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SCIENCEWEEK

Cell, Volume 130, Issue 4, 24 August 2007, Pages 663-677

Biochemistry: On Very Long-Chain Fatty Acids

A Molecular Caliper Mechanism for Determining Very Long-Chain Fatty Acid Length

Vladimir Denic and Jonathan S. Weissman

Very long-chain fatty acids (VLCFAs) are structurally diverse biological molecules with unusually long hydrocarbon chains, ranging from 20 to 36 carbons (C20–36) or more (Leonard et al., 2004). These lipids perform essential roles in a wide range of biological processes that cannot be supported by the more common shorter fatty acids (i.e., C16 and C18). For example, the chain length of certain VLCFAs allows them to simultaneously reside in both leaflets of the lipid bilayer, thereby stabilizing highly curved cellular membranes, such as those surrounding nuclear pore complexes ([Schneiter et al., 2004] and [Schneiter et al., 1996]). Notably, the variation in VLCFA chain length across different species and different tissues has enabled the numerous functional specializations of these lipids. In S. cerevisiae, for example, while C22 is able to support the essential functions of VLCFAs, C26 is specifically required for a variety of membrane-based processes, including the formation of GPI lipid anchors and the trafficking of proteins in the secretory pathway ([Dickson et al., 2006] and [Toulmay and Schneiter, 2007]). Similarly, in mammals, VLCFAs with lengths greater than C30 allow the formation of a permeability barrier that is critical for the normal structure and function of the skin ([McMahon et al., 2007] and [Vasireddy et al., 2007]). Finally, VLCFAs and their derivatives act as signaling molecules (e.g., arachidonic acid; Leonard et al., 2002) and are dominant lipid constituents of certain tissues in animals (e.g., photoreceptor cells and myelin; Poulos et al., 1992) and plants (e.g., oils and waxes; Kunst and Samuels, 2003).

Efforts to understand the mechanistic principles enabling their structural diversity have been hampered by the inability to reconstitute VLCFA synthesis from purified components ([Cinti et al., 1992], [Jakobsson et al., 2006] and [Leonard et al., 2004]). This has been largely due to the insoluble nature of the VLCFA biosynthetic machinery which has been known since the 1960s to consist of detergent-labile complexes embedded within the endoplasmic reticulum (ER) membrane (for a historical overview see Cinti et al., 1992). These pioneering studies demonstrated that VLCFAs are synthesized by the elongation of shorter fatty acids (C16 and C18) that are produced in the cytosol by the well-characterized, multienzyme complex termed the fatty acid synthase (FAS; [Jenni et al., 2007] and [Lomakin et al., 2007]). VLCFA synthesis was shown to proceed by a four-step biochemical cycle (Figure 1B; Nugteren, 1965): (1) condensation of malonyl-CoenzymeA (CoA) with an acyl-CoA precursor; (2) reduction of the resulting 3-keto intermediate; (3) dehydration of the 3-hydroxy species; and (4) reduction of the enoyl product to yield a saturated FA chain that is two carbons longer than its precursor.

The above conceptual framework facilitated more recent genetic approaches in budding yeast that identified several ER membrane proteins required for VLCFA production. Specifically, Ybr159wp and Tsc13p are strong candidates for the enzymes catalyzing the second and fourth steps of the elongation cycle, respectively, as they are homologous to known reductases and their inactivation leads to the accumulation of the expected keto and enoyl intermediates in the ER membrane ([Beaudoin et al., 2002], [Han et al., 2002] and [Kohlwein et al., 2001]). Additionally, Fen1p and Sur4p are two yeast members (Oh et al., 1997) of a large family of proteins termed the Elops (Jakobsson et al., 2006) which are required for the first (condensation) step in the elongation cycle ([Moon et al., 2001], [Paul et al., 2006] and [Westerberg et al., 2006]). Moreover, heterologous expression of Elop homologs in yeast demonstrated that these proteins determine the length of cellular VLCFA products. Hence, the proliferation and specialization of Elops has been responsible for the observed VLCFA chain length diversity across different organisms and cell types ([Jakobsson et al., 2006] and [Leonard et al., 2004]). The exact function of Elops in the elongation cycle is uncertain, however, as these proteins lack homology to known condensing enzymes. Therefore, it remains to be established whether Elops are members of a novel family of condensing enzymes or noncatalytic adapters for recruiting specific substrates to the actual condensing enzyme. Finally, no protein has been identified that is specifically required for the dehydratase reaction.

Based on these findings, we can now restate the question of VLCFA chain length determination in more specific mechanistic terms. Namely, how do Elops instruct the components of the elongation cycle to extend shorter FAs in two-carbon addition steps so as to yield VLCFA products of defined lengths? The observation that ER membranes containing a single Elop can convert substrates of different lengths to the same-length end product (Paul et al., 2006) argues against a length-determining mechanism that “counts” a fixed number of two-carbon additions. Conversely, ER membranes with different Elops allow for the elongation of the same substrate to different-length VLCFA products (Paul et al., 2006). Thus, Elops are able to specify VLCFA length in absolute terms. Efforts to understand the mechanism by which Elops achieve this and how evolutionary diversification of the Elop family has enabled the synthesis of VLCFAs of novel chain lengths have been hampered by the incomplete inventory of the components required for VLCFA synthesis (Paul et al., 2006). Moreover, the integral membrane nature of the enzymes involved has thus far obstructed the in vitro reconstitution of any step of the elongation cycle ([Cinti et al., 1992], [Jakobsson et al., 2006] and [Leonard et al., 2004]).

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