Protochlorophyllide (Pchlide) reduction is the final step to make up the spectroscopic properties of chlorophyll a in biosynthesis of chlorophyll. During evolution, photosynthetic organisms have invented two structurally unrelated Pchlide reductases; light-dependent Pchlide reductase (LPOR) and light-independent (dark-operative) Pchlide reductase (DPOR). LPOR is an NADPH-dependent enzyme operating as a key enzyme for the light-dependent greening in angiosperms, and DPOR is a nitrogenase-like enzyme that allows gymnosperms, algae, cyanobacteria and photosynthetic bacteria to produce (bacterio)chlorophylls even in the dark. We will review recent major research progresses on Pchlide reductases, especially, crystallographic structure and proposed reaction mechanism of DPOR. Differential operation and evolutionary implications of these enzymes are also discussed.
The voltage-sensor domain (VSD) is the key module for voltage sensing of transmembrane proteins. Recently, voltage-sensor only protein (VSOP) that contains a single VSD but no pore domain was discovered and functions as the voltage-gated proton channel. The fourth transmembrane segment (S4) of VSOP contains three arginines (R1, R2, R3). We found that VSOP truncated just downstream of R2 retains most channel properties. Two assays, PEGylation-protection assay and membrane insertion assay, showed that S4 inserts into the membrane, even if it is truncated between R2 and R3. These findings provide important clues to the molecular mechanism of voltage-gated proton channel.
Functions of the hippocampus is dependent on brain-derived-neurosteroids. Because estrogen is synthesized locally in the hippocampus, as well as supplied from the ovary, its functions are attracting much attention. Hippocampal sex-steroids modulate memory-related synaptic plasticity not only slowly but also rapidly. Slow actions of estradiol (E2) occur via nuclear receptors (ERα), while rapid E2 actions occur via synapse-localized ERα. Elevation or decrease of the E2 concentration changes rapidly the density and morphology of spines. Kinase networks are involved downstream of ERα. The long-term depression is modulated rapidly by changes of E2 level. The E2 level is much higher than that in blood circulation. Hippocampus-derived sex-steroids play a major role in modulation of synaptic plasticity.