LIPOLYSIS: FAT OXIDATION & KETONES

BIOC 460 - DR. TISCHLER LECTURE 33

OBJECTIVES

1. Lipolysisa) describe the pathway; b) locationc) principal enzyme d) rolee) role of albumin and FABP in transport/metabolism of FFA

2. Degradation of fatty acyl CoAa) roles of acyl CoA synthetase, CPT-I and CPT-II, and CATb) relationship of -oxidation products to energy production. c) degradation of odd- vs even-chain FAd) vitamins for metabolizing propionyl CoA to succinyl CoA

3. Ketone body metabolisma) where ketogenesis occursb) when ketogenesis occursc) role of keotgenesisd) why normal individuals do not usually develop ketacidosis

even when producing ketone bodies.

FAT FACTS

fat (lipid) makes up 37% of the calories in the American diet

energy rich and provides 9 kcal/gm

dietary lipids 90% triacylglycerols (TAGs) also include cholesterol esters, phospholipids, essential unsaturated fatty acids; fat-soluble vitamins

most dietary fat transported to adipose for storage

dietary TAGs hydrolyzed in the intestine by pancreatic lipases; then reassembled in the intestinal cells

dietary fats transported to tissues as TAG or cholesterol via chylomicrons

at peripheral tissues (e.g., adipose or muscle), FA removed from the TAG by a lipoprotein lipase in the capillary walls; released fatty acids diffuse into the cell

saturated fatty acid: CH3-(CH2)n-COOH

unsaturated fatty acid: CH3-CH=CH-(CH2)n-COOH

polyunsaturated fatty acid: CH3-CH=CH-CH2-CH=CH-(CH2)n-COOH

CH2----OOC-R1 CH2OH HOOC-R1

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R2-COO----CH CHOH HOOC-R2

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CH2----OOC-R3 CH2OH HOOC-R3

Figure 1. General structures of fatty acids and triacylglycerol. Lipolysis of stored triacylglycerol by lipases produces fatty acids plus glycerol.

Lipolysis

Triacylglycerol Glycerol Fatty acids

LIPOLYSIS

fatty acids hydrolytically cleaved from triacylglycerol

largely in adipose to release fatty acids as a fuel

may also occur in muscle or liver - smaller amounts of fatty acids are stored

hormone-sensitive (cyclic AMP-regulated) lipase initiates lipolysis – cleaves first fatty acid

this lipase and others remove remaining fatty acids

fatty acids/glycerol released from adipose to the blood

hydrophobic fatty acids bind to albumin, in the blood, for transport

MITOCHONDRION

cell membraneFA = fatty acidLPL = lipoprotein lipaseFABP = fatty acid binding protein

ACS

FABP

FABPFA

[3]

FABPacyl-CoA

[4]

CYTOPLASM

CAPILLARY

FAalbuminFA FA

FA

fromfatcell

FA

[1]

acetyl-CoA TCAcycle

-oxidation[6]

[7]

carnitinetransporter

acyl-CoA[5]

Figure 2. Overview of fatty acid degradation

ACS = acyl CoA synthetase

LPL

Lipoproteins(Chylomicrons or VLDL)

[2]

Figure 3 (top). Activation of palmitate to palmitoyl CoA (step 4, Fig. 2) and conversion to palmitoyl carnitine

IntermembraneSpace

OUTERMITOCHONDRIALMEMBRANE

palmitoyl-carnitine

CoApalmitoyl-CoA

carnitine

Cytoplasm

palmitoyl-CoA

AMP + PPiATP + CoA

palmitate

CPT-I [2]

ACS

[1]

Figure 3 (bottom). Mitochondrial uptake via of palmitoyl-carnitine via the carnitine-acylcarnitine translocase (CAT) (step 5 in Fig. 2).

Matrix

INNERMITOCHONDRIALMEMBRANE

Intermembrane Space

palmitoyl-carnitinecarnitine

CoApalmitoyl-CoA

CAT [3]

palmitoyl-carnitineCPT-II

carnitine

CoApalmitoyl-CoA

[4]

CPT-I

CAT

IntermembraneSpace

OUTERMITOCHONDRIALMEMBRANE

palmitoyl-carnitine

CoA

carnitine

Cytoplasmpalmitoyl-CoA

AMP + PPiATP + CoA

palmitate

palmitoyl-CoA

Matrix

INNERMITOCHONDRIALMEMBRANE

[3]

palmitoyl-carnitinecarnitine

CoApalmitoyl-CoA

[4]

CPT-I [2]

ACS[1]

CPT-II

Figure 4. Processing and -oxidation of palmitoyl CoA

matrix side

inner mitochondrialmembrane

2 ATP3 ATP

respiratory chain

recycle6 times

Carnitinetranslocase

Palmitoylcarnitine

Palmitoylcarnitine

Palmitoyl-CoA

+ Acetyl CoACH3-(CH)12-C-S-CoA

O

oxidationFAD

FADH2

hydration H2O

cleavage CoA

oxidationNAD+

NADH

Citricacid cycle 2 CO2

propionyl CoA carboxylase: (biotin-dependent)

propionyl CoA + ATP + CO2 methylmalonyl CoA + AMP + PPi

methylmalonyl CoA mutase: (adenosyl cobalamin-dependent)

methylmalonyl CoA succinyl CoA

Figure 5. Reactions in the metabolism of propionyl CoA derived from odd-chain fatty acids

OXIDATION OF ODD-CHAIN FATTY ACIDS

Final step of -oxidation produces:

propionyl CoA + acetyl CoA

Figure 6. Ketone body formation (ketogenesis) in liver mitochondria from excess acetyl CoA derived from the -oxidation of fatty acids

MITOCHONDRION

(excess acetyl CoA)

Hydroxymethylglutaryl CoA

HMG-CoA synthaseacetyl CoA

CoA

Acetoacetate

HMG-CoA-lyase

acetyl CoA

-Hydroxybutyrate

-Hydroxybutyratedehydrogenase

NAD+

NADH

Acetone

(non-enzymatic)

2 Acetyl CoAFatty acid-oxidation

Citric acid cycle

oxidation to

CO2

Acetoacetyl CoACoA

Thiolase

high rates of lipolysis (e.g., long‑term starvation or in uncontrolled diabetes) produce sufficient ketones in the blood to be effective as a fuel

ketones are the preferred fuel if glucose, ketones, fatty acids all available in the blood

primary tissues: using ketones, when available, are brain, muscle, kidney and intestine, but NOT the liver.

-Hydroxybutyrate + NAD+ acetoacetate + NADH -hydroxybutyrate dehydrogenase in mitochondria;

reverse of ketogenesis

KETONE BODY OXIDATION

KETOACIDOSISExcessive build-up of ketone bodies results in ketosis eventually

leading to a fall in blood pH due to the acidic ketone bodies.

In diabetic patients the events that can lead to ketosis are:

Relative or absolute (most common cause) deficiency of insulin

Mobilization of free fatty acids (from adipose lipolysis)

Increased delivery of free fatty acids to the liver

Increased uptake and oxidation of free fatty acids by the liver

Accelerated production of ketone bodies by the liver

XAdiposeTissue Free fatty

acidsLiver

Ketone BodiesInsulin

Pancreas

Figure 7. Mechanism for prevention of ketosis due to excess ketone body production that can lead to ketoacidosis