Effect on the equilibrium between the Na+-form and the K+-form of the (Na+ + K+)-ATPase of modification of the enzyme with pyridoxal 5-phosphate.
An increase in pH shifts the equilibrium between the K+-form and the Na+-form of the (Na+ + K+)-ATPase towards the Na+-form. pK for the proton effect on the equilibrium is decreased by modification of the enzyme with pyridoxal 5-phosphate. The reactivity of the enzyme towards pyridoxal 5-phosphate is increased by an increase in pH. Modification by pyridoxal 5-phosphate of epsilon-amino groups on lysine, which has a pK of about 8 with the enzyme in the K+-form and of about 7.4 in the Na+-form, shifts the equilibrium between E1Na+ and E2 towards E2, and the equilibrium between E2(K+occ) and E2 towards E2, but has no effect on the overall equilibrium between E1Na+ and E2(K+occ). An additional modification of epsilon-amino groups on lysine, which has a pK of 9.5-10 with the enzyme in the K+-form and of about 7.7 with the enzyme in the Na+-form, shifts the equilibrium between E2(K+occ) and E1Na+ towards E1Na+; this is due to a shift in the equilibrium between E2(K+occ) and E2 towards E2, but with no effect on the equilibrium between E1Na+ and E2. The results show that the transition from the K+-form to the Na+-form decreases the pK of lysine epsilon-amino groups on the enzyme, and that the protonation of these groups influences the equilibrium between the two conformations.
D000818:Animals; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D011188:Potassium / Q000378:metabolism; D011487:Protein Conformation; D011732:Pyridoxal Phosphate / Q000494:pharmacology*; D012754:Sharks; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D011732:Pyridoxal Phosphate; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium