Coenzym F430 aus Methan-Bakterien: Zusammenhänge zwischen der Struktur des hydroporphinoiden Liganden und der Redoxchemie des Nickelzentrums

The hydroporphinoid nickel complex coenzyme F430 ist the prosthetic group of methyl-coenzyme M reductase, the enzyme catalyzing the last step of biological methane formation. Although the enzyme mechanism is still unknown, it has been shown that the isolated cofactor can be reduced to the NiI-form at surprisingly positive potentials. Recent in vivo EPR studies indicate that the NiI form of F430 is indeed formed in whole cells and in highly active enzyme preparations. A comparative study of the redox chemistry of partially synthetic derivatives of F430 helped to identify the structural elements that allow coenzyme F430 to be reduced at the metal rather than at the ligand and at a physiologically accessible potential

Sodium 2-mercaptoethanesulfonate monohydrate (coenzyme M sodium salt monohydrate)

The 2-thio­ethanesulfonate anion is the smallest known coenzyme in nature (HS–CoM) and plays a key role in methano­genesis by anaerobic archaea, as well as in the oxidation of alkenes by Gram-negative and Gram-positive eubacteria. The title compound, Na+·C2H5O3S2 −·H2O, is the Na+ salt of HS–CoM crystallized as the monohydrate. Six O atoms form a distorted octa­hedral coordination geometry around the Na atom, at distances in the range 2.312 (4)–2.517 (3) Å. Two O atoms of the sulfonate group, one O atom of each of three other symmetry-related sulfonate groups plus the water O atom form the coordination environment of the Na+ ion. This arrangement forms Na–O–Na layers in the crystal structure, parallel to (100)

Two sub-states of the red2 state of methyl-coenzyme M reductase revealed by high-field EPR spectroscopy

Methyl-coenzyme M reductase (MCR) catalyzes the formation of methane from methyl-coenzyme M and coenzyme B in methanogenic archaea. The enzyme has two structurally interlinked active sites embedded in an α2β2γ2 subunit structure. Each active site has the nickel porphyrinoid F430 as a prosthetic group. In the active state, F430 contains the transition metal in the Ni(I) oxidation state. The active enzyme exhibits an axial Ni(I)-based continuous wave (CW) electron paramagnetic resonance (EPR) signal, called red1a in the absence of substrates or red1c in the presence of coenzyme M. Addition of coenzyme B to the MCR-red1 state can partially and reversibly convert it into the MCR-red2 form, which shows a rhombic Ni(I)-based EPR signal (at X-band microwave frequencies of approximately 9.4GHz). In this report we present evidence from high-field/high-frequency CW EPR spectroscopy (W-band, microwave frequency of approximately 94GHz) that the red2 state consists of two substates that could not be resolved by EPR spectroscopy at X-band frequencies. At W-band it becomes apparent that upon addition of coenzyme B to MCR in the red1c state, two red2 EPR signals are induced, not one as was previously believed. The first signal is the well-characterized (ortho)rhombic EPR signal, thus far called red2, while the second previously unidentified signal is axial. We have named the two substates MCR-red2r and MCR-red2a after their rhombic and axial signals, respectivel

Coordination and binding geometry of methyl-coenzyme M in the red1m state of methyl-coenzyme M reductase

Methane formation in methanogenic Archaea is catalyzed by methyl-coenzyme M reductase (MCR) and takes place via the reduction of methyl-coenzyme M (CH3-S-CoM) with coenzyme B (HS-CoB) to methane and the heterodisulfide CoM-S-S-CoB. MCR harbors the nickel porphyrinoid coenzyme F430 as a prosthetic group, which has to be in the Ni(I) oxidation state for the enzyme to be active. To date no intermediates in the catalytic cycle of MCRred1 (red for reduced Ni) have been identified. Here, we report a detailed characterization of MCRred1m ("m” for methyl-coenzyme M), which is the complex of MCRred1a ("a” for absence of substrate) with CH3-S-CoM. Using continuous-wave and pulse electron paramagnetic resonance spectroscopy in combination with selective isotope labeling (13C and 2H) of CH3-S-CoM, it is shown that CH3-S-CoM binds in the active site of MCR such that its thioether sulfur is weakly coordinated to the Ni(I) of F430. The complex is stable until the addition of the second substrate, HS-CoB. Results from EPR spectroscopy, along with quantum mechanical calculations, are used to characterize the electronic and geometric structure of this complex, which can be regarded as the first intermediate in the catalytic mechanis

Characterization of the MCRred2 form of methyl-coenzyme M reductase: a pulse EPR and ENDOR study

: Methyl-coenzyme M reductase (MCR), which catalyses the reduction of methyl-coenzyme M (CH3-S-CoM) with coenzyme B (H-S-CoB) to CH4 and CoM-S-S-CoB, contains the nickel porphinoid F430 as prosthetic group. The active enzyme exhibits the Ni(I)-derived axial EPR signal MCRred1 both in the absence and presence of the substrates. When the enzyme is competitively inhibited by coenzyme M (HS-CoM) the MCRred1 signal is partially converted into the rhombic EPR signal MCRred2. To obtain deeper insight into the geometric and electronic structure of the red2 form, pulse EPR and ENDOR spectroscopy at X- and Q-band microwave frequencies was used. Hyperfine interactions of the four pyrrole nitrogens were determined from ENDOR and HYSCORE data, which revealed two sets of nitrogens with hyperfine couplings differing by about a factor of two. In addition, ENDOR data enabled observation of two nearly isotropic 1H hyperfine interactions. Both the nitrogen and proton data indicate that the substrate analogue coenzyme M is axially coordinated to Ni(I) in the MCRred2 stat

Challenges in Creating Online Exercises and Exams in Organic Chemistry

e-Learning has become increasingly important in chemical education and online exams can be an attractive alternative to traditional exams written on paper, particularly in classes with a large number of students. Ten years ago, we began to set up an e-course complementing our lecture courses Organic Chemistry I and II within the open-source e-learning environment Moodle. In this article, we retrace a number of decisions we took over time, thereby illustrating the challenges one faces when creating online exercises and exams in (organic) chemistry. Special emphasis is put on the development of MOSFECCS (MOlecular Structural Formula Editor and Calculator of Canonical SMILES), our new editor for drawing structural formulae and converting them to alphanumeric SMILES codes that can be submitted as answers to e-problems. Convinced that the possibility for structure input is essential to set up sensible chemistry quizzes and exams, and realising that existing tools present major flaws in an educational context, we decided to embark on the implementation of MOSFECCS which takes into account a number of didactic aspects