glucopyranoside (OG) micelle. The key to the assemby is of constructing an artificial LH complex in lipid bilayers as well as of providing insight into structural requirements for formation of the core LH complex (LH 1). UV-vis., CD, SAXS, and DLS data indicated that the LH- a polypeptide selectively organizeds Zn-BChl a complex in OG micelle rather than BChl a complex, analogous to the LH1-type complex, depending on the temperature. Comparing the amino acid sequence in the N- and C-terminal segments of the LH- a polypeptide and its model peptides on the complex-formation with Zn-BChl a or BChl a reveals that the amino acid residues from M to F in the N-terminal segment of the LH-a polypeptide essentially account for the difference in the complex-formation between Zn-BChl a and BChl a. Interestingly, self-assembly of an artificial LH complex using the LH- a polypeptide and its model polypeptides with Zn-BChl a was observed in lipid bilayers. This is the first report that the native LH- a alone and its model peptides organize an artificial LH complex using Zn-BChl a, analogous to the LH 1-type complex of photosynthetic bacteria.

Enzymatic and chemical cleavage of the core light-harvesting polypeptides of photosynthetic bacteria: determination of the minimal polypeptide size and structure required for subunit and light-harvesting complex formation.

To ascertain the minimal structural requirements for formation of the subunit and core light-harvesting complex (LH1), the alpha- and beta-polypeptides of the LH1 from three purple photosynthetic bacteria were enzymatically or chemically truncated or modified. These polypeptides were then used in reconstitution experiments with bacteriochlorophyll a (BChla), and the formation of subunit and LH1 complexes was evaluated using absorbance and circular dichroism spectroscopies. Truncation or modification outside of the conserved core sequence region of the polypeptides had no effect on subunit or LH1 formation. However, the extent of formation and stability of the subunit and LH1 decreased as the polypeptide was shortened inside the core region within the N-terminal domain. This behavior was suggested to be due to the loss of potential ion-pairing and/or hydrogen-bonding interactions between the polypeptides. While the spectroscopic properties of the subunit complexes generated using truncated polypeptides were analogous to those obtained using native polypeptides, in some cases the resulting LH1 complex absorption was blue-shifted relative to the control. Thus, truncation within the N-terminal domain may have long-range effects on the immediate BChla binding environment, since the putative BChla binding site resides near the C-terminal end of the polypeptides. It was also demonstrated that the His located within the membrane-spanning domain on the N-terminal end of the beta-polypeptide is not participating in ligation of the BChla in the reconstituted subunit and therefore probably not in LH1.

Formation of light-harvesting complexes of photosystem II in Scenedesmus

The patterns of accumulation of photosynthetic pigments, mRNAs transcribed from the genes encoding chlorophyll a/b-binding proteins (Lhc mRNAs), and light-harvesting complex (LHC) apoproteins have been compared in dark- and light-grown cells of wild-type (WT) Scenedesmus obliquus and the mutants WT-LHC1, C-6D, C-6E, C-2A′, C-2A′-LHC1 and C-2A′-LHC2. In contrast to the WT, which developed the complete photosynthetic apparatus in darkness, mutant strains exhibited either a lack of chlorophyll (Chl) b, carotenoids or LHC formation, or a light requirement for the synthesis of Chls or carotenoids. Accumulation of LHC apoproteins strictly depended on the presence of Chl a and carotenoids. The formation of a functional light-harvesting complex additionally required Chl b. Amounts of Lhc mRNAs of dark-grown mutant cells exceeded those of the WT. Thus, accumulation of Lhc mRNAs was independent of Chl and carotenoid synthesis and of chloroplast development. Transcripts of 1.4 and 1.6 kb accumulated constitutionally during growth in darkness. Illumination increased the amounts of these mRNAs and, additionally, a 1.8-kb Lhc mRNA appeared. Hence, light altered the expression of Lhc genes both quantitatively and qualitatively. Ratios of Chl a/Lhc mRNA, Chl a/LHC apoprotein and Lhc mRNA/LHC apoprotein were identical in the greening mutants C-6D, C-2A′, C-2A′-LHC1, and C-2A′-LHC2 after 8 h illumination. Light-independent accumulation of Lhc mRNAs, by contrast, was not correlated with the amounts of either Chl a or LHC apoproteins.

Formation of light-harvesting complexes of photosystem II in Scenedesmus

The patterns of accumulation of photosynthetic pigments, mRNAs transcribed from the genes encoding chlorophyll a/b-binding proteins (Lhc mRNAs), and light-harvesting complex (LHC) apoproteins have been compared in dark- and light-grown cells of wild-type (WT) Scenedesmus obliquus and the mutants WT-LHC 1 , C-6D, C-6E, C-2A', C-2A'-LHC 1 and C-2A'-LHC 2 . In contrast to the WT, which developed the complete photosynthetic apparatus in darkness, mutant strains exhibited either a lack of chlorophyll (Ch1) b, carotenoids or LHC formation, or a light requirement for the synthesis of Chls or carotenoids. Accumulation of LHC apoproteins strictly depended on the presence of Ch1 a and carotenoids. The formation of a functional light-harvesting complex additionally required Ch1 b

Prior Accumulation of Phosphatidylglycerol high in Trans-03Hexadecenoic Acid Enhances the in vitro Stability of Oligomeric Light Harvesting Complex II

Summary The effects of cold-hardening (5 °C) and non-hardening (20 °C) temperatures on the greening of etiolated seedlings under intermittent light conditions was examined in winter rye (Secale cereale L. cv. Muskateer). Development at 5 °C resulted in 2-fold higher levels of chlorophyll a and a 3-fold higher chlorophyll a fluorescence emission at 725 nm than leaves developed at 20 °C. Exposure of etiolated seedlings to low intensity (1 OWE — m-z . s-1), intermittent light at 20 °C resulted in the formation of higher levels of the trans-A3-hexadecenoic acid content associated with phosphatidylglycerol compared to exposure of etiolated seedlings to intermittent light at 5 °C. Subsequent exposure of seedlings grown either at 20 °C or 5 °C in intermittent light to continuous light (200 /,E — m-z . s-') caused a decrease in the ratio of chlorophyll a, b from between 7-10 to about 3.0 with the concomitant appearance of light harvesting complex II. However, in vitro data indicate that the oligomeric form of light harvesting complex II predominates at 20 °C (oligomer , monomer = 1.64) whereas the monomeric form predominates at 5 °C (oligomer, monomer = 0.42). Also, at 20 °C a maximum ratio of oligomer, monomer of 1.6-1.7 was attained after only 24 h of continuous illumination in seedlings initially exposed to 48 cycles of intermittent light. In contrast, 72 h at 20 °C was required to attain the same ratio in etiolated seedlings that had not been exposed to intermittent light. We conclude that low, intermittent light can induce the accumulation of high levels of trans-A3-hexadecenoic acid in the absence of light harvesting complex II. The prior accumulation of trans-A3-hexadecenoic acid enhances the stabilization of the oligomeric form of light harvesting complex II. This is consistent with a sequential process for the accumulation of trans-A3-hexadecenoic acid and the stabilization of light harvesting complex II in its in vitro oligomeric form.