Secondary structure of heart sarcolemmal proteins during interaction with metallic cofactors of (Na+ + K+)-ATPase.
he secondary structure of membrane proteins was studied in rat heart sarcolemma by circular dichroism under conditions of interaction with metallic cofactors of (Na+ + K+)-ATPase at their optimal concentrations and under metal free conditions. Approximately 80 per cent of polypeptide chains in the membrane were organized in alpha-helical structure. Upon stabilizing the E1. Na conformation state of (Na+ + K+)-ATPase by Mg2+ and Na+ ions, only a slight increase in the protein alpha-helix content (to 83 per cent) was observed. On the other hand, simultaneous addition of Mg2+ and K+ ions resulting in the establishment of the E2 . K conformational state of the enzyme, was followed by a significant decrease in the membrane protein helicity (to 72 per cent). The presence of all three metallic cofactors of (Na+ + K+)-ATPase did not induce any further conformational change in sarcolemmal proteins as compared to the state induced by the interaction with Mg2+ and Na+ ions. In contrast to results obtained with Mg2+ ions, the interaction of Na+ with the sarcolemmal membranes led to a considerable decrease and that of K+ to a significant increase in alpha-helicity of the membrane polypeptides. These findings have confirmed the regulatory role of magnesium in transition of the conformational state from E1 to E2 in the reaction sequence of (Na+ + K+)-ATPase. Specific modulation by Na+ and K+ of the helicity of sarcolemmal proteins in the presence of Mg2+ and in the absence of ATP might be considered as a preprint of conformational changes which will occur in the presence of ATP.
D000818:Animals; D002942:Circular Dichroism; D066298:In Vitro Techniques; D008274:Magnesium / Q000378:metabolism; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D009124:Muscle Proteins / Q000378:metabolism*; D009206:Myocardium / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D011487:Protein Conformation; D051381:Rats; D011919:Rats, Inbred Strains; D012508:Sarcolemma / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D008565:Membrane Proteins; D009124:Muscle Proteins; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D008274:Magnesium; D011188:Potassium