Hydrocyanic Acid / Thiocyanate Inhibition of Thyroperoxidase: Rutin Mediation by Kinetic and Docking Studies

The present study explores the effects of phenolics and flavonoids as potential therapeutic agents for management of raised serum levels of some cassava food toxicants, SCN / HCN as reported in our previous study. Michaelis-Menten kinetics, together with Lineweaver-Bulk plot are employed to determine the kinetic constants of porcine thyroid peroxidase (pTPO), a principal enzyme in thyroid hormone synthesis; while the Schrodinger Suite (2020) is used to dock phenolics and flavonoids test compounds against the human TPO (PDB ID: 1PRX), with ascorbic acid and methylimidazole serving as the control compounds. In the absence of inhibitory compounds, TPO exhibits typical Michaelis-Menten kinetics, with values for Vmax 0.014126Ms−1, Km 0.001297M, and Kcat 5.389s−1. Upon the addition of some individual phytochemicals, all the kinetic parameters altered, (much less so when the SCN was introduced), exhibiting an overall catalytic efficiency as noncompetitive or mixed-type inhibitors, thereby modulating enzyme function and catalysis. The kinetics and or docking studies suggest epicatechin, chlorogenic acid, quercetin, gallic acid and the toxicants SCN and HCN act as noncompetitive inhibitors and exhibited similar binding with a key amino acid residue, ALA-151, as that of the control inhibitor compound, ascorbic acid – an interaction deemed to affect enzyme’s structural conformation and activity, contributing to the phytochemicals’ potential as modulators / regulators of enzyme activities. Contrarily, rutin acts as an allosteric activator, increasing the rate of breakdown of ES complex to products and so a potential remedy for conditions of hypothyroidism. This kinetic behaviour also aligns with its binding interactions with amino acid residues different from those of its counterpart inhibitor components. The studied phytochemicals including rutin, show higher binding affinity (docking scores -6.52 to -5.06 Kcal/mol), displaying the most stable binding conformation and supporting their potential roles as enzyme modulators than those obtained from the inhibitor toxicants (-3.58 to -3.04 Kcal/mol) and the standard ascorbic acid ligand (4.27) Kcal/mol. The results from chlorogenic acid and epicatechin suggest both possess therapeutic relevance in hyperthyroidism and in TPO homeostasis respectively. More understanding into the complex interplay between enzyme, substrate, and regulatory / modulatory factors in thyroid hormone synthesis, function, advancement of diagnostics and therapeutic approaches for thyroid disorders and associated health conditions, has been provided. These findings have significant implications for drug design as activators / inhibitors / modulators of enzyme functions and in nutraceutics preparations.