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Isoprenoids, also known as terpenes or terpenoids, represent a large family of natural products composed of five‐carbon isopentenyl diphosphate or its isomer dimethylallyl diphosphate as the building blocks. Isoprenoids are structurally and functionally diverse and include dolichols, steroid hormones, carotenoids, retinoids, aromatic metabolites, the isoprenoid side‐chain of ubiquinone, and isoprenoid attached signaling proteins. Productions of isoprenoids are catalyzed by a group of enzymes known as prenyltransferases, such as farnesyltransferases, geranylgeranyltransferases, terpenoid cyclase, squalene synthase, aromatic prenyltransferase, and cis‐ and trans‐prenyltransferases. Because these enzymes are key in cellular processes and metabolic pathways, they are expected to be potential targets in new drug discovery. In this review, six distinct subsets of characterized prenyltransferases are structurally and mechanistically classified, including (1) head‐to‐tail prenyl synthase, (2) head‐to‐head prenyl synthase, (3) head‐to‐middle prenyl synthase, (4) terpenoid cyclase, (5) aromatic prenyltransferase, and (6) protein prenylation. Inhibitors of those enzymes for potential therapies against several diseases are discussed. Lastly, recent results on the structures of integral membrane enzyme, undecaprenyl pyrophosphate phosphatase, are also discussed. [ABSTRACT FROM AUTHOR]

The ba3 cytochrome c oxidase from Thermus thermophilus is a B-type oxygen-reducing heme-copper oxidase and a proton pump. It uses only one proton pathway for transfer of protons to the catalytic site, the KB pathway. It was previously shown that the ba3 oxidase has an overall similar reaction sequence to that in mitochondrial-like A-type oxidases. However, the timing of loading the pump site, and formation and decay of catalytic intermediates is different in the two types of oxidases. In the present study, we have investigated variants in which two amino acids of the KB proton pathway leading to the catalytic site were exchanged; Tyr-248 (located ∼23 Å below the active site towards the cytoplasm) in subunit I (Y248T) and Glu-15 (∼26 Å below the active site, ∼16 Å from Tyr-248) in subunit II (E15IIQ). Even though the overall catalytic turnover in these two variants is similar and very low (<1 % of wildtype), the substitutions had distinctly different effects on the kinetics of proton transfer to the catalytic site. The results indicate that the Glu-15II is the only essentially crucial residue of the KB pathway, but that the Tyr-248 also plays a distinct role in defining an internal proton donor and controlling the dynamics of proton transfer to the pump site and the catalytic site. [ABSTRACT FROM AUTHOR]

The heme-copper oxygen reductases are redox-driven proton pumps that generate a proton motive force in both prokaryotes and mitochondria. These enzymes have been divided into 3 evolutionarily related groups: the A-. Band C-families. Most experimental work on proton-pumping mechanisms has been performed with members of the A-family. These enzymes require 2 proton input pathways (Dand K-channels) to transfer protons used for oxygen reduction chemistry and for proton pumping, with the D-channel transporting all pumped protons. In this work we use site-directed mutagenesis to demonstrate that the ba3 oxygen reductase from Thermus thermophilus, a representative of the B-family, does not contain a D-channel. Rather, it utilizes only 1 proton input channel, analogous to that of the A-family K-channel, and it delivers protons to the active site for both 02 chemistry and proton pumping. Comparison of available subunit I sequences reveals that the only structural elements conserved within the oxygen reductase families that could perform these functions are active-site components, namely the covalently linked histidinetyrosine, the Cu8 and its ligands, and the active-site heme and its ligands. Therefore, our data suggest that all oxygen reductases perform the same chemical reactions for oxygen reduction and comprise the essential elements of the proton-pumping mechanism (e.g., the proton-loading and kinetic-gating sites). These sites, however, cannot be located within the D-channel. These results along with structural considerations point to the A-propionate region of the active-site heme and surrounding water molecules as the proton-loading site. [ABSTRACT FROM AUTHOR]

The role of ascorbate (AsA) recycling via dehydroascorbate reductase (DHAR) in the tolerance of Chlamydomonas reinhardtii to photo-oxidative stress was examined. The activity of DHAR and the abundance of the CrDHAR1 (Cre10.g456750) transcript increased after moderate light (ML; 750 µmol m -2 s -1 ) or high light (HL; 1,800 µmol m -2 s -1 ) illumination, accompanied by dehydroascorbate (DHA) accumulation, decreased AsA redox state, photo-inhibition, lipid peroxidation, H 2 O 2 overaccumulation, growth inhibition and cell death. It suggests that DHAR and AsA recycling is limiting under high-intensity light stress. The CrDHAR1 gene was cloned and its recombinant CrDHAR1 protein was a monomer (25 kDa) detected by Western blot that exhibits an enzymatic activity of 965 µmol min -1   mg -1 protein. CrDHAR1 was overexpressed driven by a HSP70A:RBCS2 fusion promoter or down-regulated by artificial microRNA (amiRNA) to examine whether DHAR-mediated AsA recycling is critical for the tolerance of C. reinahartii cells to photo-oxidative stress. The overexpression of CrDHAR1 increased DHAR protein abundance and enzyme activity, AsA pool size, AsA:DHA ratio and the tolerance to ML-, HL-, methyl viologen- or H 2 O 2 -induced oxidative stress. The CrDHAR1-knockdown amiRNA lines that have lower DHAR expression and AsA recycling ability were sensitive to high-intensity illumination and oxidative stress. The glutathione pool size, glutathione:oxidized glutathione ratio and glutathione reductase and ascorbate peroxidase activities were increased in CrDHAR1-overexpressing cells and showed a further increase after high-intensity illumination but decreased in wild-type cells after light stress. The present results suggest that increasing AsA regeneration via enhanced DHAR activity modulates the ascorbate-glutathione cycle activity in C. reinhardtii against photo-oxidative stress., (© The Author 2016. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)