Question OF THE Month l PDF

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bioTEC_ Cellular Biology 1º Grado en Biotecnología Facultad de Ciencias Experimentales Universidad Pablo de Olavide

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QUESTION OF THE MONTH: ION CARRIERS

Question 1: A synthetic lipid vesicle suspension was prepared in the laboratory. This suspension contained vesicles composed by different phospholipids and multiple identical copies of a single protein embedded within the lipids, the ATP-dependent Na+/K+ antiporter. In a first experiment all proteins have been intentionally oriented with the right side in (i.e. the cytosolic domains of the proteins are facing to the inside of the vesicles). Please predict what may happen under the following circumstances. a) Both the internal (liquid trapped within the vesicles) and external (liquid outside of the vesicles) aqueous solutions contain sodium and potassium but not ATP. b) Both the internal and external aqueous solutions contain sodium, potassium and ATP. c) The internal aqueous solution contains both sodium and potassium. The external aqueous solution just ATP. d) The external aqueous solution contains both sodium and potassium. The internal aqueous solution just ATP. e) The internal aqueous solution contains ATP and potassium. The external solution just sodium.

Answers

a) Under these circumstances, even though there’s Na+ and K+, the pumping won’t occur because there’s no ATP which is needed for this process. b) Under these circumstances, the pumping will occur. Pumps will hydrolyze ATP, leading to phosphorylation of the pump and release of ADP. 3 Na+ will be transported to the cytosol and 2 K+ to the exterior of the vesicles.

c) Under these circumstances, even if there’s ATP, the pumping won’t occur. The pump needs to transport the Na+ from the external to the internal face. If there’s no Na+ in the external aqueous solution, there won’t be transportation and therefore no pumping. d) Under these circumstances, the pumping won’t occur because the ATP is located in the internal region. The ATP needs to be located in the external region, where the ATP binding place is placed.

e) As explained before, the pumping won’t occur because ATP needs to be located at least in the external face, where the ATP binding place is.

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Question 2 In a parallel separate second experiment, Na+/K+ antiporter had a random distribution (50% inside out, 50% right-side in). Please predict what may happen under the circumstances described under experimental conditions b, c and e.

Answers

b) Under these circumstances, as ATP is located in both the external and internal face, the pumping will occur. Pumps will hydrolyze ATP, leading to phosphorylation of the pump and release of ADP. In the 50% inside out pumps, 3 Na+ will be transported to the cytosol and 2 K+ to the exterior of the vesicles. In the 50% right-side in pumps, 3 Na+ will be transported to the exterior of the vesicles and 2 K+ to the cytosol.

c) Under these circumstances, in the 50% inside out pumps, the pumping won’t occur, because Na+ needs to be transported from the external to the internal face, so Na+ in the external aqueous solution is needed. In the 50% right-side in pumps, the pumping won’t occur either, because ATP needs to be located in the internal aqueous solution, as it’s where the ATP binding place is.

e) Under these circumstances, in the 50% inside out pumps, the pumping won’t occur, because ATP needs to be located at least in the external face, where the ATP binding place is. In the 50% right-side in pumps, the pumping won’t occur either because, even though the ATP is located where there’s the ATP binding place, K+ must be located in the external face and Na+ in the internal face so they can be transported.

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