title: Actin-bound nucleotide/divalent cation interactions.
abstract: At this point, it may be worthwhile to list, in summary form, the important aspects of divalent cation and nucleotide binding to actin that have been reviewed here: 1) High affinity divalent cation binding to actin is very tight, with equilibrium dissociation constant KCa approximately 1 nM and KMg approximately 5 nM at pH 7.0. 2) The binding kinetics of Ca++ are diffusion limited. Dissociation is slow, with k-Ca approximately 0.015 sec at pH 7.0 (and low ionic strength). 3) The binding kinetics of Mg++ are limited by the characteristics of the Mg++ aquo-ion and are much slower than for Ca++; k-Mg approximately 0.0012 at pH 7.0. 4) Increase in pH or ionic strength weakens divalent cation binding at the high affinity site, primarily by increasing k-Ca and k-Mg. 5) Exchange of Mg++ for Ca++ (or vice versa) at the high affinity site is by a competitive pseudo-first order process with an apparent rate constant (kapp) intermediate between k-Ca and k-Mg and dependent upon the cation concentration ratio [Ca]/[Mg] present. 6) High affinity ATP binding is modulated by the high affinity divalent cation. The cation concentration range over which this modulation occurs is about 100-fold higher for Mg++ than for Ca++, again because of the different characteristics of the Mg++ and Ca++ aquo-ions. 7) At low divalent cation concentrations, ATP dissociation from actin is limited by dissociation of the tightly-bound divalent cation. 8) At high divalent cation concentrations, ATP dissociation probably occurs via dissociation of the divalent cation-nucleotide complex and is quite slow, with dissociation rate constant approximately 0.0005 sec-1. 9) Competitive nucleotide exchange on actin may be described by a pseudo-first order model analogous to that for divalent cation exchange. The pseudo-first order rate constants depend upon the divalent cation concentration. The overall nucleotide exchange rate constant kex depends upon these constants and the solution nucleotide concentration ratio, e.g. [ATP]/[ADP]. The following circumstances develop from the characteristics of the high affinity binding of divalent cation and nucleotide to actin: 1) The standard methods for actin preparation convert in vivo Mg-actin into Ca-actin. 2) Converting Ca-actin back to Mg-actin is not easy. A very low ratio of [Ca]/[Mg] is necessary, which usually requires the use of Ca-cheltors, and a long time (5-10 min) must be allowed for complete exchange.(ABSTRACT TRUNCATED AT 400 WORDS)
authors: Research Service, Stratton VA Medical Center, Albany, New York 12208.
authors: L C
authors: LC
authors: Gershman
authors: L A
authors: LA
authors: Selden
authors: H J
authors: HJ
authors: Kinosian
authors: J E
authors: JE
authors: Estes
mesh_terms: D000199:Actins / Q000378:metabolism*; D000244:Adenosine Diphosphate / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D002413:Cations, Divalent / Q000378:metabolism*; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism; D009711:Nucleotides / Q000378:metabolism*
chemical_list: D000199:Actins; D002413:Cations, Divalent; D009711:Nucleotides; D000244:Adenosine Diphosphate; D008274:Magnesium; D002118:Calcium