Protein arginine deiminase 4 (PAD4): Current understanding and future therapeutic potential

Abstract

The protein arginine deiminases (PADs), and in particular PAD4, have emerged as potential therapeutic targets for the treatment of rheumatoid arthritis (RA). In this review, evidence linking dysregulated PAD activity to the onset and progression of RA is presented, and the potential role of such aberrant activity in other human diseases, such as multiple sclerosis and cancer, is discussed. The known physiological roles of the PADs, particularly PAD4, and current knowledge regarding PAD structure, catalysis and inhibition are also described.

Figure 1

PADs catalyze the hydrolytic deimination of protein arginine to produce protein citrulline and ammonia.

Figure 2

Crystal structure of the PAD4•Ca²⁺ •Benzoyl Arginine Amide (BAA) complex. The C-terminal catalytic domain is shown in blue. The N-terminal domain contains two immunoglobulin-like subdomains (red and green, respectively). There are five Ca²⁺ binding sites (black). Insert shows the location of the RA-associated single nucleotide polymorphisms (SNPs), S55G, A82V, and A112G.

Figure 3

Active site of PAD4. A) Position of active site residues in the apoenzyme (dark grey) and calcium-bound form of the enzyme (light grey). B) Proposed catalytic mechanism. Adapted from [53].

Figure 3

Active site of PAD4. A) Position of active site residues in the apoenzyme (dark grey) and calcium-bound form of the enzyme (light grey). B) Proposed catalytic mechanism. Adapted from [53].

Figure 4

Diagram of PAD4 bound to a histone H3 tail mimic. Critical binding contacts are indicated by the dashed lines. Adapted from [54].

Figure 5

PAD4 inhibitors. ^aEnzyme was incubated at 52 °C for 30 min with 50 mM HEPES (pH 7.6), CaCl₂(5 mM), DTT (2 mM), and BAEE (5 mM). ^bAfter preincubating the enzyme with inhibitor, 100 mM Tris-HCl (pH 7.6), CaCl₂ (10 mM), DTT (2 mM), and NaCl (50 mM) at 37 °C for 15 min, BAEE (1 mM final) was added. ^cReactions were carried out in 100 mM Tris-HCl (pH 7.6), CaCl₂ (10 mM), DTT (1 mM), and Bz-Arg (0.1 mM) at 37 °C for 40 or 60 min. ^dReactions were carried out in 100 mM Tris-HCl (pH 7.6), CaCl₂ (10 mM), DTT (5 mM), and BAEE (5 mM) at 37 °C for 10 min. ^eReactions were carried out in 100 mM HEPES (pH 7.6), CaCl₂ (10 mM), TCEP (0.5 mM), NaCl (50 mM), and BAEE (10 mM final) at 37 °C for 15 min.