Compared with fatty acid metabolism, which takes place in the mitochondrial matrix, fatty acid synthesis takes place in the cytosol (fatty-acid == lipid). Fatty acids synthesis also yses the acyl carrier protein (ACP), instead of coenzyme A used in fatty acid breakdown. In humans, the enzymes of fatty acid synthesis are joined in a single polypeptide chain called fatty acid synthase (one large protein), while the degradative enzymes are apparently not associated, with the exception of the trifunctional protein. The reductant in fatty acid synthesis is NADPH, whereas the oxidants in fatty acid degradation are NAD+ and FAD. Fatty acid synthesis uses malonyl-CoA, which is synthesized from acetyl-CoA by acetyl-CoA carboxylase, the mechanism of which is similar to pyruvate carboxylase. This reaction is both the committed step and rate limiting step in fatty acid synthesis. It also consumes one ATP. In both cases, the addition of CO2 activates the metabolite for the subsequent reaction.
Each round of 2-carbon addition consumes one malonyl-CoA, two NADPH’s, and releases one CO2. Decarboxylation of the malonyl group drives the first step, a condensation reaction. NADPH is oxidized to NADP+ in both reduction steps, and water is generated in the dehyration step, naturally. Malonyl-CoA acts as a 2-carbon donor.
Fatty acid synthase is one large polypeptide with multiple activities. It’s really big (dimer of identifal 2,500-residue monomers), and has 5 different “activities”, each of which is used during two-carbon addition cycle. We need a swining arm, the ACP, to visit each activity site.
ACP is attached to KR. Acetyl-CoA goes first to ACP and then to KS, which catalyzes the condensation reaction. Next, Kr reduces carbonyl to a hydroxy group. Then, DH removes water. Then, ER reduces the double bond. Finally, the butyryl group is translocated to Cys on KS. To being the next round, MAT catalyzes the reacharging of teh ACP with anther malonyl group. This video (may) be helpful.
The NADPH needed for fatty acid comes from 2 sources. The pentose phosphate pathway generates NADPH, and the conversion of malate to pyruvate by malic enzyme also generates NADPH. The acetyl-CoA needed for FA synthesis in the cytosol is transported out of the matrix via citrate, since there is no acetyl-CoA transporter on the inner mitochondrial membrane. The digram below shows this process.
This method also provides the additional benefit of generating NADPH in the cytosol with malic enzyme. As it turns out, citrate plays an important regulatory role in both fatty acid synthesis and glycolysis. Citrate can inhibit PFK-1, which inhibits glycolysis. This should make sense as an excess of citrate means glycolysis should slow down. Citrate also activates acetyl-CoA carboxylase, which is however inhibited by glucagon and epinephrine, as well as palmitoyl-CoA. This makes sense since we don’t want to make new fatty acids in the fasting state or when there’s an exces of downstream product. Malonyl-CoA also inhibits β-oxidation, which makes sense because we don’t want to run FA synthesis and metabolism at the same time.
Let’s consider how fatty acid synthesis and breakdown is regulated under high blood sugar and low blood sugar conditions.
Under high blood sugar conditions, we would observe an increase in insulin levels. Insulin would activate phosphatase, which dephosphorylates acetyl-CoA carboxylase (ACC), activating it. Activated ACC would convert acetyl-CoA into malonyl-CoA. Malonyl-CoA would inhibit fatty acid breakdown by inhibiting the carnitine shuttle. Malonyl-CoA would also be used for fatty acid synthesis. Under low blood sugar conditions, we would observe an increase in glucagon, which activates PKA. PKA phosphorylates ACC, deactivating it. This prevents acetyl-CoA being converted to malonyl-CoA. Without malonyl-CoA, the carnitine shuttle is uninhibited and FA breakdown runs.
To synthesis triacylglycerols, we need glycerol 3-phosphate. Glycerol 3-phosphate can by synthesized from glucose or glycerol. Synthesis from glycerol simply involves phosphorylation with ATP and is catalyzed by glycerol kinase. Synthesis from glucose involves dihydroxyacteone phosphate (DHAP) generated from glycolysis. DHAP can be reduced by NADH + H+, generating glycerol 3-phosphate and NAD+. This reaction is catalyzed by glycerol 3-phosphate dehydrogenase. Acyl transferase then adds fatty acids onto glycerl 3-phosphate.
Lipids are transported in the bloodstream differently. Fatty acids mobilized from adipocytes are transported bound to surm albumin, but triacylglycerols, cholesterol, and cholesteryl esters are transported in lipoproteins. Lipoproteins are globular groups of lipids surrounded by a phospholipid monolayer.