Carbamylation of N-Terminal Proline

Abstract

Protein carbamylation is of great concern both in vivo and in vitro. Here, we report the first structural characterization of a protein carbamylated at the N-terminal proline. The unexpected carbamylation of the R-amino group of the least reactive codified amino acid has been detected in high-resolution electron density maps of a new crystal form of the HIV-1 protease/saquinavir complex. The carbamyl group is found coplanar to the proline ring with a trans conformation. The reaction of N-terminal with cyanate ion derived from the chaotropic agent urea was confirmed by mass spectra analysis on protease single crystals. Implications of carbamylation process in vitro and in vivo are discussed.KEYWORDS Carbamylation, N-terminal proline, HIV-1 protease/saquinavir complex, single crystals T he HIV protease (PR) is an aspartic protease that shares sequence homology around the active site with other retroviral proteases, as the conserved Asp-ThrGly catalytic triad. 1 PR is required for proteolytic cleavage of viral Gag and Gag-Pol polyproteins into individual structural and functional proteins during viral maturation. 2 In the absence of this proteolysis, immature noninfectious virions are produced, thus making PR a prime target for structureassisted drug design in antiviral therapy. 3,4 The structure-assisted drug design and discovery process utilizes techniques such as protein crystallography, nuclear magnetic resonance (NMR), and computational biochemistry to guide synthesis of potential drugs. PR has been widely characterized biochemically and structurally, which has led to the discovery of HIV protease inhibitors (PIs). Their utilization in highly active antiretroviral therapy (HAART) has been a major turning point in the management of HIV/acquired immune-deficiency syndrome (AIDS). 5 Recently, we have demonstrated that high-resolution X-ray crystallography can be used as a powerful method to identify the most potent PR inhibitor present in an epimeric mixture. 6 The pseudosymmetric peptidomimetic inhibitor based on a novel Phe-Pro isostere core matched the 2-fold symmetry of the homodimeric structure of the PR. Fourier maps obtained by high-resolution diffraction data (1.3 Å) clearly showed the catalytic site fully occupied by a single ordered stereoisomer. On the contrary, several X-ray crystal structures of PR complexed with more asymmetric FDAapproved inhibitors, like darunavir, nelfinavir, and saquinavir (SQV), have the catalytic site occupied by the inhibitor oriented in two almost equivalent positions related by a pseudo-2-fold symmetry. [7][8][9][10][11][12] To investigate this order/disorder phenomenon and its possible relationship with crystal packing, we have undertaken a systematic search for new crystal forms of wild-type PR in complex with SQV. Single crystals of the PR/SQV complex were obtained by a vapor diffusion technique and analyzed by X-ray diffraction. These crystals belong to a new monoclinic crystal form, which contains two dimers of the complex in the asymmetric unit. Other crystal...

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