TABLE II A Summary of the Functional... | Rockefeller University Press

TABLE II

A Summary of the Functional Outcomes of the Various Binding States of the Luminal L-Site and the Cytoplasmic A- and I₂-Sites Derived from the Data

A		A-site
		Ca²⁺	empty	Mg²⁺
L-site	Ca²⁺	H	L	C
	empty	H	C	C
	Mg²⁺	H	C	C
B		I₂-site
		Ca²⁺	empty	Mg²⁺
		I	O	I

A		A-site
		Ca²⁺	empty	Mg²⁺
L-site	Ca²⁺	H	L	C
	empty	H	C	C
	Mg²⁺	H	C	C
B		I₂-site
		Ca²⁺	empty	Mg²⁺
		I	O	I

(A) Three levels of channel activity are promoted by the A- and L-sites. H, high F_o state; L, low F_o state; C, closed channels. Ca²⁺ binding to the A-site lead to high activity, whereas Mg²⁺ binding causes channel closure regardless of the state of the L-site. In the absence of ion binding to the A-site, Ca²⁺ binding to the L-site generates low activity, whereas Mg²⁺ binding leaves the channel closed. (B) The binding of either Ca²⁺ or Mg²⁺ to the open channel (O) is considered to promote channel inactivation (I). The activity of the channel depends on the number of subunits in each state (see Tables III and IV).

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