Interactions of a fluorescently labeled peptide with kringle domains in proteins

Covalent Modification of Biomolecules through Maleimide-Based Labeling Strategies

Since their first use in bioconjugation more than 50 years ago, maleimides have become privileged chemical partners for the site-selective modification of proteins via thio-Michael addition of biothiols and, to a lesser extent, via Diels-Alder (DA) reactions with biocompatible dienes. Prominent examples include immunotoxins and marketed maleimide-based antibody-drug conjugates (ADCs) such as Adcetris, which are used in cancer therapies. Among the key factors in the success of these groups is the availability of several maleimides that can be N-functionalized by fluorophores, affinity tags, spin labels, and pharmacophores, as well as their unique reactivities in terms of selectivity and kinetics. However, maleimide conjugate reactions have long been considered irreversible, and only recently have systematic studies regarding their reversibility and stability toward hydrolysis been reported. This review provides an overview of the diverse applications for maleimides in bioconjugation, highlighting their strengths and weaknesses, which are being overcome by recent strategies. Finally, the fluorescence quenching ability of maleimides was leveraged for the preparation of fluorogenic probes, which are mainly used for the specific detection of thiol analytes. A summary of the reported structures, their photophysical features, and their relative efficiencies is discussed in the last part of the review.

Structural/functional properties of the Glu1-HSer57 N-terminal fragment of human plasminogen: conformational characterization and interaction with kringle domains

The Glu1-Val79 N-terminal peptide (NTP) domain of human plasminogen (Pgn) is followed by a tandem array of five kringle (K) structures of approximately 9 kDa each. K1, K2, K4, and K5 contain each a lysine-binding site (LBS). Pgn was cleaved with CNBr and the Glul-HSer57 N-terminal fragment (CB-NTP) isolated. In addition, the Ile27-Ile56 peptide (L-NTP) that spans the doubly S-S bridged loop segment of NTP was synthesized. Pgn kringles were generated either by proteolytic fragmentation of Pgn (K4, K5) or via recombinant gene expression (rK1, rK2, and rK3). Interactions of CB-NTP with each of the Pgn kringles were monitored by 1H-NMR at 500 MHz and values for the equilibrium association constants (Ka) determined: rK1, Ka approximately 4.6 mM(-1); rK2, Ka approximately 3.3 mM(-1); K4, Ka approximately 6.2 mM-'; K5, K, 2.3 mM(-1). Thus, the lysine-binding kringles interact with CB-NTP more strongly than with Nalpha-acetyl-L-lysine methyl ester (Ka < 0.6 mM(-l), which reveals specificity for the NTP. In contrast, CB-NTP does not measurably interact with rK3. which is devoid of a LBS. CB-NTP and L-NTP 1H-NMR spectra were assigned and interproton distances estimated from 1H-1H Overhauser (NOESY) experiments. Structures of L-NTP and the Glul-Ile27 segment of CB-NTP were computed via restrained dynamic simulated annealing/energy minimization (SA/EM) protocols. Conformational models of CB-NTP were generated by joining the two (sub)structures followed by a round of constrained SA/EM. Helical turns are indicated for segments 6-9, 12-16, 28-30, and 45-48. Within the Cys34-Cys42 loop of L-NTP, the structure of the Glu-Glu-Asp-Glu-Glu39 segment appears to be relatively less defined, as is the case for the stretch containing Lys5O within the Cys42-Cys54 segment, consistent with the latter possibly interacting with kringle domains in intact Glul-Pgn. Overall, the CB-NTP and L-NTP fragments are of low regular secondary structure content-as indicated by UV-CD spectra- and exhibit fast amide 1H-2H exchange in 2H2O, suggestive of high flexibility.

Lysine-50 is a likely site for anchoring the plasminogen N-terminal peptide to lysine-binding kringles

Interactions between the kringle 4 (K4) domain of human plasminogen (Pgn) and segments of the N-terminal Glu1-Lys77 peptide (NTP) have been investigated via 1H-NMR at 500 MHz. NTP peptide stretches devoid of Lys residues but carrying an internal Arg residue show negligible affinity toward K4 (equilibrium association constant Ka < 0.05 mM(-1)). In contrast, while most fragments containing an internal Lys residue exhibit affinities comparable to that shown by the blocked Lys derivative Nalpha-acetyl-L-lysine-methyl ester (Ka approximately 0.2 mM(-1), peptides encompassing Lys50O consistently show higher Ka values. Among the investigated linear peptides, Nalpha-acetyl-Ala-Phe-Tyr-His-Ser-Ser-Lys5O-Glu-Gln-NH2 (AcAFYHSK5OEQ-NH2) exhibits the strongest interaction with K4 (Ka approximately 1.4 mM(-1)), followed by AcYHSK50EQ-NH2 (Ka approximately 0.9 mM(-1)). Relative to the wild-type sequence, mutated hexapeptides exhibit lesser affinity for K4. When a Lys50 --> Ser mutation was introduced (==> AcYHSS50EQ-NH2), binding was abolished. The Ile27-lle56 construct (L-NTP) contains the Lys50 site within a loop constrained by two cystine bridges. The propensity of recombinant Pgn K1 (rK1) and K2 (rK2) modules, and of Pgn fragments encompassing the intact K4 and K5 domains, for binding L-NTP, was investigated. We find that L-NTP interacts with rK1, rK2, K4, and K5-all lysine-binding kringles-in a fashion that closely mimics what has been observed for the Glul-HSer57 N-terminal fragment of Pgn (CB-NTP). Thus, both the constellation of kringle lysine binding site (LBS) aromatic residues that are perturbed upon complexation of L-NTP and magnitudes of kringle-L-NTP binding affinities (rK1, Ka approximately 4.3 mM(-1); rK2, Ka approximately 3.7 mM(-1; K4, Ka approximately 6.4 mM(1); and K5, Ka approximately 2.1 mM(-1)) are essentially the same as for the corresponding kringle-CB-NTP pairs. Molecular modeling studies suggest that the Glu39-Lys50 stretch in NTP generates an area that complements, both topologically and electrostatically, the solvent-exposed kringle LBS surface.