CoA-independent pathway

Synthesis of 3-HP From Glycerol Through the CoA-Independent Pathway... 

FIGURE 14.3 Biochemical reactions showing production of 3-HP from glycerol by CoA-dependent and CoA-independent pathways (Xue et al., 2008 Ashok et al., 2011 Luo et al.,... 

It was reported that both glycerol dehydratase dhaB)-mediated CoA-dependent and CoA-independent pathways require an external supply of Vitamin B12 (or) Coenzyme Bi2, as the majority of microorganisms, including K. pneumoniae, are not able to synthesize... 

Fig. 9. Biosynthesis of platelet-activating factor (PAF) via the remodeling pathway. Lyso-PAF, the immediate precursor of PAF, can be formed from l-alkyl-2-acyl-sn-glycero-3-phosphocholine through the direct action of (I) PLAj or (II) CoA-independent transacylase. The lysoplasmenylethanolamine (or other potential ethanolamine- and choline-containing lysoglycerophospholipids) is thought to be generated by (III) a PLAj that exhibits a high degree of selectivity for substrates with an atachidonoyl moiety at the sn-2 position. The transacylase (II) appears to possess both acyl transfer and PLAj hydrolytic activities. Lyso-PAF produced by either the transacylation (II) or direct PLAj (I) action can be acetylated to form PAF by (IV) an acetyl-CoA acetyltransferase.

Processes affecting the carbon-isotopic compositions of isoprenoid lipids. The isoprene carbon skeleton is indicated schematically in Figure 27. The corresponding biosynthetic reactant—equivalent in its role to acetyl-CoA—is isopentenyl pyrophosphate. As shown in Figure 29, this compound can be made by two different and fully independent pathways. The mevalonic-acid pathway was until recently thought to be the only route to isoprenoids. The deoxyxylulose-phosphate, or methylerythritol-phosphate, pathway was first discovered in Bacteria by Rohmer and coworkers (Flesch and Rohmer... 

The accumulation of storage lipids in plants includes de novo biosynthesis of FA in the plastid stroma with their subsequent involvement in the glycerolipid metabolism resulting in TAG formation in the ER [68], The two pathways of TAG biosynthesis were called as acyl-CoA-dependent and acyl-CoA-independent. 

Fig. 117.9 An overview of the cytosolic mevalonate pathway and plastidial mevalonate-independent pathway for the biosynthesis of terpenoids (isoprenoids) in plant cells [99] (DMAPP, dimethylallyl diphosphate DXP, l-deoxy-D-xylulose-5-phosphate DXS, DXP synthase DXR, DXP reductoisomerase MEP, 2-C-methyl-D-erythritol 4-phosphate GASP, gfyc-eraldehyde-3-phosphate HMG-CoA, 3-hydroxy-3-methylglutaryl-CoA HMGR, HMGR-CoA reductase IPP, isopentenyl diphosphate. Mevinolin and fosmidomycin are inhibitors of HMGR and DXR, respectively)...

The decomposition reaction proceeds in nature by two biosynthetic routes the mevalonate pathway, via mevalonic acid (MVA) (Scheme 102.2), and the recently discovered mevalonate-independent pathway, via deoxyxylulose phosphate (DXP) (Scheme 102.3), the latter taking place in plants. In Scheme 102.2, a schematic representation of the mevalonate pathway is shown in which three molecules of acetyl-coenzyme A derived by carbohydrate, fat, or protein catabolism yield by aldol-type reaction p-hydroxy-(3-methylglutaryl-coenzyme A (HMG-CoA), which is irreversibly reduced to -mevalonic acid (MVA), whose phosphorylation and elimination reaction afford IPP. This molecule is converted by an isomerase into an equilibrium mixture with DMAPP. 

In the reductive pathway, 3-HPA, the main intermediate of 3-HP production, can be produced from glycerol by introduction of CoA-independent dha and CoA-dependent pdu regulons. 

In CoA-dependent and CoA-independent mechanisms, the major rate-limiting step is the external supply of coenzyme B12 to the culture medium, which increases the cost of 3-HP for the industrial level production. Moreover, the bacterial cells are sensitive to low pH, which is a key rate-limiting step in 3-HP production. As the concentration of 3-HP increases in the culture medium there is a constant decline in pH. As a result, it requires a large volume of base titrant to maintain a constant pH of the culture medium (Van Maris et al., 2004). S. cerevisiae acts as an attractive alternate for low pH tolerance when compared to a bacterial cell. The malonyl-CoA pathway has attracted attention for the production of 3-HP and serves as an alternative for the coenzyme Bi2-dependent glycerol pathway (Lynch et al., 2011 Rathnasingh et al., 2012). 

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