This study presents the first structural characterization of the biarylitude cross-linking P450 enzyme, P450Blt, in complex with its pentapeptide substrate MRYLH. The structure reveals key insights into how P450Blt coordinates with the histidine residue of the substrate and how this contributes to its specificity for forming a C–N bond between tyrosine and histidine residues in the MRYLH substrate. The structure also highlights the importance of the I-helix residues in substrate binding and catalysis. The P450Blt structure was determined using X-ray crystallography and molecular replacement, with a resolution of 1.8 Å. The structure shows that the MRYLH peptide is positioned perpendicular to the I-helix, with its C-terminal carboxylate group nested between the I- and G-helices. The N-terminal methionine residue is situated in a hydrophobic cavity formed by residues in the protein N-terminus and other regions. The structure also reveals that the P450Blt enzyme forms hydrogen bonds with key residues in the MRYLH peptide, including Ser-239, which is central to oxygen activation in P450s. The structure further shows that the P450Blt enzyme forms a hydrogen bond network with the MRYLH peptide, which is essential for its catalytic activity. The study also includes molecular dynamics simulations that support the stability of the MRYLH-P450Blt complex and reveal the dynamic nature of the peptide binding. The results suggest that the P450Blt enzyme uses a combination of hydrogen bonding and water-mediated interactions to coordinate the MRYLH peptide. The study also includes biochemical experiments that confirm the role of key residues in the P450Blt enzyme in substrate binding and catalysis. The results highlight the importance of the I-helix residues in the P450Blt enzyme in coordinating the substrate and in determining the specificity of the enzyme for forming a C–N bond between tyrosine and histidine residues in the MRYLH substrate. The study also compares the structure of P450Blt with other P450 enzymes and shows that it shares some structural similarities with other P450 enzymes, but has a unique active site that is essential for its catalytic activity. The study concludes that the P450Blt enzyme is a key player in the biosynthesis of biarylitude cross-linked peptides and that its structure provides important insights into the mechanism of P450-catalyzed cross-linking reactions.This study presents the first structural characterization of the biarylitude cross-linking P450 enzyme, P450Blt, in complex with its pentapeptide substrate MRYLH. The structure reveals key insights into how P450Blt coordinates with the histidine residue of the substrate and how this contributes to its specificity for forming a C–N bond between tyrosine and histidine residues in the MRYLH substrate. The structure also highlights the importance of the I-helix residues in substrate binding and catalysis. The P450Blt structure was determined using X-ray crystallography and molecular replacement, with a resolution of 1.8 Å. The structure shows that the MRYLH peptide is positioned perpendicular to the I-helix, with its C-terminal carboxylate group nested between the I- and G-helices. The N-terminal methionine residue is situated in a hydrophobic cavity formed by residues in the protein N-terminus and other regions. The structure also reveals that the P450Blt enzyme forms hydrogen bonds with key residues in the MRYLH peptide, including Ser-239, which is central to oxygen activation in P450s. The structure further shows that the P450Blt enzyme forms a hydrogen bond network with the MRYLH peptide, which is essential for its catalytic activity. The study also includes molecular dynamics simulations that support the stability of the MRYLH-P450Blt complex and reveal the dynamic nature of the peptide binding. The results suggest that the P450Blt enzyme uses a combination of hydrogen bonding and water-mediated interactions to coordinate the MRYLH peptide. The study also includes biochemical experiments that confirm the role of key residues in the P450Blt enzyme in substrate binding and catalysis. The results highlight the importance of the I-helix residues in the P450Blt enzyme in coordinating the substrate and in determining the specificity of the enzyme for forming a C–N bond between tyrosine and histidine residues in the MRYLH substrate. The study also compares the structure of P450Blt with other P450 enzymes and shows that it shares some structural similarities with other P450 enzymes, but has a unique active site that is essential for its catalytic activity. The study concludes that the P450Blt enzyme is a key player in the biosynthesis of biarylitude cross-linked peptides and that its structure provides important insights into the mechanism of P450-catalyzed cross-linking reactions.