Functional role of Thr-312 and Thr-315 in the proton-transfer pathway in ba3 Cytochrome c oxidase from Thermus thermophilus

Cytochrome ba(3) from Thermus thermophilus is a member of the family of B-type heme-copper oxidases, which have a low degree of sequence homology to the well-studied mitochondrial-like A-type enzymes. Recently, it was suggested that the ba(3) oxidase has only one pathway for the delivery of protons to the active site and that this pathway is spatially analogous to the K-pathway in the A-type oxidases [Chang, H.-Y., et al. (2009) Proc. Natl. Acad. Sci. U.S.A. 106, 16169-16173]. This suggested pathway includes two threonines at positions 312 and 315. In this study, we investigated the time-resolved reaction between fully reduced cytochrome ba(3) and O(2) in variants where Thr-312 and Thr-315 were modified. While in the A-type oxidases this reaction is essentially unchanged in variants with the K-pathway modified, in the Thr-312 --> Ser variant in the ba(3) oxidase both reactions associated with proton uptake from solution, the P(R) --> F and F --> O transitions, were slowed compared to those of wild-type ba(3). The observed time constants were slowed approximately 3-fold (for P(R) --> F, from 60 to approximately 170 mus in the wild type) and approximately 30-fold (for F --> O, from 1.1 to approximately 40 ms). In the Thr-315 --> Val variant, the F --> O transition was approximately 5-fold slower (5 ms) than for the wild-type oxidase, whereas the P(R) --> F transition displayed an essentially unchanged time constant. However, the uptake of protons from solution was a factor of 2 slower and decoupled from the optical P(R) --> F transition. Our results thus show that proton uptake is significantly and specifically inhibited in the two variants, strongly supporting the suggested involvement of T312 and T315 in the transfer of protons to the active site during O(2) reduction in the ba(3) oxidase.

[Micronutrients and their relevance for the eye--function of lutein, zeaxanthin and omega-3 fatty acids]

Micronutrients play an important role in function and health maintenance for the eye. Especially lutein, zeaxanthin and omega-3 fatty acids perform remarkable functions: lutein together with zeaxanthin forms the macular pigment, these carotenoids filter out the damaging blue light component from the sunlight as well as the ultraviolet light which leads to improved contrast sensitivity and less problems with screen glare. Furthermore, the macular pigment has antioxidant and anti-inflammatory effects. The omega-3 fatty acids also possess anti-inflammatory effects and, when converted into neuroprotectin, they protect against oxidative induced apoptosis in the retina. They are also responsible for the fluidity and supply to the photoreceptor membrane. These properties are important for the prevention and treatment of degenerative eye diseases like age-related macular degeneration. However, older people are often not sufficiently supplied of micronutrients in their diet. Because the supply of nutrients can hardly be achieved by dietary change, the additional intake in the form of food supplements is useful in this age group. Scientific studies have shown the positive effects of supplementation with micronutrients such as lutein/zeaxanthin, vitamin C, vitamin E, zinc and omega-3 fatty acids, docosahexaenoic acid and eicosapentaenoic acid (DHA and EPA). Currently available nutritional products are based in part on the ingredients of the ARED study (Age Related Eye Disease Study). According to more recent studies formulations containing lutein and omega-3 fatty acids in physiologically meaningful doses without additional beta-carotene should be preferred. 10 to 20 mg of lutein and zeaxanthin represent a safe daily dose Regarding to the context above, beta-carotene in high doses plays a minor role to the eye and is especially critical for the health of smokers. This paper summarises the functions of the presented micronutrients in the eye and can assist ophthalmologists in advising their patients.

Substrate control of internal electron transfer in bacterial nitric-oxide reductase

Nitric -oxide reductase (NOR) from Paracoccus denitrificans catalyzes the reduction of nitric oxide (NO) to nitrous oxide (N(2)O) (2NO + 2H(+) + 2e(-) -->N(2)O + H(2)O) by a poorly understood mechanism. NOR contains two low spin hemes c and b, one high spin heme b(3), and a non-heme iron Fe(B). Here, we have studied the reaction between fully reduced NOR and NO using the "flow-flash" technique. Fully (four-electron) reduced NOR is capable of two turnovers with NO. Initial binding of NO to reduced heme b(3) occurs with a time constant of approximately 1 micros at 1.5 mM NO, in agreement with earlier studies. This reaction is [NO]-dependent, ruling out an obligatory binding of NO to Fe(B) before ligation to heme b(3). Oxidation of hemes b and c occurs in a biphasic reaction with rate constants of 50 s(-1) and 3 s(-1) at 1.5 mM NO and pH 7.5. Interestingly, this oxidation is accelerated as [NO] is lowered; the rate constants are 120 s(-1) and 12 s(-1) at 75 microM NO. Protons are taken up from solution concomitantly with oxidation of the low spin hemes, leading to an acceleration at low pH. This effect is, however, counteracted by a larger degree of substrate inhibition at low pH. Our data thus show that substrate inhibition in NOR, previously observed during multiple turnovers, already occurs during a single oxidative cycle. Thus, NO must bind to its inhibitory site before electrons redistribute to the active site. The further implications of our data for the mechanism of NO reduction by NOR are discussed.

Whole-body high-field MRI shows no skeletal muscle degeneration in young patients with recessive myotonia congenita

BACKGROUND

Muscle magnetic resonance imaging (MRI) is the most sensitive method in the detection of dystrophic and non-dystrophic abnormalities within striated muscles. We hypothesized that in severe myotonia congenita type Becker muscle stiffness, prolonged transient weakness and muscle hypertrophy might finally result in morphologic skeletal muscle alterations reflected by MRI signal changes.

AIM OF THE STUDY

To assess dystrophic and/or non-dystrophic alterations such as fatty or connective tissue replacement and muscle edema in patients with severe recessive myotonia congenita.

METHODS

We studied three seriously affected patients with myotonia congenita type Becker using multisequence whole-body high-field MRI. All patients had molecular genetic testing of the muscle chloride channel gene (CLCN1).

RESULTS

Molecular genetic analyses demonstrated recessive CLCN1 mutations in all patients. Two related patients were compound heterozygous for two novel CLCN1 mutations, Q160H and L657P. None of the patients showed skeletal muscle signal changes indicative of fatty muscle degeneration or edema. Two patients showed muscle bulk hypertrophy of thighs and calves in line with the clinical appearance.

CONCLUSIONS

We conclude that (i) chloride channel dysfunction alone does not result in skeletal muscle morphologic changes even in advanced stages of myotonia congenita, and (ii) MRI skeletal muscle alterations in myotonic dystrophy must be clear consequences of the dystrophic disease process.

Pattern of skeletal muscle involvement in primary dysferlinopathies: a whole-body 3.0-T magnetic resonance imaging study

OBJECTIVES AND METHODS

Mutations in the gene encoding dysferlin cause limb girdle muscular dystrophy type 2B (LGMD2B), distal Miyoshi myopathy (MM), and a rare form of distal anterior compartment myopathy. To study the correlations between clinical manifestations and muscle imaging changes we conducted a 3.0-T magnetic resonance imaging (MRI) study in six German patients with primary dysferlinopathies defined by absence of dysferlin expression in muscle (MM, n = 3; LGMD2B, n = 2; hyperCKemia without clinical symptoms, n = 1).

RESULTS

Patients with manifest myopathy had widespread muscular pathology. In analogy to previous imaging studies, we confirmed an involvement of the anterior and posterior thigh compartments and a predominant involvement of posterior lower legs. However, our whole-body MRI study further provided evidence of signal alterations in the glutei, erector spinae and shoulder girdle muscles. Correlation of clinical findings with imaging demonstrated the potential of MRI to detect subclinical muscle pathology.

CONCLUSIONS

Whole-body 3.0-T MRI is a non-invasive method to demonstrate various degrees of skeletal muscle alterations and disease progression in muscular dystrophies. Furthermore, whole-body high-field MRI may serve as a helpful diagnostic tool in differentiating primary dysferlinopathies from other forms of LGMD and distal myopathies.

Exploring the terminal region of the proton pathway in the bacterial nitric oxide reductase

The c-type nitric oxide reductase (cNOR) from Paracoccus (P.) denitrificans is an integral membrane protein that catalyzes NO reduction; 2NO+2e(-)+2H(+)-->N(2)O+H(2)O. It is also capable of catalyzing the reduction of oxygen to water, albeit more slowly than NO reduction. cNORs are divergent members of the heme-copper oxidase superfamily (HCuOs) which reduce NO, do not pump protons, and the reaction they catalyse is non-electrogenic. All known cNORs have been shown to have five conserved glutamates (E) in the catalytic subunit, by P. denitrificans numbering, the E122, E125, E198, E202 and E267. The E122 and E125 are presumed to face the periplasm and the E198, E202 and E267 are located in the interior of the membrane, close to the catalytic site. We recently showed that the E122 and E125 define the entry point of the proton pathway leading from the periplasm into the active site [U. Flock, F.H. Thorndycroft, A.D. Matorin, D.J. Richardson, N.J. Watmough, P. Adelroth, J. Biol. Chem. 283 (2008) 3839-3845]. Here we present results from the reaction between fully reduced NOR and oxygen on the alanine variants of the E198, E202 and E267. The initial binding of O(2) to the active site was unaffected by these mutations. In contrast, proton uptake to the bound O(2) was significantly inhibited in both the E198A and E267A variants, whilst the E202A NOR behaved essentially as wildtype. We propose that the E198 and E267 are involved in terminating the proton pathway in the region close to the active site in NOR.

Distinct muscle imaging patterns in myofibrillar myopathies

OBJECTIVE

To compare muscle imaging findings in different subtypes of myofibrillar myopathies (MFM) in order to identify characteristic patterns of muscle alterations that may be helpful to separate these genetic heterogeneous muscular disorders.

METHODS

Muscle imaging and clinical findings of 46 patients with MFM were evaluated (19 desminopathy, 12 myotilinopathy, 11 filaminopathy, 1 alphaB-crystallinopathy, and 3 ZASPopathy). The data were collected retrospectively in 43 patients and prospectively in 3 patients.

RESULTS

In patients with desminopathy, the semitendinosus was at least equally affected as the biceps femoris, and the peroneal muscles were never less involved than the tibialis anterior (sensitivity of these imaging criteria to detect desminopathy in our cohort 100%, specificity 95%). In most of the patients with myotilinopathy, the adductor magnus showed more alterations than the gracilis muscle, and the sartorius was at least equally affected as the semitendinosus (sensitivity 90%, specificity 93%). In filaminopathy, the biceps femoris and semitendinosus were at least equally affected as the sartorius muscle, and the medial gastrocnemius was more affected than the lateral gastrocnemius. The semimembranosus mostly showed more alterations than the adductor magnus (sensitivity 88%, specificity 96%). Early adult onset and cardiac involvement was most often associated with desminopathy. In patients with filaminopathy, muscle weakness typically beginning in the 5th decade of life was mostly pronounced proximally, while late adult onset (>50 years) with distal weakness was more often present in myotilinopathy.

CONCLUSIONS

Muscle imaging in combination with clinical data may be helpful for separation of distinct myofibrillar myopathy subtypes and in scheduling of genetic analysis.

Distinct muscle imaging patterns in myofibrillar myopathies

OBJECTIVE

To compare muscle imaging findings in different subtypes of myofibrillar myopathies (MFM) in order to identify characteristic patterns of muscle alterations that may be helpful to separate these genetic heterogeneous muscular disorders.

METHODS

Muscle imaging and clinical findings of 46 patients with MFM were evaluated (19 desminopathy, 12 myotilinopathy, 11 filaminopathy, 1 alphaB-crystallinopathy, and 3 ZASPopathy). The data were collected retrospectively in 43 patients and prospectively in 3 patients.

RESULTS

In patients with desminopathy, the semitendinosus was at least equally affected as the biceps femoris, and the peroneal muscles were never less involved than the tibialis anterior (sensitivity of these imaging criteria to detect desminopathy in our cohort 100%, specificity 95%). In most of the patients with myotilinopathy, the adductor magnus showed more alterations than the gracilis muscle, and the sartorius was at least equally affected as the semitendinosus (sensitivity 90%, specificity 93%). In filaminopathy, the biceps femoris and semitendinosus were at least equally affected as the sartorius muscle, and the medial gastrocnemius was more affected than the lateral gastrocnemius. The semimembranosus mostly showed more alterations than the adductor magnus (sensitivity 88%, specificity 96%). Early adult onset and cardiac involvement was most often associated with desminopathy. In patients with filaminopathy, muscle weakness typically beginning in the 5th decade of life was mostly pronounced proximally, while late adult onset (>50 years) with distal weakness was more often present in myotilinopathy.

CONCLUSIONS

Muscle imaging in combination with clinical data may be helpful for separation of distinct myofibrillar myopathy subtypes and in scheduling of genetic analysis.

A pathway for protons in nitric oxide reductase from Paracoccus denitrificans

Nitric oxide reductase (NOR) from P. denitrificans is a membrane-bound protein complex that catalyses the reduction of NO to N(2)O (2NO+2e(-)+2H(+)-->N(2)O+H(2)O) as part of the denitrification process. Even though NO reduction is a highly exergonic reaction, and NOR belongs to the superfamily of O(2)-reducing, proton-pumping heme-copper oxidases (HCuOs), previous measurements have indicated that the reaction catalyzed by NOR is non-electrogenic, i.e. not contributing to the proton electrochemical gradient. Since electrons are provided by donors in the periplasm, this non-electrogenicity implies that the substrate protons are also taken up from the periplasm. Here, using direct measurements in liposome-reconstituted NOR during reduction of both NO and the alternative substrate O(2), we demonstrate that protons are indeed consumed from the 'outside'. First, multiple turnover reduction of O(2) resulted in an increase in pH on the outside of the NOR-vesicles. Second, comparison of electrical potential generation in NOR-liposomes during oxidation of the reduced enzyme by either NO or O(2) shows that the proton transfer signals are very similar for the two substrates proving the usefulness of O(2) as a model substrate for these studies. Last, optical measurements during single-turnover oxidation by O(2) show electron transfer coupled to proton uptake from outside the NOR-liposomes with a tau=15 ms, similar to results obtained for net proton uptake in solubilised NOR [U. Flock, N.J. Watmough, P. Adelroth, Electron/proton coupling in bacterial nitric oxide reductase during reduction of oxygen, Biochemistry 44 (2005) 10711-10719]. NOR must thus contain a proton transfer pathway leading from the periplasmic surface into the active site. Using homology modeling with the structures of HCuOs as templates, we constructed a 3D model of the NorB catalytic subunit from P. denitrificans in order to search for such a pathway. A plausible pathway, consisting of conserved protonatable residues, is suggested.