6 Peroxisomes

Abstract

I. INTRODUCTION
Peroxisomes are the major group in the microbody family of organelles, which also includes glyoxysomes, glycosomes, and the catalase-free microbodies. The “peroxisome” was defined by de Duve (de Duve and Baudhuin 1966) on the basis of biochemical function: It contains at least one enzyme that produces H₂O₂ (an oxidase) together with catalase, which decomposes H₂O₂ either catalatically (2H₂O₂→2H₂O + O₂) or peroxidatically (RH₂ + H₂O₂→R + 2H₂O). This combination of enzymes allows the peroxisome to carry out respiration, an essential cell function in an aerobic world. Peroxisomal respiration does not conserve energy as ATP and may have arisen in evolution before the more elaborate mitochondrial respiratory system involving oxidative phosphorylation (de Duve 1969). A second major peroxisomal function is the β-oxidation of fatty acids, which is conserved (Kunau et al. 1996) among peroxisomes of evolutionarily diverse organisms, including animals (van den Bosch et al. 1992; Reddy and Mannaerts 1994), plants (Beevers 1979; Tolbert 1981), and fungi (Tanaka et al. 1982; Kunau et al. 1987). Interestingly, mitochondria also catalyze β-oxidation in animals but not in plants or fungi (Kunau et al. 1987). A third important cell function to which the peroxisorne contributes in plant and fungal cells is the glyoxylate cycle, which is required for the net conversion of two-carbon compounds (and of fat, which is catabolized to acetyl-CoA) to carbohydrate. The “glyoxysome” of castor bean was given this name because it contains all of the enzymes of the glyoxylate cycle (Beevers 1982). The glyoxysome also catalyzes H₂O₂-based respiration...