Human peptidylarginine deiminase type III (PAD3) is a key enzyme in the regulation of protein citrullination, a post-translational modification that alters protein charge, stability, and interactions. Expressed primarily in epidermal keratinocytes and hair follicles, PAD3 plays a critical role in maintaining skin barrier function and hair shaft integrity through selective citrullination of structural proteins such as S100A3, trichohyalin, and filaggrin. Its unique substrate specificity—targeting specific arginine residues with high precision—sets it apart from other PAD isozymes and underscores its importance in both physiological and pathological processes.
This study presents a comprehensive structural analysis of human PAD3 across six distinct functional states: the apo wild-type (WT), Ca²⁺-bound active form, non-productive Ca²⁺-bound state, catalytically inactive C646A mutant in both apo and Ca²⁺-bound forms, and the complex with the pan-PAD inhibitor Cl-amidine. These structures reveal a dynamic activation mechanism governed by calcium-dependent conformational changes. In the absence of Ca²⁺, the active site remains disordered, indicating that full ion occupancy is required for structural maturation. Upon binding of five Ca²⁺ ions per subunit, the enzyme undergoes a transition to a stable, catalytically competent conformation, enabling substrate access and covalent modification.
The most striking feature of the PAD3 structure is the presence of a large, hydrophobic cavity adjacent to the active site, centered around Gly374. This residue occupies a position equivalent to Arg374 in PAD4, creating a significantly more spacious and less polar environment. Unlike PAD4, where the arginine side chain contributes to hydrogen bonding and electrostatic interactions, the small glycine in PAD3 allows for greater flexibility and accommodation of bulky substituents. This cavity is not only structurally distinct but also functionally significant—its size and chemical properties are directly linked to PAD3’s narrow substrate selectivity. The fact that this space remains underutilized in the absence of ligands suggests it may serve as a natural docking site for inhibitors or regulatory partners.
Cl-amidine binds covalently to Cys646 within the active site, forming a stable adduct confirmed by electron density maps and refined geometry. Additional stabilization arises from hydrogen bonding between the inhibitor’s carbamoyl group and the main-chain carbonyl of Leu640 and the side chain of Asp350. However, subtle differences in the surrounding residues—particularly Gly374 instead of Arg374—result in altered hydrogen-bonding networks compared to PAD4. Notably, no water molecule bridges Arg372 and the inhibitor in PAD3, a feature observed in PAD4, likely due to the increased hydrophobicity introduced by Gly374. This loss of hydration further enhances the potential for selective inhibition by hydrophobic compounds.
Importantly, the non-productive Ca²⁺-bound form reveals how structural dysregulation can occur. Despite the presence of five Ca²⁺ ions, the active site remains disordered, and the catalytic cysteine adopts an aberrant conformation.AKT1 Antibody In stock A short helix forms near Asp350, disrupting the proper alignment of key residues.NDUFA4L2 Antibody Autophagy Furthermore, Arg372 shifts position and forms a salt bridge with Asp369, a configuration absent in the active state.PMID:34982483 This misfolding is attributed to crystallization under supraphysiological Ca²⁺ concentrations (~200 mM), which may mimic pathological conditions and trap the enzyme in a non-functional state. This observation highlights the importance of physiological ion levels in maintaining enzymatic activity and warns against overinterpreting structures obtained under extreme conditions.
The comparison of PAD3 with PAD1, PAD2, and PAD4 reveals a clear correlation between active site architecture and substrate specificity. While PAD1 exists as a monomer and exhibits broad reactivity, PAD2 and PAD3 form homodimers with similar overall folds but divergent cavity depths. PAD2 has a shallow active site due to flexible loops and distinct side-chain orientations, leading to lower selectivity. In contrast, PAD3 maintains a deep, rigid pocket stabilized by a conserved α-helix near the catalytic center—a feature shared with PAD4 but not PAD2. This rigidity likely restricts conformational sampling and enforces precise substrate alignment, explaining why PAD3 selectively modifies Arg51 in S100A3 despite steric constraints.
Biochemical assays confirm that PAD3-mediated citrullination of S100A3 leads to tetramer formation and enhanced metal ion binding, suggesting a functional feedback loop in hair follicle maturation. Interestingly, PAD4 shows similar reactivity toward S100A3 in vitro, yet does not colocalize with it in vivo, implying that cellular context, rather than intrinsic catalytic preference, dictates substrate access. Nevertheless, the structural basis for PAD3’s selectivity lies in its unique combination of cavity size, depth, and dynamic stability.
These findings provide a robust foundation for developing next-generation PAD3-selective inhibitors. By exploiting the hydrophobic cavity around Gly374 and incorporating bulky, lipophilic groups, it may be possible to design compounds that achieve high potency and selectivity while minimizing off-target effects on PAD2 and PAD4. Future studies will focus on solving structures of PAD3 bound to native substrates like S100A3 peptides, offering deeper insights into the molecular recognition process.
In conclusion, this work elucidates the structural dynamics, activation mechanism, and substrate recognition principles of human PAD3. It establishes a direct link between enzyme architecture and biological function, demonstrating how subtle sequence differences translate into profound functional outcomes. These insights are invaluable for advancing targeted therapeutics in autoimmune diseases, neurodegeneration, and cancer, where dysregulated citrullination plays a central role.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
