Enzyme-independent catabolism of cysteine with pyridoxal-5′-phosphate

Abstract

Pyridoxal-5′-phosphate (PLP) is a versatile cofactor that assists in different types of enzymatic reactions. PLP has also been reported to react with substrates and catalyze some of these reactions independent of enzymes. One such catalytic reaction is the breakdown of cysteine to produce hydrogen sulfide (H2S) in the presence of multivalent metal ions. However, the enzyme-independent catalytic activity of PLP in catabolizing cysteine in the absence of multivalent ions is unknown. In this study, we show that PLP reacts with cysteine to form a thiazolidine product, which is supported by quantum chemical calculations of the absorption spectrum. The reaction of PLP with cysteine is dependent on ionic strength and pH. The thiazolidine product slowly decomposes to produce H2S and the PLP regenerates to its active form with longer reaction times (> 24 h), suggesting that PLP can act as a catalyst. We propose an enzyme-independent plausible reaction mechanism for PLP catalyzed cysteine breakdown to produce H2S, which proceeds through the formation of thiazolidine ring intermediates that later hydrolyzes slowly to regenerate the PLP. This work demonstrates that PLP catalyzes cysteine breakdown in the absence of enzymes, base, and multivalent metal ions to produce H2S.

Cystine rather than cysteine is the preferred substrate for β-elimination by cystathionine γ-lyase: implications for dietary methionine restriction

“…Cysteine inhibits CGL by forming a thiazolidine with the catalytic PLP aldimine in the active site of the enzyme. This mechanism has now been verified by UV–vis spectrometry and 1 H-NMR using the CGL of various species and PLP (this work and [ 69 , 71 , 72 , 81 – 83 ]). Moreover, the formation of a thiazolidine accounts for the inhibition of other PLP-containing enzymes by cysteine [ 81 , 84 ].…”

“…The dissociation rates of cysteine or its metabolites from either the CGL bound or free thiazolidine are not known. Studies of the thiazolidine adduct report on the appearance of PLP or pyridoxine after days rather than a rate per se [ 71 , 73 , 83 ]. In the same vein, Terzuoli et al calculated an equilibrium constant for the dissociation of the thiazolidine of 36.8 M −1 at 37 °C for a PLP:cysteine stoichiometry of 3:4 [ 73 ].…”