Degradation of lipids, ketone bodies Josef Fontana EC - 56
Overview of the lecture • Energetic importance of TAG • Pathways of lipid metabolism – Li...
Degradation of lipids, ketone bodies Josef Fontana EC - 56
Overview of the lecture • Energetic importance of TAG • Pathways of lipid metabolism – Lipids as a source of energy - TAG degradation in cells, β-oxidation – Synthesis and utilization of ketone bodies
Energetic importance of TAG
Triacylglycerols (TAG) • TAG store a large amount of chemical energy • TAG are excelent for energy storage - 1g of fat has 6 times more energy than 1g of hydrated glycogen • Complete oxidation of 1g of FA = 38 kJ • Complete oxidation of 1g of saccharides or proteins only 17 kJ
Triacylglycerols (TAG) • 70 kg man has an energy reserve of 420 000 kJ in TAG, 10 000 kJ in proteins (muscle), 2 500 kJ in glycogen and 170 kJ in glucose. (Total weight of TAG is around 11 kg) • Glycogen and glucose are sufficient to supply the body one day, TAG many weeks • The main site of TAG accumulation is adipocyte cytoplasm
Pathways of lipid metabolism Lipids as a source of energy TAG degradation in cells, βoxidation
Lipids as an energy source • The use of lipids has three stages: • 1) Mobilization of lipids - TAG hydrolysis to fatty acids and glycerol, transport in blood • 2) Activation of fatty acids in cytosol and their transport to the matrix of mitochondria • 3) β-oxidation: FA degradation to acetyl~CoA – enters the Krebs cycle
Mobilization of lipids • Hormone sensitive lipase • TAG → 3 FFA + glycerol • FFA are bound to plasma albumin • Glycerol is used in liver • Inhibition by insulin • Activation by epinephrine and glucagon
Glycerol is converted to intermediates of glycolysis CH2OH HO
C
H
CH2OH
Glycerolkinasa ATP
HO
C
H
L-Glycerol-3-fosfát
+
NAD
C
G
H 2-
CH2O PO3
L-Glycerol-3-fosfát
Glycerol-3-fosfátdehydrogenasa
2CH2O PO3
HO
ADP
Glycerol
CH2OH
CH2OH
+
NADH + H
CH2OH O
C 2-
CH2O PO3
Dihydroxyacetonfosfát
FA transport to the cell • SCFA (↓12C) – simple diffusion • FA with longer chain – different transport systems in the membrane - facilitated diffusion: – FATP (fatty acid transport protein) – FAT/CD36 (fatty acid translocase)
Activation of FA • Cytosol • Beginning of the metabolism • Constant concentration gradient of FA • Ester bond: FA + HSCoA: AcylCoAsynthetase • MK + ATP + HS-CoA → acyl-CoA + AMP + 2 Pi
FA oxidation • β-oxidation in the mitochondrial matrix has the major role • ω- a α- oxidation – ER membranes • Greek letters determine the carbon atom which the reactions
Entry of FA in the matrix • Acyl-CoA can go through the outer mitochondrial membrane but not through the inner • FA leaves CoA – carnitine • Carnitine acyltransferases (CAT) – transfer of FA between CoA and carnitine
Carnitine
Carnitine esterified with FA
Acylcarnitine synthesis - CAT I • Cytosolic side of the outer mitochondrial membrane • Transfer of acyl from HSCoA to carnitine
Carnitine acyltransferases II • Mitochondrial matrix • Transfer of FA from acylcarnitine to HSCoA • Free carnitine leaves the matrix by translocase – exchange with a new acylcarnitine • Acyl-CoA in the mitochondrial matrix - βoxidation
β-oxidation • Acyl~CoA dehydrogenase (prosthetic group is FAD) • Enoyl~CoA hydratase • L-3-Hydroxyacyl~CoA dehydrogenase (coenzyme is NAD+) • β-Ketothiolase
Acyl-CoA dehydrogenase R
C H2
H2 C
O C H2
C
S
CoA
• FAD → FADH2
Acyl-CoA FAD OXIDACE FADH2
R
C H2
H C
O C H
C
• New double bond between 2 (α) and 3 (β) carbon
S
Trans-∆ 2-Enoyl-CoA
CoA
• There are specific AcylCoA-dehydrogenases for FA with: – short (4-6 C) – medium (6-10 C) – long (12-18 C) chain
Enoyl-CoA hydratase • Addidion of water
R
• New OH- group
C H2
H C
O C
C H
S
CoA
Trans-∆ 2-Enoyl-CoA
H2O
HO R
C H2
HYDRATACE
O
H C
C
C H
S
H
L-3-Hydroxyacyl-CoA
CoA
Hydroxyacyl-CoA dehydrogenase HO
• Oxidation of OHgroup to keto group • NAD+ → NADH + H+
R
C H2
O
H C
C
C
S
CoA
H H L-3-Hydroxyacyl-CoA NAD+ OXIDACE H+ + NADH
R
C H2
O
O
C
C
C H
S
H
3-Ketoacyl-CoA
CoA
β-Ketothiolase R
C H2
O
O
C
C
C
H H 3-Ketoacyl-CoA
CoA
HS
S
C H2
C
• SH group of HSCoA attacks βketo carbon
THIOLÝZA
O
O R
• Catalyzes thiolytic cleavage
CoA
S
CoA
+
Acyl-CoA (zkrácený o 2 uhlíkové atomy)
H
C
C H
S
H Acetyl-CoA
CoA
• AcCoA + Acyl~CoA (2C shorter)
One rotation of β-oxidation • β-oxidation is a cyclic process: • acyl-CoA + FAD + NAD+ + HS-CoA → acyl-CoA (2 C shorter) + FADH2 + NADH + H+ + Ac-CoA • Acyl-CoA (2 C shorter) enters next rotation • FA has mostly an even number of C - last rotation converts butyryl-CoA to 2 AcCoA
Regulation of β-oxidation • At the entrance of FA to MIT - CAT I • Malonyl-CoA inhibits - (intermediate of FA synthesis) • Principle: • 1) synthesis of FA is in the cytosol, as well as reaction of CAT I • 2) malonyl-CoA is produced in the first reaction of FA synthesis • 3) cross regulation prevents the simultaneous FA synthesis and FA degradation
Omegaoxidation of fatty acids (endoplasmic reticulum; minority pathway for long chain FA)
The figure was found at http://www.biocarta.com/pathfiles/omegaoxidationPathway.asp (January 2007)
Pathways of lipid metabolism Synthesis and utilization of ketone bodies
Synthesis and function of ketone bodies • Acetoacetate, β-hydroxybutyrate and acetone • Liver mitochondria • Ketone bodies are water-soluble form of FA
Synthesis and function of ketone bodies • Entry of AcCoA to KC depends on the availability of oxaloacetate • OAA is produced by pyruvate carboxylation • During starvation or DM is OAA consumed in gluconeogenesis • Lack of saccharides – reduction of OAA – decrease of KC rate
Synthesis and function of ketone bodies • ↑ lipolysis (HSL)→ ↑ FA → β-oxidation → excess of AcCoA → ketogenesis • Condensation of 2 AcCoA → acetoacetyl~CoA • Reaction with another AcCoA → 3-hydroxy3-methylglutaryl~CoA (HMG~CoA) • Cleavage of HMG-CoA → AcCoA a acetoacetate
β-Ketothiolase • The last step of β-oxidation – reverse reaction in ketone bodies synthesis • 2 → AcCoA ~ acetoacetyl CoA O H3C
C
O S
Acetyl-CoA
CoA
+
H3C
C
S
Acetyl-CoA
CoA
1 H3C CoA
O
O
C
C
C H2
S
Acetoacetyl-CoA
CoA
3-hydroxy-3-methylglutaryl-CoA synthase H3C
O
O
C
C
C H2
O S
Acetoacetyl-CoA
CoA
+
H3C
C
S
Acetyl-CoA
O
2
CoA
-
O H2O
CoA
C
HO C H2
CH3 C
C H2
O C
S
CoA
3-Hydroxy-3-methylglutaryl-CoA
• Catalyzes the condensation on the 3rd carbon in AcAcCoA
3-hydroxy-3-methylglutaryl-CoA lyase O -
O
C
HO C H2
CH3 C
C H2
O C
S
3
CoA
-
O O
3-Hydroxy-3-methylglutaryl-CoA H3C
C
O
O
C
C
C H2
Acetoacetát S
CoA
HMG-CoA is cleaved to acetoacetate and AcCoA
CH3
β-hydroxybutyratedehydrogenase • Catalyzes the reversible conversion of ketone bodies: acetoacetate and β-hydroxybutyrate • Massive formation of KB - βhydroxybutyrate is quantitatively the most important KB in the blood • Acetoacetate → acetone (spontaneously)
β-hydroxybutyratedehydrogenase O -
O
C
O C H2
C
Acetoacetát
O
4
-
CH3
O H+ + NADH
NAD+
C
H C H2
OH C
CH3
D-3-Hydroxybutyrát
O H3C CO2
C Aceton
CH3
Ketone bodies activation Acetoacetát • Extrahepatic: reconversion to AcCoA – to the KC CoA-transferasa
• Acetoacetate activated by CoA – transfer from Suc~CoA • Cleavage by thiolase – two AcCoA • Transferase is not in liver!
Sukcinyl-CoA Sukcinát
Acetoacetyl-CoA CoA Thiolasa
2 Acetyl-CoA
Role of acetoacetate • Cardiac muscle and kidney cortex prefer acetoacetate before glucose • Brain adapts to acetoacetate during starvation (long term starvation – up to 50% of energy from acetoacetate) • Regulatory role: high levels of acetoacetate in blood - signal of presence of large amounts of AcCoA – decrease in lipolysis
Regulation of ketogenesis • 1) HSL – lipolysis in adipose tissue • 2) CAT I – FA entry to MIT (βoxidation) • 3) Use of AcCoA: in KC or in ketogenesis • 4) Mitochondrial HMG-CoA synthase