Photosynthesis Research (v.96, #3)

Memories: from protein synthesis to photosynthesis by Anthony San Pietro (185-199).
Emphasis herein is on the early years of my scientific life, primarily in graduate school and at the McCollum-Pratt Institute, Johns Hopkins University, as techniques learned and research performed then became the basis for future scientific endeavors. Studies on the mechanism of conversion of light energy into chemical free energy were a logical consequence of earlier investigations on enzyme-catalyzed hydrogen transfer reactions and pyridine nucleotide coenzyme biochemistry. Identification of several protein factors involved in pyridine nucleotide reduction by illuminated chloroplasts is described and, hopefully, adequately and honestly referenced to complementary research in other laboratories. Coupled with progress were changes in nomenclature of the protein factors and are so noted. In particular, David Wharton proposed the descriptive name, ferredoxin, for the non-heme iron and labile sulfide-containing proteins which serve as redox cofactors in a variety of energy conserving reactions. The inclusion of “Lessons” is adapted from Efraim Racker (1976, A new look at mechanisms in bioenergetics. Academic Press, NY). They are lessons that I learned and are included herein solely for graduate students.
Keywords: Ferredoxin; Ferredoxin–NADP oxidoreductase; Non-Heme Iron Proteins; Photosynthesis; Robert (Robin) Hill; David Shemin; Erwin Chargaff

A blue diode PAM (Pulse Amplitude Modulation) fluorometer was used to measure rapid Photosynthesis (P) versus Irradiance (E) curves (P vs. E curves) in Synechococcus (classical cyanobacteria), Prochlorothrix (prochlorophyta), Chlorella (chlorophyta), Rhodomonas (cryptophyta), Phaeodactylum (bacillariophyta) Acaryochloris (Chl d/a cyanobacteria) and Subterranean Clover (Trifolium subterraneum, Papilionaceae, Angiospermae). Effective quantum yield (ΦPSII) versus irradiance curves could be described by a simple exponential decay function (ΦPSII = ΦPSII, maxe−kE) although Log/Log transformation was sometimes found to be necessary to obtain the best fits. Photosynthesis was measured as relative Electron Transport Rate (rETR) standardised on a chlorophyll basis. P versus E curves were fitted to the waiting-in-line function (an equation of the form P = Pmax · k · E · e−kE) allowing half-saturating and optimal irradiances (Eoptimum) to be estimated. The second differential of the equation shows that at twice optimal light intensities, there is a point of inflection in the P versus E curve. Photosynthesis is inhibited 26.4% at this point of inflection. The waiting-in-line model was found to be a very good descriptor of photosynthetic light saturation curves and superior to hyperbolic functions with an asymptotic saturation point (Michaelis–Menten, exponential saturation and hyperbolic tangent). The exponential constants (k) of the ΦPSII versus E and P versus E curves should be equal because rETR is directly proportional to ΦPSII × E. The conventionally calculated Non-Photochemical Quenching (NPQ) in Synechococcus was not significantly different to zero but NPQ versus E curves for the other algae could be fitted to an exponential saturation model. The kinetics of NPQ does not appear to be related to the kinetics of ΦPSII or rETR.
Keywords: Acaryochloris ; Chlorella ; Subterranean clover; Phaeodactylum ; Prochlorothrix ; Rhodomonas ; Synechococcus ; Trifolium subterraneum ; Oxyphotobacteria; Photosynthesis; PAM fluorometry; Modulated fluorometry; Effective quantum yield; Electron transport rate; Non-photochemical quenching; Light saturation curves; P versus E; PE

Quantitative characteristics of photosynthetic electron transport were evaluated in vivo on the basis of the multi-exponential analysis of OJIP fluorescence transients induced by saturating actinic light. The OJIP fluorescence curve F(t), measured in Chlamydomonas reinhardtii cells, was transformed into the (1 − F O/F(t)) × (F V /F M)−1 transient, which is shown to relate to PS 2 closure. We assumed that kinetics of PS 2 closure during OJIP rise reflects time-separated processes related to the establishment of redox equilibrium at the PS 2 acceptor side (OJ), PQ pool (JI), and beyond Cyt b/f (IP). Three-exponential fitting was applied to (1 − F O/F(t)) × (F V /F M)−1 transient to obtain lifetimes and amplitudes of the OJ, JI, and IP components of PS 2 closure, which were used to calculate overall rates of reduction and re-oxidation of the PS 2 acceptor side, PQ pool, and intermediates beyond Cyt b/f complex. The results, obtained in the presence of inhibitors, oxidative reagents, and under different stress conditions prove the suggested model and characterize the introduced parameters as useful indicators of photosynthetic function.
Keywords: Chlorophyll fluorescence; OJIP transient; Chlamydomonas reinhardtii

Molecular mechanics calculations and exciton theory have been used to study pigment organization in chlorosomes of green bacteria. Single and double rod, multiple concentric rod, lamella, and Archimedean spiral macrostructures of bacteriochlorophyll c molecules were created and their spectral properties evaluated. The effects of length, width, diameter, and curvature of the macrostructures as well as orientations of monomeric transition dipole moment vectors on the spectral properties of the aggregates were studied. Calculated absorption, linear dichroism, and polarization dependent fluorescence-excitation spectra of the studied long macrostructures were practically identical, but circular dichroism spectra turned out to be very sensitive to geometry and monomeric transition dipole moment orientations of the aggregates. The simulations for long multiple rod and spiral-type macrostructures, observed in recent high-resolution electron microscopy images (Oostergetel et al., FEBS Lett 581:5435–5439, 2007) gave shapes of circular dichroism spectra observed experimentally for chlorosomes. It was shown that the ratio of total circular dichroism intensity to integrated absorption of the Q y transition is a good measure of degree of tubular structures in the chlorosomes. Calculations suggest that the broad Q y line width of chlorosomes of sulfur bacteria could be due to (1) different orientations of the transition moment vectors in multi-walled rod structures or (2) a variety of Bchl-aggregate structures in the chlorosomes.
Keywords: Antenna; Chlorosome; Green bacteria; Rod element; Lamella; Spiral; Light harvesting; Molecular modeling