Lipid Metabolism
Prof. Dr. Zeliha Büyükbingöl
Synhesis of Fatty Acids (FAs) and Triacylglycerols (TAGs
)
Fatty acids (nonessential) and TAG are also synhesized in the body.
FA and TAG synhesis occurs in the cytoplasm, but its precursor, acetylCoA, is formed in the mitochondrial matrix.
Excess dietary carbohydrate is the major carbon source for FA synthesis.
FAs are synthesized by the repetitive condensation of two- carbon units derived from malonyl CoA.
Pathway Reaction Steps in Fatty Acid Synthesis – From Acetyl CoA to Palmitate
1.Four reactions shuttle Acetyl-CoA from mitochondria to cytoplasm 2.Fatty acid polimerisation starts with condensation of acetyl and malonyl groups 3. Fatty acid synthase (FAS) carries out the chain elongation steps of fatty acid biosynthesis
4.Acyl carrier protein (ACP), a region of the FAS protein.
Acetyl CoA Shuttle
Citrate synthase
Cytrate lyase
Malate dehydrogenase
Malic enzyme
Fatty Acid Polimerization
Acetyl CoA- Acyl Carrier Protein (ACP) Transacylase
Acetyl CoA carboxylase (irreversible step in FAS, is controlled by two mechanisms)
Malonyl CoA-ACP Transacylase
3-Ketoacyl synthase
Beta-carbonyl reduction
3-Ketoacyl reductase
Dehydratase
Enoyl reductase
Elongation Cycle
Thioesterase
Elongation and Desaturation of Fatty Acids
Elongation of FAs
After synhesis on the FAS
complex, palmitate is activated (palmitoyl -CoA) and other
activated long-chain FAs, can be elongated, two carbons at a
time, by a series of reactions that occur in the endoplasmic reticulum. Malonyl CoA serves as the donor of the two carbon
units.
e.g. Palmitoyl-CoA---Stearoyl-CoA
Desaturation of FAs
Desaturation of FAs involves a process that require molecular oxygen, NADH and cytochrome b5. Human desaturases cannot introduce doble bonds between C9 and methyl end.
Synthesis of TAGs (Lipogenesis)
TAGs are synthesized fromphosphatidic acd by dephosphorylation to 1-2- diglyceride.
In the fed state, FAs synthesized in the liver or released from CMs and VLDL by capillary lipase, are used to synthesize TAG in liver and adipose tissue. Glycerol 3-phosphate is produced from glucose in both
tissues. It is also produced from glycerol in liver but not in adipose tissue, which lacks glycerol kinase.
Synthesis of Phospholipids
The biosynthesis phospholipids depends on the syntesis of diacylglycerol. Diacylglycerol in liver
comes from phosphatidic acid. Phosphatidic acid can be derived from glycerol.
Sphingolipids constitute a class of lipids defined by
their eighteen carbon amino-alcohol backbones which are synthesized in the ER from nonsphingolipid
precursors.
TAG Mobilization (Lipolysis) and FA Oxidation
FAs are mobilized in the fasting state by activating hormone-sensitive lipase.
Long chain FAs are shuttled into the
mitochondrial matrix by formation of acyl- carnitine esters.
Beta-oxidation FAs consist of repeating sequence of four enzymes to produce acetyl-CoA.
Glycerol and Free Fatty Acids
Glycerol released during lipolizis is transported to the liver.
Free FAs released from adipose tissue are carried in the bloodstream and are delivered to all tissues ( liver, skeletal muscle, heart, kidney) except brain and erytrocytes.
In the cell cytoplasm, they are acetylated by fatty acyl CoA synthetase.
Carnitine transports acetylated FAs across mitochondrial membrane.
Hormone –sensitive lipase is the only point in FA oxidation.
Fatty acids as Fuels
Energy Content of Nutrients : - 1 gram of carbohydrates = 4 kilocalories
1 gram of protein = 4 kilocalories 1 gram of fat = 9 kilocalories
In addition to carbohydrates, protein and fat, alcohol can also provide energy (1 gram alcohol = 7 kilocalories)
FAs are a major fuel for humans and supply our energy needs between meals and during periods of increased demand, such as exercise. During overnight fasting, fatty acids become the major fuel for cardiac muscle, skeletal muscle and liver.
ATPis generated from oxidation of FAs in the pathway of beta-oxidation. Between meals and during overnight fasting long chain FAs are released from adipose tissue TAGs. They circulate through blood bound to albumin. In cells they are converted to fatty acylCoA by AcylCoA sythetases. The activated acyl grup is transported into the mitochondria bound to carnitine. In beta-oxidation, fatty acyl group is sequentially oxidized to yield FADH2, NADH and acyl CoA.Water soluble medium chain long FA
metabolism does not require carnitine and only in the liver. Odd chain long FA s undergo the beta- oxidation terminal three-C propionyl CoA , which entersthe TCA as succinyl CoA. FAs that do not readily undergo mitochondrial beta -oxidation are oxidize first by alternative routes that convert themto amore soluble substrates or tourinary excretion products. Excess FA s may undergo microsomal omega-
oxidation which converts them to dicarboxylic acids that occurs in urine. Very long chain FA s are
whittled down to size in peroxizomes. Peroxisomal alpha and beta oxidation generates H2O2 NADH, acyl CoA or propionyl CoA and a short tomedium chain length acyl CoA.
In the liver, much of acylCoA is converted to keton bodies which enter the blood. In other tissues, keton bodies are converted to acetyl CoA--- TCA cycle. The liver sythesizes keton bodies but not use them as fuel.
β-oxidation of Fatty Acids
Release of the energy of fatty acids is accomplished primarily by
chopping them into two-carbon units of Acetyl-CoA, which enters
the TCA cycle.
Fatty acid oxidation: Pathway Reaction Steps
Lipolysis : Free fatty acid and glycerol release
Free fatty acid transport to tissues
Fatty acyl CoA formation in cytosol
Carnitine shuttle
Oxidation system, consist of four enzymes that act sequentially to yield a fatty acyl CoAthat is two carbons shorter than the original fatty acid
Repetition of these four reactions
Acetyl CoA enters the TCA
Regulation: Hormone-sensitive lipase
Energy yield of beta-oxidation of palmitic acid
Palmitoyl-CoA + 7 FAD + 7 NAD++ 7 CoA + 7 H2O →
8 Acetyl-CoA + 7 FADH2+ 7 NADH + 7 H+
The net yield from the complete oxidation of a mole of palmitate is 131 – 2 = 129 ATP molecules.
The efficiency of energy conservation in fatty acid oxidation can be estimated from thenumber of ATP ormed and from the free energy of oxidation of palmitic acid to CO2and H2O,as determined by calorietry. The standard free energy of
hydrolysis of 129 ATP is 129 ×– 7.3kcal = – 941.7 kcal or roughly – 942 kcal.
