Chapter 2. Review of the literature
2.1. Uptake and activation of fatty acids prior to β -oxidation
For fatty acids to be utilizable as an energy source, they need to be released from triacylglycerols by lipases (for a review, see Gibbons et al. 2000), transported bound to albumin in blood to the target tissues and taken up by the cells either by diffusion or by transport proteins in the plasma membrane (for reviews, see van Nieuwenhoven et al. 1996, Hamilton 1998). In cytosol, fatty acids are bound to fatty acid binding proteins and delivered to appropriate subcellular sites (reviewed by Bernlohr et al. 1997).
Before fatty acids can enter the β -oxidation cycle, they must be activated to their CoA esters by the ATP-driven acyl-CoA synthetase (Fig. 1). Acyl-CoA synthetases are present in mitochondria, peroxisomes and the endoplasmic reticulum (ER), and they vary in substrate specificity (reviewed by Van Veldhoven & Mannaerts 1999). For mitochondrial β -oxidation, long-chain fatty acids are activated by a long-chain acyl-CoA synthetase located on the outer mitochondrial membrane with its active site exposed to the cytosolic side (Aas 1970, Hesler et al. 1990). Long-chain acyl-CoAs cannot readily traverse the inner mitochondrial membrane, while instead, the acyl moiety is coupled to carnitine by the malonyl-CoA-sensitive carnitine acyltransferase I on the outer mitochondrial membrane, then shuttled across the inner mitochondrial membrane by a carnitine acylcarnitine translocase in exchange for a carnitine molecule from the mitochondrial matrix (Pande 1975, Pande & Parvin 1976), after which the acyl moiety is linked back to a CoA molecule by a carnitine acyltransferase II located on the matrix side of the inner mitochondrial membrane (Fig. 1) (for a review, see Kerner & Hoppel 2000). Carnitine acyltransferases are commonly also called carnitine palmitoyltransferases (CPT) because of their substrate chain length specificity. Short- and medium-chain fatty acids do not require such a transport system for mitochondrial import, and they are activated in the mitochondrial matrix by short- and medium-chain acyl-CoA synthetases (Webster et al. 1965, Aas & Bremer 1968, Aas 1970, Eaton et al. 1996).
For peroxisomal β -oxidation, fatty acids are activated at different subcellular locations. Long-straight-chain and 2-methyl-branched-chain fatty acids are activated by acyl-CoA synthetases on the cytoplasmic side of the peroxisomal membrane, on the outer mitochondrial membrane and in ER (Krisans et al. 1980, Mannaerts et al. 1982, Vanhove et al. 1991, Wanders et al. 1992). The same long-chain acyl-CoA synthetase is probably also responsible for the activation of branched-chain fatty acids (Wanders et al. 1992, Watkins et al. 1996). Very-long-chain acyl-CoAs (>C20) are generated only in peroxisomes and ER (Singh & Poulos 1988, Lazo et al. 1990) by a very-long-chain fatty acyl-CoA synthetase (Uchida et al. 1996, Uchiyama et al. 1996). The peroxisomal very-long-chain acyl-CoA synthetase is located on the matrix side of the peroxisomal membrane, in contrast to the peroxisomal long-chain acyl-CoA synthetase (Mannaerts et al. 1982), and, in addition to the straight-chain fatty acids, it also activates branched-chain fatty acids, such as pristanic acid (Steinberg et al. 1999). The peroxisomal very-long-chain acyl-CoA synthetase could thus have an important role in the intraperoxisomal reactivation of pristanic acid (Steinberg et al. 1999), which is the α-oxidation product of phytanic acid (see “β -Oxidation of α-methyl-branched-chain fatty acids”). Derivatives of fatty acids oxidized in peroxisomes, namely dicarboxylic fatty acids, prostaglandins and the carboxylic side chains of bile acid intermediates are activated to their CoA esters by ER enzymes (Vamecq et al. 1985, Prydz et al. 1988, Schepers et al. 1988, Schepers et al. 1989). Peroxisomes do not use the carnitine-coupled transport system present in mitochondria for the import of acyl-CoA esters. It is not completely clear how activated fatty acids enter the peroxisomal matrix for degradation by β -oxidation. Long-chain and very-long-chain fatty acyl-CoAs probably reach the matrix via a membrane-bound transporter containing an ATP-binding cassette (ABC) motif, as shown in S. cerevisiae (Hettema et al. 1996). The homologous protein in human is affected in adrenoleukodystrophy, a peroxisomal disease, in which the metabolism of very-long-chain fatty acids is impaired (Aubourg et al. 1993, Mosser et al. 1993, 1994). How CoA esters of dicarboxylic fatty acids, prostaglandins and bile acid intermediates reach peroxisomes is not known yet.
In the yeast S. cerevisiae, medium-chain fatty acids first enter the peroxisome, the exclusive site of β -oxidation in yeast, and then get activated by an intraperoxisomal acyl-CoA synthetase, whereas long-chain fatty acids are activated in the cytosol and transported via Pat1p and Pat2p, the ABC transporter proteins (Hettema et al. 1996, Trotter 2001).