Rate Enhancement of Compound I Formation of Barley Peroxidase by Ferulic Acid, Caffeic Acid and Coniferyl Alcohol

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Abstract
Reactions of barley peroxidase 1 were studied using transient-state and steady-state kinetics at pH 3.96, 25 degrees C, and 0.1 M ionic strength, in both the presence and the absence of 1 mM calcium ion. The rate of compound I formation from barley peroxidase 1 and hydrogen peroxide in the absence of reducing substrate is very slow, with or without calcium. When each of the three reducing substrates ferulic acid, caffeic acid, and coniferyl alcohol is added individually, there is a striking enhancement of the rate of compound I formation by a factor of 10-40 depending on the substrate. These unique rate enhancements can be explained by the effect of tight substrate binding to the native enzyme, and they may be indicative of an activating effect of reducing substrate on barley peroxidase 1 under physiological conditions. All steady-state kinetic results can be explained by an initial tight binding of reducing substrate AH to the barley peroxidase, Peroxidase + AH reversible Peroxidase-AH, and substitution of the peroxidase-AH complex for native enzyme in the standard modified ping-pong mechanism for peroxidase reactions [Dunford, H. B. (1991) in Peroxidases in Chemistry and Biology (Everse, J., Everse, K. E., & Grisham, M. B., Eds.) Vol. II, pp 1-24, CRC Press, Boca Raton, FL]. The dissociation constant of barley peroxidase 1 and ferulic acid was 1.4 +/- 0.6 microM as determined by the change in the absorbance at the Soret maximum at the conditions mentioned above.(ABSTRACT TRUNCATED AT 250 WORDS)

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