Despite the prevalence of nitrogen and sulfur in biological systems, and in the proteinogenic amino acids, S-N bonds are not widely found in biological systems. Indeed, currently, only approximately 100 S-N containing biomolecules are known. Many of these are metabolites, but it is now known that this bond is also exploited in proteins. For instance, protein tyrosine phosphatase 1B reversibly forms a cyclic sulfenylamide to protect its active site cysteinyl from oxidative damage. More recently, however, Type IV Collagen has been found to contain a sulfilimine (S=N) bond crosslink between a methionine (Met93) and a lysine (Lys211) or hydroxylated lysine (Hyl211). Presently, this is the only known such occurrence of this bond in nature. Collagen IV is the most abundant molecular species of the basement membrane and is a critical component of the lamina densa [1], a sub-layer of the basement membrane [2]. In particular, it forms a network that provides a scaffold for other components of the basement membrane [3]. It has been suggested that collagen IV has critical roles in wound healing[4] and embryogenesis [5], while its Improper functioning is associated with several diseases [6,7]. Notably, the sulfilimine crosslink in Collagen IV has been implicated as having played a critical role in evolution. Unfortunately, despite its ubiquitous occurrence and biological importance, much remains unclear or unknown about Collagen IV's sulfilimine bond, including its formation, the impact of involving Lys211 or Hyl211, and its very nature. Indeed, synthetic sulfilimine bonds have been discussed as potential superbases, yet in biological systems it is generally indicated as being neutral. We have applied a multi-scale computational approach to systematically examine the mechanisms by which a sulfilimine crosslink could be formed, and the nature of the resulting bond. In addition, we have examined the impact of the bond forming between Met93 and Lys211 versus Met93 and Hyl211. We will be presenting some of the results we have obtained.

[1] Kalluri, R. Basement membranes: structure, assembly and role in tumour angiogenesis. Nat. Rev. Cancer 2003, 3, 422-33. [2] Paulsson, M. Basement-Membrane Proteins - Structure, Assembly, and Cellular Interactions. Crit. Rev. Biochem. Mol. 1992, 27, 93-127. [3] Wu, Y.; Ge, G. Complexity of type IV collagens: from network assembly to function. Biol. Chem. 2019, 400, 565-574. [4] Tracy, L. E.; Minasian, R. A.; Caterson, E. J. Extracellular Matrix and Dermal Fibroblast Function in the Healing Wound. Adv. Wound Care 2016, 5, 119-136. [5] Bai, X.; Dilworth, D. J.; Weng, Y. C.; Gould, D. B. Developmental distribution of collagen IV isoforms and relevance to ocular diseases. Matrix Biol. 2009, 28, 194-201. [6] Petty, R. E.; Hunt, D. W.; Rosenberg, A. M. Antibodies to type IV collagen in rheumatic diseases. J. Rheumatol. 1986, 13, 246-53. [7] Abreu-Velez, A. M.; Howard, M. S. Collagen IV in Normal Skin and in Pathological Processes. N. Am. J. Med. Sci. 2012, 4, 1-8.