Abstract
Intracellular oxygenation is key to energy metabolism as well as tumor radiation therapy. Although integral proteins are ubiquitous in membranes, few studies have considered their effects on molecular oxygen permeability. Published experimental work with rhodopsin and bacteriorhodopsin has led to the hypothesis that integral proteins lessen membrane oxygen permeability, as well as the permeability of the lipid region. The current work uses atomistic molecular dynamics simulations to test the influence of an ungated potassium channel protein on the oxygen permeability of palmitoyloleoylphosphatidylcholine (POPC) bilayers with and without cholesterol. Consistent with experiment, whole-membrane oxygen permeability is cut in half upon adding 30 wt% potassium channel protein to POPC, and the apparent permeability of the lipid portion of the membrane decreases by 40%. Unexpectedly, oxygen is found to interact directly with the protein surface, accompanied by a 40% reduction of the apparent whole-membrane diffusion coefficient. Similar effects are seen in systems combining the potassium channel with 1:1 POPC/cholesterol, but the magnitude of permeability reduction is smaller by ~30%. Overall, the simulations indicate that integral proteins can reduce oxygen permeability by altering the diffusional path and the local diffusivity. This effect may be especially important in the protein-dense membranes of mitochondria.
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Notes
- 1.
PL for POPC or POPC/Chol was adjusted to reflect the thickness of the respective protein-incorporating bilayer, using the relation in Eq. 1 under the condition PM = PL for lipid alone. Each thickness-adjusted PL value was then used with Eq. 3, along with aL from Table 2, to predict PM for the protein-incorporating membrane. For example, for K+chan/POPC:
PL, KchanPOPC = (KP, POPC ∙ DM, POPC)/hKchanPOPC = (7.8)(3.8 × 10−6 cm 2/s)/6.2 nm = 47.8 cm/s; PM, KchanPOPC = aL, KchanPOPC ∙ PL, KchanPOPC = (0.77)(47.8 cm/s) = 37 cm/s.
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Acknowledgments
The authors thank Harold Swartz for encouraging our inclusion of membrane proteins in the simulation models, as well as Richard Pastor and James Kindt for valuable comments on the manuscript. The work was funded by the National Institutes of Health (NIH) under National Institute of General Medical Sciences (NIGMS) grant P20GM103451 and by a gift from the Glendorn Foundation.
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Dotson, R.J., Pias, S.C. (2018). Reduced Oxygen Permeability upon Protein Incorporation Within Phospholipid Bilayers. In: Thews, O., LaManna, J., Harrison, D. (eds) Oxygen Transport to Tissue XL. Advances in Experimental Medicine and Biology, vol 1072. Springer, Cham. https://doi.org/10.1007/978-3-319-91287-5_65
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