## 2 Cell organelles and metabolic pathways that contribute to HCV infection
### Fatty acid and cholesterol biosynthesis  
Biosynthesis of fatty acids and cholesterol is of fundamental importance to maintenance of cellular homeostasis. Fatty acid synthesis initiates through the irreversible carboxylation of acetyl-CoA by acetyl-CoA carboxylase to generate malonyl-CoA (Figure 1A). Thereafter, fatty acid synthase performs a cycle of elongation steps, utilizing one acetyl-CoA and seven malonyl-CoA moieties, to generate palmitic acid [29]. Fatty acid synthase is a large, homodimer of 552 kDa that catalyses each of the sequential reactions [30]. Palmitic acid is then employed as a substrate for further elongation or desaturation. Elongation is mediated by a family of elongase enzymes, referred to as ELOVLS (Elongation of very-long-chain fatty acids) [31] while the desaturases are responsible for introducing double bonds at specific positions in the acyl chain of fatty acids. Mammalian cells express three desaturases (\(\Delta\)9, \(\Delta\)6 and \(\Delta\)5); the \(\Delta\)9-desaturase is commonly referred to as stearoyl-CoA desaturase while the \(\Delta\)6- and \(\Delta\)5-desaturases are termed fatty acid desaturases. Elongases exhibit preferential specificity for saturated, mono- and poly-unsaturated fatty acids and hence there is complex interplay between the ELOVLS and desaturases. Incorporation of fatty acids into triacylglycerglycerols (TAGs), which are major components of the hydrophobic core in lipid droplets (see below), begins with the addition of an acyl-CoA ester by long-chain acyl-CoA synthetase (ACS; Figure 1A). Thereafter, two fatty acyl-CoAs are used to sequentially acylate glycerol-3-phosphate, firstly forming lysophosphatidic acid (catalyzed by glycerol-3-phosphate acyltransferase) and then phosphatidic acid (catalyzed by lysophosphatidic acid acyltransferase). Phosphatidic acid is converted to diacylglycerol by phosphatidate phosphorhydrolase-1 before final addition of another fatty acyl-CoA to yield TAG. This final stage in the pathway is performed by diacylglycerol acyltransferase (DGAT). Two DGAT enzymes, DGAT1 and DGAT2, have been identified, which appear to have distinct intracellular roles [32]. It has been hypothesized that DGAT1 may be involved in esterification of exogenous fatty acids or the re-esterification of fatty acids after their hydrolysis. By contrast, DGAT2 may incorporate endogenously synthesized fatty acids into triglycerides. Cholesterol biosynthesis is more complex than the pathway for production of fatty acids and involves at least 30 distinct enzymatic reactions. Synthesis can be divided into five major steps (Figure 1B). Firstly, HMG-CoA synthase catalyzes a condensation reaction between acetyl-CoA and acetoacetyl-CoA to yield 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). The second stage is the  
Figure 1: Scheme for triacylglycerolglycerol **(a)** and cholesterol ester synthesis **(b)**. For the synthesis of triacylglycerolglycerols, note that there is an alternative monoacylglycerolglycerol pathway for TAG synthesis in addition to the glycerol phosphate pathway shown in **a**[33]. In addition, fatty acids from diet are utilized for production of fatty acyl-CoAs as well as endogenously synthesized fatty acids. Abbreviations are as follows: ACAT, acyl-coenzyme A:cholesterol transferase; ACC, acetyl-CoA carboxylase; ACS, long-chain acyl-CoA synthetase; DGAT, diacylglycerolglycerol acyltransferase; FAS, fatty acid synthase; FPP, farnesyl pyrophosphate; GPAT, glycerol-3-phosphate acyltransferase; GPP, geranyl pyrophosphate; HMG-CoA, 3-hydroxy-3-methylglutaryl-CoA; IPP, isopentenyl pyrophosphate; LPAAT, lysophosphatidic acid acyltransferase; PPH-1, phosphatidate phosphohydrolase-1. production of prevalent acid from HMG-CoA by HMG-CoA reductase, the rate-limiting step in the cholesterol synthesis pathway and the target for statins. Following a combination of phosphorylation and decarboxylation reactions, mevalonate gives rise to isopentenyl pyrophosphate (IPP), which is then converted to squalene by a series of condensation and reduction reactions.