*4.2.5.1 Fatty acid synthesis*

FAS is chiefly mediated by the enzyme acetyl-CoA carboxylase 1 (ACC1), catalyzing the rate-limiting step in FA biosynthesis, i.e., the carboxylation reaction of acetyl-CoA to malonyl-CoA in the cellular cytoplasm. Acetyl-CoA needed for *de novo* FAS is made available in the cytoplasm through the citrate-pyruvate shuttle that first transfers pyruvate (final glycolytic product) from the cytoplasm into mitochondria to form acetyl-CoA and then transfers a substantial portion of citrate (one of the intermediates generated in the Kreb's cycle by the combination of acetyl-CoA with oxaloacetate) from the mitochondria into the cytosol. In the cytoplasm, citrate lyase generates cytosolic acetyl-CoA from citrate that subsequently powers downstream FA, cholesterol, and lipid biosynthesis [45, 46]. Another important enzyme participating in the initial steps of SFA/MUFA/PUFA generation is fatty acid synthase (FASN), a multienzyme complex that operates downstream of ACC1 and mediates the conversion of acetyl-CoA and malonyl-CoA to saturated long-chain FAs [47].

CD5 antigen-like (CD5L) protein, a member of the scavenger receptor cysteine-rich superfamily inhibits the *de novo* synthesis of SFA through direct binding to FASN, thus helping maintain the intracellular lipidome saturation by modulating PUFA versus SFA levels. CD5L is more than a general inhibitor as it regulates the quantity as well as the quality of fatty acids being generated inside the cell, fine-tuning the FA composition in T cells, causing elevation of PUFA and alterations in specific lipid species, including cholesterol metabolites, guiding post-translational modifications [13].

Acetyl-CoA is a central intermediate in lipid metabolism. In addition to FAS, cytosolic acetyl-CoA can be catalyzed in the mevalonate-cholesterol synthetic pathway, generating cholesterol and its derivatives (desmosterol, 4α-carboxy, 4β-methylzymosterol, oxysterols *viz.* 7β, 27-dihydroxy-cholesterol <7β, 27–OHC > and 7α, 27– OHC, 20α-OHC, 22R-OHC, 25-OHC) [15, 48]. All these lipid biosynthetic processes are delicately regulated by coordinated actions of sterol response element binding proteins (SREBPs), the transcription factors which activate all genes necessary for lipid synthesis with SREPB1 inducing genes involved in *de novo* lipogenesis whereas SREBP2 activating genes necessary for cholesterol synthesis and uptake [49].

#### *4.2.5.2 Fatty acid oxidation*

Long-chain free FAs enter the metabolizing cells *via* specific transport proteins (SLC27) where they are acted upon by long-chain fatty acid-CoA ligase resulting in a fatty acyl-adenylate, which then reacts with free coenzyme A (CoA) in the presence of acyl-CoA synthetase (ACS) to give an acyl-CoA molecule. Acyl-CoA enters the mitochondria through carnitine transporter, which itself is directed by 3 enzymes including carnitine palmitoyl transferase I (CPT I) located on the cytosolic faces of the outer and inner mitochondrial membranes, and carnitine-acylcarnitine translocase (CAT) and carnitine palmitoyl transferase II (CPT II) located on the interior face of the inner mitochondrial membrane. In the mitochondrial matrix, beta-oxidation cuts the long-chain FAs (now in the form of acyl-CoA molecules) into a series of two-carbon acetate units, which, combined with CoA, form acetyl CoA. This is how acetyl-CoA is added to the cycle, which will be dissipated as carbon dioxide and water, releasing a substantial quantity of energy - captured in the form of ATP—with each beta oxidative cut of the acyl-CoA molecule yielding 5 ATP molecules. It has been calculated that complete β-oxidation of a single palmitate molecule can potentially yield over 100 ATP molecules.

Effector and regulatory/tolerogenic immune cells employ different metabolic modules to fulfill their energy requirements. Activated immune cells and effector immune cell subsets including Th1, Th2, Th17 cells and M1 macrophages upregulate glucose and AA transporters to increase their uptake and rely on aerobic glycolysis, glutaminolysis, PPP and FAS to support pro-inflammatory cytokine secretion while regulatory cells including T regs, memory T cells and M2 macrophages predominantly utilize FAO and OXPHOS to meet their ATP requirements [43].
