Dailey H A, Strittmatter P
J Biol Chem. 1981 Apr 25;256(8):3951-5.
The present data show that the carboxyl terminal end of the membrane binding segment (nonpolar peptide) of cytochrome b5 is present on the same side of phospholipid bilayers as the hydrophilic, heme-containing, NH2-terminal segment. This orientation was determined by observing rapid ionization of both tyrosyl residues at positions 5 and 8 from the carboxyl terminus upon addition of sodium hydroxide to the outer aqueous phase of vesicle preparations, and the reaction of one of these residues with a polar, impermeant reagent, diazotized sulfanilic acid. The rate of ionization of both aromatic residues occurred at least 1 order of magnitude faster than ionization of indigo trisulfonate trapped in the inner aqueous compartment of the vesicles. These data and consideration of our earlier characterization of cytochrome b5 structure and binding to membranes support a model for the membrane binding segment that is highly structured, penetrates to the middle of the bilayer, and loops back to the outer surface to place both the NH2 and the carboxyl termini on the same surface of the bilayer.
目前的数据表明，细胞色素b5膜结合片段（非极性肽）的羧基末端与含血红素的亲水性氨基末端片段位于磷脂双层的同一侧。这种取向是通过观察在向囊泡制剂的外部水相中加入氢氧化钠后，羧基末端第5位和第8位的两个酪氨酸残基的快速离子化，以及其中一个残基与极性、非渗透性试剂重氮化磺胺酸的反应来确定的。两个芳香族残基的离子化速率比被困在囊泡内部水相中的靛蓝三磺酸盐的离子化速率至少快1个数量级。这些数据以及我们早期对细胞色素b5结构和与膜结合的表征的考虑，支持了一个膜结合片段的模型，该模型结构高度有序，穿透到双层膜的中间，并折回外表面，使氨基末端和羧基末端都位于双层膜的同一表面。
