MECHANISM OF ATP SYNTHESIS IN SARCOPLASMIC RETICULUM

Abstract

Vesicles derived from the sarcoplasmic reticulum (SR) retain a membrane bound ATPase that can catalyze both the hydrolysis and the synthesis of adenosine triphosphate (ATP). When suspended in a medium containing ATP and Mg , these vesicles can reduce the calcium (Ca) concentration of the medium from 10 -4 to less than 10 -7 M. In this process, the ATPase uses the chemical energy derived from the hydrolysis of ATP to pump Ca 2+ into the vesicles and to maintain a steep Ca 2+ concentration gradient that is formed across the vesicle membrane· In 1971, it was shown that the entire process of Ca 2+ transport can be reversed. When a Ca 2+ concentration gradient is formed across the vesicle membrane, depending on the conditions used, the ATPase can catalyze either the synthesis of ATP or a rapid ATP⇆ P i exchange. During ATP ⇆ P i exchange, the ATPase operates simultaneously forward (ATP hydrolysis) and backwards (ATP synthesis from ADP and P i) . When vesicles previously loaded with calcium are incubated in a medium containing EGTA, Ca 2+ flows out of the vesicles at a slow rate due to the low Ca 2+ permeability of the membrane.