Photoaffinity labeling of human sex hormone-binding globulin using 17alpha-alkylamine derivatives of 3beta-androstanediol substituted with azidonitrophenylamido, azidonitrophenylamino, or trifluoroazidonitrophenylamino chromophores. Localization of Trp-84 in the vicinity of the steroid-binding site

Photoaffinity labeling of plasma proteins

Photoaffinity labeling is a powerful technique for identifying a target protein. A high degree of labeling specificity can be achieved with this method in comparison to chemical labeling. Human serum albumin (HSA) and α₁-acid glycoprotein (AGP) are two plasma proteins that bind a variety of endogenous and exogenous substances. The ligand binding mechanism of these two proteins is complex. Fatty acids, which are known to be transported in plasma by HSA, cause conformational changes and participate in allosteric ligand binding to HSA. HSA undergoes an N-B transition, a conformational change at alkaline pH, that has been reported to result in increased ligand binding. Attempts have been made to investigate the impact of fatty acids and the N-B transition on ligand binding in HSA using ketoprofen and flunitrazepam as photolabeling agents. Meanwhile, plasma AGP is a mixture of genetic variants of the protein. The photolabeling of AGP with flunitrazepam has been utilized to shed light on the topology of the protein ligand binding site. Furthermore, a review of photoaffinity labeling performed on other major plasma proteins will also be discussed. Using a photoreactive natural ligand as a photolabeling agent to identify target protein in the plasma would reduce non-specific labeling.

Why do p-nitro-substituted aryl azides provide unintended dark reactions with proteins?

Aryl azide-mediated photo cross-linking has been widely used to obtain structural features in biological systems, even though the reactive species generated upon photolysis in aqueous solution have not been well characterized. We have established a mechanistic framework for the formation of adducts between photoactivated 5-azido-2-nitrobenzoyl reagents and protein functional groups. Photolysis of the aryl azide tethered to biotin via an amide linkage yields a cross-link with streptavidin. The ability of the pre-irradiated reagent to form a similar cross-link indicates that it is the long-lived reactive intermediate that contributes to the cross-link formation. The reactive intermediate forms an adduct with tryptophan. The sequence of the labeled peptide is found to be GlyTrp(*)ThrValAlaTrp(*)LysAsn, corresponding to residues 74-81 of the streptavidin sequence, where Trp(*) designates the modified Trp-75 and Trp-79. A peak at m/z 1455.1 corresponding to the calculated [M(peptide)+aryl nitrene+2O](+) molecular ion value has been observed for the labeled peptide. Product structure identification experiments support the assignment that the long-lived reactive intermediate is a p-nitro-N-arylhydroxylamine, which undergoes a number of transformations in aqueous solution leading to nitroso derivatives. A plausible mechanism of the interaction between tryptophan and nitroso compound is discussed.

Synthesis of an Agonistic, Difluoro-Azido Photolabel of Angiotensin II and Labeling of the AT1Receptor: Transmembrane Domains 3, 6, and 7 Form the Ligand-Binding Pocket

p-Azido-phenylalanine has been frequently used for photoaffinity labeling of target proteins such as the angiotensin receptors. However, chemical studies showed that simple aryl nitrenes first react intramolecularly, forming a semistable cyclic keteneimine and then reacting with nucleophile residues in the target structure like those of lysine and arginine. We synthesized 3,5-difluoro-4-azidophenylalanine where the formation of the keteneimine is prevented and where photoincorporation should be due to nonselective nitrene insertion only. This new amino acid was introduced in position 8 of angiotensin II and compared with the corresponding azidophenylalanine peptide using human AT1 receptor as target. The new photolabel maintained full agonist activity and a similar yield of photolabeling but without the previously observed gradual hydrolysis. Several selective proteolyses of the labeled receptor indicate that the new photolabel forms three simultaneous contact regions on the hAT1 receptor, suggestive of a nonselective behavior of the photolabel. A major contact was established in the sixth transmembrane domain but also in the third and seventh domain. Our results are in excellent agreement with those recently obtained from methionine proximity assay studies.

Steroid ligands bind human sex hormone-binding globulin in specific orientations and produce distinct changes in protein conformation

The amino-terminal laminin G-like domain of human sex hormone-binding globulin (SHBG) contains a single high affinity steroid-binding site. Crystal structures of this domain in complex with several different steroid ligands have revealed that estradiol occupies the SHBG steroid-binding site in an opposite orientation when compared with 5 alpha-dihydrotestosterone or C19 androgen metabolites (5 alpha-androstan-3 beta,17 beta-diol and 5 alpha-androstan-3 beta,17 alpha-diol) or the synthetic progestin levonorgestrel. Substitution of specific residues within the SHBG steroid-binding site confirmed that Ser(42) plays a key role in determining high affinity interactions by hydrogen bonding to functional groups at C3 of the androstanediols and levonorgestrel and the hydroxyl at C17 of estradiol. Among residues participating in the hydrogen bond network with hydroxy groups at C17 of C19 steroids or C3 of estradiol, Asp(65) appears to be the most important. The different binding mode of estradiol is associated with a difference in the position/orientation of residues (Leu(131) and Lys(134)) in the loop segment (Leu(131)-His(136)) that covers the steroid-binding site as well as others (Leu(171)-Lys(173) and Trp(84)) on the surface of human SHBG and may provide a basis for ligand-dependent interactions between SHBG and other macromolecules. These new crystal structures have also enabled us to construct a simple space-filling model that can be used to predict the characteristics of novel SHBG ligands.