Direct and continuous assay for prolyl 4-hydroxylase

Abstract

Prolyl 4-hydroxylase (P4H) is a nonheme iron dioxygenase that catalyzes the posttranslational hydroxylation of (2S)-proline (Pro) residues in protocollagen strands. The resulting (2S,4R)-4-hydroxyproline (Hyp) residues are essential for the folding, secretion, and stability of the collagen triple helix. P4H uses alpha-ketoglutarate and O2 as cosubstrates, and forms succinate and CO2 as well as Hyp. Described herein is the first assay for P4H that continuously and directly detects turnover of the proline-containing substrate. This assay is based on (2S,4S)-4-fluoroproline (flp), a proline analogue that is transformed into (2S)-4-ketoproline (Kep) and inorganic fluoride by P4H. The fluoride ion, and thus turnover by P4H, is detected by a fluoride ion-selective electrode. Using this assay, steady-state kinetic parameters for the human P4H-catalyzed turnover of a flp-containing peptide were determined and found to be comparable to those obtained with a discontinuous HPLC-based assay. In addition, this assay can be used to characterize P4H variants, as demonstrated by a comparison of catalysis by D414A P4H and the wild-type enzyme. Finally, the use of the assay to identify small-molecule inhibitors of P4H was verified by an analysis of catalysis in the presence of 2,4-pyridine dicarboxylate, an analogue of alpha-ketoglutarate. Thus, the assay described herein could facilitate biochemical analyses of this essential enzyme.

Fig. 1

Reaction catalyzed by prolyl 4-hydroxylase when (A) a Pro-containing peptide or (B) a flp-containing peptide is the substrate.

Fig. 2

Fluoride ion-detection assay for P4H (90 nM). Assays were performed in the presence or absence of PEG-Gly–Tyr–Yaa–GlyOEt, where Yaa = flp (1.0 mM) or Flp (0.50 mM). The dashed line shows the fit of the Yaa = flp data by linear-regression analysis. Assay conditions are as described in the Materials and methods section.

Fig. 3

Dependence of the rate of fluoride-ion release from PEG-Gly–Tyr–flp–GlyOEt (1.0 mM) on the concentration of P4H. Assay conditions are as described in the Materials and methods section. Reactions were performed in duplicate. Data were fitted by linear-regression analysis. v = ∂[F⁻]/∂t.

Fig. 4

Catalysis of fluoride ion-release from PEG-Gly–Tyr–flp–GlyOEt by P4H (90 nM). Assay conditions are as described in the Materials and methods section. Individual points are the average (±SE) of three reactions. Data were fitted to the Michaelis–Menten equation: v = ∂[F⁻]/∂t = k_cat[P4H][peptide]/(K_M + [peptide]).

Fig. 5

Catalysis of fluoride ion-release from PEG-Gly–Tyr–flp–GlyOEt (0.20 mM) by a P4H variant or in the presence of a small-molecule inhibitor. (A) Catalysis by wild-type P4H (90 nM) and its D414A variant (0.90 μM). (B) Inhibition of catalysis by 2,4-pyridine dicarboxylate. Assay conditions are as described in the Materials and methods section. v = ∂[F⁻]/∂t.