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Biochemistry
Published in Ronald Fayer, Lihua Xiao, Cryptosporidium and Cryptosporidiosis, 2007
Pyruvate may be converted to acetyl-CoA by a bifunctional pyruvate, pyruvate:NADP+ oxidoreductase (PNO), which contains a pyruvate-ferredoxin oxidoreductase (PFO) domain and an NADPH-cytochrome P450 reductase domain (CPR) (Rotte et al., 2001). The architecture of PNO is unique to Cryptosporidium, as it is not present in any other apicomplexans examined so far, but found only in a distant free-living protist, Euglena gracilis (Rotte et al., 2001). Whereas Euglena PNO apparently contains a signal peptide sequence and is localized in the mitochondria, CpPNO is found in the cytosol (Ctrnacta et al., 2006). Acetyl-CoA can be converted by acetyl-CoA carboxylase (ACC) to malonyl-CoA, which serves as the building block in synthesizing fatty acids and polyketides. This parasite possesses only one cytosolic ACC, lacking the plastid ortholog found in Toxoplasma and Plasmodium (Jelenska et al., 2001, 2002; Gornicki, 2003). At least two organic end products can be formed from acetyl-CoA, including acetate by an acetate-CoA ligase (AceCL, also referred to as acetyl-CoA synthetase), in which an extra ATP molecule can be generated from AMP and PPi, and ethanol, by a bifunctional type E alcohol dehydrogenase (adhE) that first makes aldehyde and then ethanol. Ethanol may also be produced from pyruvate by pyruvate decarboxylase (PDC) coupled with a monofunctional ADH1. Pyruvate may also be converted to lactate by lactate dehydrogenase (LDH).
Biochemical Pathways for the Biofuel Production
Published in Debabrata Das, Jhansi L. Varanasi, Fundamentals of Biofuel Production Processes, 2019
Debabrata Das, Jhansi L. Varanasi
TAGs are the predominant component of the oils extracted from the seeds or fruits of oleaginous plants like sunflower, oilseed rape, maize, and soybean. TAGs of higher plants contain acyl groups of palmitic acid (16:0), stearic acid (18:0), oleic acid (18:1), linolenic acid (18:2), and α-linolenic acid (18:3). In these organisms, the fatty acid biosynthesis occurs in the stroma of plastids. The subsequent incorporation of fatty acids into the glycerol backbone leads to the biosynthesis of TAGs in the endoplasmic reticulum (ER) (Figure 4.1a). During fatty acid synthesis, the acetyl-CoA carboxylases (ACC) catalyze the irreversible carboxylation of acetyl-CoA to form malonyl-CoA. The malonyl group is then transferred to the acyl carrier protein (ACP), giving rise to malonyl-ACP, which is the primary substrate for fatty acid synthase complex. Repeated condensation of malonyl-ACP by the action of fatty acid synthase complex with the subsequent addition of two carbon units after each elongation leads to the formation of fatty acid-ACP complexes (like 16:0 ACP, 18:0 ACP, and 18:1 ACP). The type of acyl chain bound to ACP depends upon the type of enzyme present (mostly ketoacyl synthases I, II, and III) and varies in different plant species. The termination of fatty acid elongation is catalyzed by ACP thioesterases, which catalysis the hydrolysis of acyl-ACP to produce free fatty acids that are able to cross the plastidial envelope. These free fatty acids are then reactivated as acyl-CoAs in the cytosol and form the acyl-CoA pool, which provides the acyl donor for the acyl transferase reactions of the TAG assembly in the ER.
Oil from Algae
Published in Prasenjit Mondal, Ajay K. Dalai, Sustainable Utilization of Natural Resources, 2017
Prasenjit Mondal, Preety Kumari, Jyoti Singh, Shobhit Verma, Amit Kumar Chaurasia, Rajesh P. Singh
In algae, the lipids are basically classified into polar lipids, that is, phospholipids, and neutral lipids, that is, triglycerides. Polar lipids are used as structural component whereas the triglycerides are the main material in the production of biodiesel. The synthesis routes of triglycerides in algae mainly consist of the following steps: (i) the formation of acetyl coenzyme A (acetyl-CoA) in the cytoplasm; (ii) the elongation and de-saturation of carbon chain of fatty acids; and (c) the biosynthesis of triglycerides in the microalgal biological system (Huang et al. 2010). A simplified overview of lipid biosynthetic pathway is shown in Figure 8.2. Acetyl-CoA provided by photosynthetic reactions acts as a precursor for fatty acid synthesis in the chloroplast. Acetyl-CoA carboxylase (ACCase) catalyzes the first committed step in fatty acid biosynthesis by converting acetyl-CoA to malonyl-CoA. Two types of ACCase are identified in algae, the heteromeric and homomeric. The heteromeric form is present in plastids whereas the homomeric form in cytosol. Further, the malonyl-CoA is transferred to the acyl carrier protein (ACP) by malonyl-CoA:ACP transacetylase, which then introduced in fatty acid synthesis cycle during the 3-ketoacyl-ACP synthase (KAS). The formation of 16- or 18-carbon chain of fatty acids is principally dependent on the reaction of two enzyme systems together with acetyl-CoA carboxylic enzyme along with fatty acid synthase in many algae. Fatty acid elongation occurs in ER and their synthesis requires specific classes of desaturases and enolases enzymes. The desaturates enzymes introduce double bonds at specific carbon atoms in the fatty acid chain in addition to the fatty acid elongation system that elongates the precursors in two-carbon increments (Gong and Jiang 2011). TAG biosynthesis and assembly is a complex process in algae can occur via the direct glycerol pathway, that is, Kennedy pathway where the acyltransferases residing in the ER catalyze sequential transport of the acyl group from the acetyl-CoA to glycerol 3-phosphate backbone and determine the final content of TAG as shown in Figure 8.2. TAG is then deposited into the cytosol as ER-derived lipid droplets (De Bhowmick et al. 2015).
Mechanisms of beneficial effects of exercise training on non-alcoholic fatty liver disease (NAFLD): Roles of oxidative stress and inflammation
Published in European Journal of Sport Science, 2019
Parvin Farzanegi, Amir Dana, Zeynab Ebrahimpoor, Mahdieh Asadi, Mohammad Ali Azarbayjani
Numerous studies showed that exercise training significantly decreases intrahepatic fat contents. The underlying mechanism involves β-oxidation and lipogenesis (Figure 1). Recent investigations have indicated that exercise activity can regulate hepatic lipid metabolism by regulating hepatic β-oxidation and lipogenesis (Jeppesen & Kiens, 2012). Rector et al. (2008) indicated that exercise training increases hepatic fatty acid oxidation in NAFLD rats by increasing acetyl-coenzyme A carboxylase (ACC) phosphorylation and cytochrome c contents, indicating a possible enhancement of the final steps of oxidative phosphorylation. They also showed that exercise training decreases hepatic lipogenesis by reducing fatty acid synthase (FAS) and ACC contents (Rector et al., 2008). ACC is the committed step in fatty acid synthesis, catalysing the carboxylation of acetyl-CoA to form malonyl-CoA. ACC phosphorylation inhibits its activity and reduces formation of malonyl-CoA. Reduced content of malonyl-CoA is associated with increase in fatty acid oxidation and decrease in available substrate for FAS. Koves et al. (2005) found that exercise training increased the flux of both β-oxidation and tricarboxylic acid (TCA) cycle in the skeletal muscle. These data suggest that exercise training can enhance the complete oxidation of lipids in both the liver and the skeletal muscle.