TY - JOUR
T1 - Apparent Heat Capacity Change Accompanying a Nonspecific Protein-DNA Interaction. Escherichia coli SSB Tetramer Binding to Oligodeoxyadenylates
AU - Ferrari, Marilyn E.
AU - Lohman, Timothy M.
PY - 1994/11/1
Y1 - 1994/11/1
N2 - We have examined the effects of temperature on the equilibrium constant, Kobs, for Escherichia coli SSB tetramer binding to a series of single-stranded (ss) oligodeoxyribonucleotides, dT(pT)n, dC(pC)n, and dA(pA)n (n = 34, 55, and 69) in order to investigate the thermodynamic basis for the strong preference of E. coli SSB (and other SSB proteins) for binding polypyrimidine stretches of ss-DNA. In addition to the expected base-dependent differences in the magnitude of Kobs, we also observe qualitatively different temperature dependencies for the binding of the SSB tetramer to oligodeoxyadenylates. Linear van't Hoff plots are obtained for SSB tetramer binding to dT(pT)n and dC(pC)n, with ΔH°obs ranging from −50 to −100 kcal/mol depending on the oligodeoxynucleotide length and salt concentration. In contrast, all van't Hoff plots for SSB tetramer binding to dA(pA)N are distinctly nonlinear with maxima in Kobs occurring near 25 °C, indicative of an apparent large negative change in molar heat capacity ΔAC°p,obs < 0). Thus for the SSB-dA(pA)n interaction, ΔH°obs and ΔS°obs are both highly temperature dependent, but compensate such that ΔG°obs is relatively insensitive to temperature. These nonlinear van't Hoff plots are not due to coupling of SSB assembly to dA(pA)N binding or to temperature-dependent shifts in the formation of other SSB-DNA binding modes. The nonlinear van't Hoff plots for SSB tetramer binding to dA(pA)N appear to result from the coupling of two processes: (1) the unstacking of the dA(pA)N bases (occurring with ΔH° > 0 and ΔC°p = 0) and (2) the binding of SSB to the unstacked DNA (occurring with ΔH° < 0 and ΔC°p = 0). Therefore, although each isolated equilibrium occurs with ΔC°p ≈ 0, the overall equilibrium displays an apparent ΔC°p,obs < 0 due to the coupled equilibrium. The binding of SSB to dT(pT)N and dC(pC)N occurs with ΔH° < 0 and ΔC°p,obs = 0, since the bases in these ss-DNA molecules do not stack appreciably. These results indicate that a nonspecific protein-DNA interaction can display a large negative apparent ΔC°p; however, this effect appears not to be due to the hydrophobic effect, but rather to a temperaturedependent conformational transition in the DNA that is coupled to protein binding. Implications of these observations for other protein-nucleic acid systems are discussed.
AB - We have examined the effects of temperature on the equilibrium constant, Kobs, for Escherichia coli SSB tetramer binding to a series of single-stranded (ss) oligodeoxyribonucleotides, dT(pT)n, dC(pC)n, and dA(pA)n (n = 34, 55, and 69) in order to investigate the thermodynamic basis for the strong preference of E. coli SSB (and other SSB proteins) for binding polypyrimidine stretches of ss-DNA. In addition to the expected base-dependent differences in the magnitude of Kobs, we also observe qualitatively different temperature dependencies for the binding of the SSB tetramer to oligodeoxyadenylates. Linear van't Hoff plots are obtained for SSB tetramer binding to dT(pT)n and dC(pC)n, with ΔH°obs ranging from −50 to −100 kcal/mol depending on the oligodeoxynucleotide length and salt concentration. In contrast, all van't Hoff plots for SSB tetramer binding to dA(pA)N are distinctly nonlinear with maxima in Kobs occurring near 25 °C, indicative of an apparent large negative change in molar heat capacity ΔAC°p,obs < 0). Thus for the SSB-dA(pA)n interaction, ΔH°obs and ΔS°obs are both highly temperature dependent, but compensate such that ΔG°obs is relatively insensitive to temperature. These nonlinear van't Hoff plots are not due to coupling of SSB assembly to dA(pA)N binding or to temperature-dependent shifts in the formation of other SSB-DNA binding modes. The nonlinear van't Hoff plots for SSB tetramer binding to dA(pA)N appear to result from the coupling of two processes: (1) the unstacking of the dA(pA)N bases (occurring with ΔH° > 0 and ΔC°p = 0) and (2) the binding of SSB to the unstacked DNA (occurring with ΔH° < 0 and ΔC°p = 0). Therefore, although each isolated equilibrium occurs with ΔC°p ≈ 0, the overall equilibrium displays an apparent ΔC°p,obs < 0 due to the coupled equilibrium. The binding of SSB to dT(pT)N and dC(pC)N occurs with ΔH° < 0 and ΔC°p,obs = 0, since the bases in these ss-DNA molecules do not stack appreciably. These results indicate that a nonspecific protein-DNA interaction can display a large negative apparent ΔC°p; however, this effect appears not to be due to the hydrophobic effect, but rather to a temperaturedependent conformational transition in the DNA that is coupled to protein binding. Implications of these observations for other protein-nucleic acid systems are discussed.
UR - https://www.scopus.com/pages/publications/0028071996
U2 - 10.1021/bi00209a022
DO - 10.1021/bi00209a022
M3 - Article
C2 - 7947696
AN - SCOPUS:0028071996
SN - 0006-2960
VL - 33
SP - 12896
EP - 12910
JO - Biochemistry
JF - Biochemistry
IS - 43
ER -