Lipid metabolism

Pavla Balínová

LipidsLipids dissolve well in organic solvents but they are insoluble in water.

Biological roles of lipids:● lipids are important source of energy – they serve as metabolic fuel● amphipathic lipids are building blocks of cellular membranes● some of them are substrates for synthesis of other compounds (eicosanoids, bile acids)● lipids are excellent insulators

Classification of lipids

I. Simple lipids● Triacylglycerols TAG (fats)● Waxes

II. Complex lipids● Phospholipids● Sphingophospholipids● Glycolipids

III. Isoprenoids and steroidsIsoprenoids: vitamins A, D, E, KSteroids: sterols, bile acids, steroid hormones

Figure is found on http://en.wikipedia.org/wiki/Triacylglycerol

The figure was adopted from: J.Koolman, K.H.Röhm / Color Atlas of Biochemistry, 2nd edition

Fatty acids (FA)

Degradation of fats in adipose tissue

Adipose tissue (fat cells) = fat storageDegradation of TAG in adipose tissue (lipolysis) is catalyzed by hormone sensitive lipase (HSL). This enzyme is activated by epinephrine and glucagon and inhibited by insulin.

Figure is found on http://web.indstate.edu/thcme/mwking/fatty-acid-oxidation.html

Utilization of FA within the cell

Tissues take up FA from the blood to rebuild fats or to obtain energy from their oxidation.Metabolism of FA is especially intensive in the liver.„Free“ fatty acids (FFA) are transferred with albumin in the blood.

FA in blood → enter to the cell → in the cytoplasm FA are converted to their CoA derivatives by enzyme acyl-CoA-synthetase (ATP is consumed) → acyl-CoAs

Fatty acid + ATP + CoA ---> Acyl-CoA + PPi + AMP

Transfer of acyl-CoAs from cytoplasm to the mit. matrix is performed by a carnitine transporter

Figure is found on http://web.indstate.edu/thcme/mwking/fatty-acid-oxidation.html

β-oxidation of fatty acids

• substrate: acyl-CoA• product: n acetyl-CoA, n NADH + H+, n

FADH2

• function: gain of energy from fatty acids• subcelullar location: matrix of mitochondria• organ location: liver, skeletal muscles and

other tissues with expection to CNS • regulatory enzyme: carnitine

acyltransferase I

SummaryFor complete degradation of palmitic acid 7 cycles are required.Degradation of palmitic acid (16 C) gives 106 ATP in total.

Regulation of β-oxidation of FA:Regulatory enzyme is carnitine acyltransferase I – it is inhibited by malonyl-CoA (intermediate of FA synthesis).

Synthesis of ketone bodies (ketogenesis)

• substrate: acetyl-CoA• product: acetoacetate, 3-hydroxybutyrate, acetone• function: energy substrate for extrahepatal tissues• subcelullar location: matrix of mitochondria• organ location: liver

Excessive production of ketone bodies is typical during starvation or diabetes mellitus:↑ lipolysis → ↑ FA → β-oxidation of FA → excess of acetyl-CoA → ↑ ketogenesis

Synthesis of ketone bodies (ketogenesis)

Figure is found at http://themedicalbiochemistrypage.org/fatty-acid-oxidation.html#ketogenesis

Use of ketone bodies by the extrahepatal tissues

• acetoacetate and 3-hydroxybutyrate are reconverted to acetyl-CoA (→ citric acid cycle)

• is located in matrix of mitochondria of the peripheral tissues

• is significant in skeletal muscles, heart and also in the brain if lack of Glc occurs

Use of ketone bodies by the extrahepatal tissues

Figure is found at http://themedicalbiochemistrypage.org/fatty-acid-oxidation.html#ketogenesis

Liver lacks this enzyme therefore is unable to use ketone bodies as fuel

Fatty acid synthesis

•substrate: acetyl-CoA, NADPH + H+

•product: palmitate (= endproduct of FA synthesis)•function: de novo synthesis of FA which are stored as TAG

•subcelullar location: cytosol•organ location: mainly liver and adipose tissue and also other tissues

•regulatory enzyme: acetyl-CoA carboxylase

The committed step in FA synthesis

Formation of malonyl-CoA from acetyl-CoA and HCO3- is

catalyzed by enzyme acetyl-CoA carboxylase (a key regulatory enzyme). Citrate is an allosteric stimulator and palmitoyl-CoA inhibits this enzyme.Hormonal regulation: glucagon and epinephrine - inhibition insulin - stimulation

The growing fatty acids are linked to a phosphopantetheine group of an acyl carrier protein (ACP) of FA synthase.Acetyl-CoA is carboxylated by HCO3

- to yield malonyl-CoA → condensation between the acetyl-ACP and the malonyl-ACP → acetoacetyl-ACP is formed.

FA synthesis is performed through a cycle of four

reactions:

Figure is found on http://138.192.68.68/bio/Courses/biochem2/FattyAcid/FASynthesis.html

Biosynthesis of TAG

Biosynthesis of TAG occurs in cytoplasm and ER of liver and fat cells but also in other tissues.

Figure is found on http://web.indstate.edu/thcme/mwking/lipid-synthesis.html#phospholipids

Complex lipids(phospholipids, sphingophospholipids,

glycolipids)Phospholipids = glycerol + 2 FA + phosphate group + hydrophilic compound

Phosphatidylethanolamine (cephalin)PhophatidylinositolPhosphatidylcholine (lecithin)

choline

Sphingophospholipids= sphingosine + FA + phosphate residue + amino alcohol or sugar alcoholCeramide = sphingosine + fatty acidSphingomyelin

acyl residue

Glycolipids= sphingosine + FA + sugar or oligosaccharide residueThe phosphate group is absent.Galactocerebroside

Degradation of phospholipids

Phospholipases are divided according to the type of the bond which is cleaved.

Figure is found on http://web.indstate.edu/thcme/mwking/lipid-synthesis.html#phospholipids

Synthesis of cholesterol

• substrate: acetyl-CoA• product: cholesterol• function: de novo synthesis of endogenous

cholesterol• subcelullar location: cytosol and endoplasmic

reticulum• organ location: liver, intestine, adrenal cortex,

ovaries, testes and placenta make the largest contributions to the body´s cholesterol pool

Cholesterol is a constituent of cellular membranes and it is present in all animal tissues.

Biosynthesis of isoprenoids and steroids

Figure is found on http://web.indstate.edu/thcme/mwking/cholesterol.html

Regulation of cholesterol synthesis

• reduction of HMG-CoA to mevalonic acid is catalyzed by HMG-CoA reductase – rate-limiting and key regulatory step

• expression of HMG-CoA reductase gene is inhibited by cholesterol

● activity of HMG-CoA reductase is increased by insulin and thyroxine but glucagone has the opposite effect

Figure was adopted from textbook T. M. Devlin et al.: Textbook of Biochemistry With Clinical Correlations, 4th ed.

Lipoproteins

● Chylomicrons carry TAG (fat) from the intestine to the liver and adipose tissue● VLDL carry (newly synthesized) TAG from the liver to peripheral tissues● IDL are intermediate between VLDL and LDL● LDL carry cholesterol from the liver to cells of the body● HDL collects cholesterol from body´s tissues and brings it back to the liver

Figure was assumed from http://http://www.britannica.com/EBchecked/topic-art/720793/92254/Cutaway-view-of-a-low-density-lipoprotein-complex-The-LDL

Metabolism of lipoproteins

Figure was assumed from www.media-2.web.britannica.com/eb-media/42/96842-...

Enzyme lipoprotein lipase (LPL)

is present on the surface of the vascular endothelia. This enzyme hydrolyses TAG in chylomicrons → chylomicrons remnants → liverLPL also catalyzes cleavage of TAG in VLDL → IDL.LPL synthesis is stimulated by insulin.

Enzyme lecithin cholesterol acyltransferase (LCAT) catalyzes esterification of cholesterol with acyl. It is included in HDL formation.

Figure was adopted from textbook T. M. Devlin et al.: Textbook of Biochemistry With Clinical Correlations, 4th ed.