Peroxisomes import reaction

Peroxisomes are membrane-bounded organelles that are specialized for carrying out oxidative reactions. Their internal proteins are imported from the cytoplasm. The peroxisome import signal is the tripeptide Ser-Lys-Leu located near the carboxyl terminus of the protein. Peroxisome protein import is a critical cellular process, as evidenced by the severe tissue abnormalities found in patients with the inherited disease Zelweyer syndrome. These individuals are characterized as having protein-free peroxisomes. 

Mevalonate diphosphate decarboxylase (MVP EC4.1.1.33) catalyzes the conversion of mevalonate diphosphate to isopentenyl diphosphate, a key building block for a large family of functionally important terpenoids. Fig. (6). This reaction is the third step in the biosynthesis of steroids and terpenoids from the mevalonate pathway, and the last well characterized step in the mevalonate pathway for the biosynthesis of isopentenyl pyrophophaste, the isoprenoids precursor [296-298]. Some reports showed that MVP is located predominantly in the cytosolic fraction and its expression is independent of peroxisome proliferation [299-300]. 

It has long been recognized that fatty acids may be partially degraded with the subsequent esterification of chain-shortened products. For example, in 1964, Verdino et al. (20) reported that when 4,7,10,13,16,19-22 5 was fed to rats raised on a diet devoid of fat, there was a large increase in esterified arachi-donic acid. In 1970 Stoffel, et al. (21) showed that this partial degradative reaction was associated with a mitochondrial fraction however, these smdies were carried out before the importance of peroxisomal fatty acid 3-oxidation was recognized. In 1993, Christensen et al. (22) reported that when labeled [3-14C] 7,10,13,16-22 4 and 7,10,13,16,19-22 5 were incu-... 

The remainder of the biosynthesis takes place in the peroxisomes. The substrate is imported by the cassette-transporter comatose (CTS). The hydrogenation of the endocyclic double bond is carried out by 12-oxophytodienic acid reductase this enzyme belongs to a small group of flavin-dependent oxidoreductases. In accord with the reaction mechanism, which was suggested for the related Old Yellow Enzyme of yeast, two hydrogen bridges from histidine (His-186 and His-189 in the 12-oxophytodienic acid reductase from mouse-ear cress Arabidopsis thaliana)) towards the carbonyl group polarise the double bond, so that a hydride of the reduced flavin cofactor can be transferred to C-10. The carbanion in the 11 -position is then protonated by Tyr-191. 

These are small vesicles which are present in almost all eukaryotic cells. They contain a variety of oxidative enzymes and are a major site of O2 utilization. Although hydrogen peroxide is produced, this is prevented from accumulating because catalase is present. Peroxisomes are believed to play an important part in detoxication reactions in the liver and kidney, e.g. in the conversion of alcohol to acetaldehyde. 

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