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Chronic obstructive pulmonary disease (COPD) is one of the leading causes of morbidity and mortality worldwide. Besides abnormities within the respiratory system COPD is also associated with effects outside the lungs, so called systemic effects. One systemic effect that has been highlighted is skeletal muscle dysfunction which has also been associated with reduced exercise capacity. Apart from changes in muscle morphology, low levels of physical activity have also been suggested as a plausible mediator of reduced exercise capacity in COPD. The aim of this thesis was to study muscle morphology and physical activity patterns in patients with different degrees of COPD and to examine the associations between muscle morphology, physical activity and exercise capacity in these patients. Skeletal muscle morphology was found to shift towards a more glycolytic muscle profile in COPD patients and changes in muscle morphology were found to be correlated to disease severity and to exercise capacity. Muscle capillarization was also found to be lower in COPD compared with healthy subjects and to be correlated to disease severity and exercise capacity. When studying signalling pathways involved in muscle capillarization, an overexpression of VHL was found in patients with mild and moderate COPD compared with healthy subjects. Furthermore, COPD patients were found to be less physically active compared with healthy subjects and the level of physical activity was associated with exercise capacity.In conclusion, changes in skeletal muscle morphology and low levels of physical activity are present in COPD patients and may partly explain the lower exercise capacity observed in these patients. The more glycolytic muscle profile in COPD is suggested to be mediated by hypoxia and low levels of physical activity in this patient group. Furthermore, increased levels of VHL may lead to impaired transduction of the hypoxic signalling pathway, which may contribute to the decreased muscle capillarization observed in COPD.

Oxygen homeostasis is essential to the life of aerobes, which is regulated in humans by Hypoxia Inducible Factor-1α (HIF-1α). Under hypoxic conditions, HIF-1α transactivates over a hundred genes related angiogenesis, erythropoiesis, etc. HIF-1α level and function is regulated by four HIF hydroxylase enzymes: three isoforms of prolyl hydroxylase domain (PHD1, PHD2 and PHD3) and factor inhibiting HIF-1α (FIH). PHD2 is the focus of this research. PHD2 is a non-heme Fe(II) 2-oxoglutarate dependent dioxygenase, which controls HIF-1α levels by hydroxylating two proline residues within the ODD domain of HIF-1α, then the hydroxylated prolines are recognized by pVHL, which targets HIF-1α for proteasomal degradation. Under hypoxic conditions PHD2 cannot hydroxylate HIF-1α and its level rises in cells. The aims of this research include understanding how PHD2 chooses its substrate, how the O2 activation occurs, and how certain transition metals inhibit PHD2. Our results revealed that electrostatics play a role in substrate selectivity of PHD2 by provoking a change in the opening and closing rate of β2β3 loop for NODD and CODD substrates. Mutational studies of second coordination sphere residues combined with kinetic studies indicated that decarboxylation of 2OG is the slow step in the chemical mechanism. The removal of a hydrogen-bond by the Thr387aAla mutation revealed a rate 15 times faster than WT-PHD2 by making O2 a better nucleophile. Our results indicate that this hydrogen bonding is essential for proper O2 activation. Previous reports show that certain metals increase HIF-1α levels by inhibiting PHD2. However there are conflicts about how this inhibition occurs, either through metal replacement from the active site or metals binding to a different site causing inhibition. Our competitive and non-competitive kinetic assays showed different inhibition profiles. Under competitive conditions Zn2+, Co2+, Mn2+, and Cu2+ can bind to the enzyme active site and lead to inhibition but under non-competitive conditions Zn2+, Co2+, and Mn2+ partially inhibit PHD2 suggesting that these metals cannot displace the Fe2+ from the active site. XAS experiments with Zn2+ and Fe3+ indicate that Zn2+ binds to the surface of PHD2 in a six-coordinate manner composed of two Cys201, 208, His205, Tyr197 and two water ligands.