Thurlkill Richard L, Grimsley Gerald R, Scholtz J Martin, Pace C Nick
Department of Molecular and Cellular Medicine, Texas A&M University, System Health Science Center, College Station, Texas 77843, USA.
Protein Sci. 2006 May;15(5):1214-8. doi: 10.1110/ps.051840806. Epub 2006 Apr 5.
We have used potentiometric titrations to measure the pK values of the ionizable groups of proteins in alanine pentapeptides with appropriately blocked termini. These pentapeptides provide an improved model for the pK values of the ionizable groups in proteins. Our pK values determined in 0.1 M KCl at 25 degrees C are: 3.67+/-0.03 (alpha-carboxyl), 3.67+/-0.04 (Asp), 4.25+/-0.05 (Glu), 6.54+/-0.04 (His), 8.00+/-0.03 (alpha-amino), 8.55+/-0.03 (Cys), 9.84+/-0.11 (Tyr), and 10.40+/-0.08 (Lys). The pK values of some groups differ from the Nozaki and Tanford (N & T) pK values often used in the literature: Asp (3.67 this work vs. 4.0 N & T); His (6.54 this work vs. 6.3 N & T); alpha-amino (8.00 this work vs. 7.5 N & T); Cys (8.55 this work vs. 9.5 N & T); and Tyr (9.84 this work vs. 9.6 N & T). Our pK values will be useful to those who study pK perturbations in folded and unfolded proteins, and to those who use theory to gain a better understanding of the factors that determine the pK values of the ionizable groups of proteins.
我们使用电位滴定法来测量具有适当封闭末端的丙氨酸五肽中蛋白质可电离基团的pK值。这些五肽为蛋白质中可电离基团的pK值提供了一个改进的模型。我们在25℃下于0.1M KCl中测定的pK值为:3.67±0.03(α-羧基)、3.67±0.04(天冬氨酸)、4.25±0.05(谷氨酸)、6.54±0.04(组氨酸)、8.00±0.03(α-氨基)、8.55±0.03(半胱氨酸)、9.84±0.11(酪氨酸)和10.40±0.08(赖氨酸)。一些基团的pK值与文献中常用的野崎和坦福德(N&T)pK值不同:天冬氨酸(本研究为3.67,而N&T为4.0);组氨酸(本研究为6.54,而N&T为6.3);α-氨基(本研究为8.00,而N&T为7.5);半胱氨酸(本研究为8.55,而N&T为9.5);以及酪氨酸(本研究为9.84,而N&T为9.6)。我们的pK值将对研究折叠和未折叠蛋白质中pK扰动的人员以及使用理论来更好地理解决定蛋白质可电离基团pK值的因素的人员有用。