Lipid Catabolism

CH339K

Fats are stored in lipid droplets

Lipid droplets in a rat adipocyte

Glucagon Epinephrine

Human Serum Albumin

• 30-50 g/l of blood• 67 kDal• 585 amino acids• Can bind up to 10 fatty acids• Different binding sites have different affinities• Also binds thyroid hormones

FADL – An E. coli Fatty Acid Transporter

1) b-Barrel Transmembrane Protein2) 14 Antiparallel b-sheets3) N-terminal “hatch” domain4) Conformational change on substrate

binding opens hatch

a) Ribbon drawing of intact proteinb) “Hatch” domainc) Cutaway view to show hatch in central

channeld) Cytoplasmic space-filling view to show

hatch plugging channel

van den Berg, B. (2005) Current Opinion in Structural Biology 15(4): 401-407.

Fate of Glycerol• Not wasted• Shuttled to liver in blood• Catabolized there

Glycolysis

Gluconeogenesis

Activation of Fatty Acids

Keq = 337Keq = 1589

Overall:

Keq = 535,000

Transport into the Mitochondrion

(Carnitine-AcylcarnitineTranslocase)

-oxidation

•Mitochondrial matrix•Oxidizes fatty acyl CoA’s at the carbon•Sequentially cleaves off acetyl CoAs•Acetyl CoA is processed through Krebs and ETC

2 Systems for -oxidation• ≥ 12 carbons:

• TFP – last 3 enzymes in multienzyme complex

• < 12 carbons• 4 soluble matrix enzymes

• Palmitate weighs ~256 g/mol (about 42% more than glucose)• Oxidation yields 108 ATPs, versus 32ish for glucose (about

340% more)

MonounstauratedFatty Acids

• Need one extra enzyme• Converts double bond

1

Polyunsaturated Fatty Acids

• Need two extra enzymes• Reduce conjugated double bonds to a single double bond 1

2

1

Odd-numbered Fatty Acids

• Left with 3 carbons• Add inorganic carbon• Convert to succinate• Throw into Krebs Cycle

Pernicious Anemia• B12 is produced only by several genera of bacteria, obtained

from animal food• daily requirement is about 2-3 g/day• Gastric mucosa produces a protein called intrinsic factor• Lack of intrinsic factor results in impaired B12 absorption,

pernicious anemia, death in 1-3 years• Original treatment (1920’s) was ½ lb. of raw liver daily• Concentrated liver juice (yum) became available in 1928• B12 isolated in 1948, synthesized in 1973• Now treated with large doses (several mg) B12• Sources: fish, meat, poultry, eggs, milk, especially liver and

mollusks (clams, oysters, etc.)

Liver Juice! Ummm!!!

(ACC = Acetyl CoA Carboxylase)

Regulation

Peroxisomes• -Oxidation also occurs in peroxisomes (major site in plants)• In critters, peroxisomes are primary organelles for oxidation of very long chain and branched fatty acids (cerotic acid, phytanic acids)

Catalase

Glucose

Acyl CoADehydrogenase

Acyl CoAOxidase

Catalases• Once again, a heme-containing enzyme• Overall reaction: 2 H2O2 ⇄ O2 + 2 H2O• First step: produces porphyrin cation radical• Second step: HOOH acts as electron donor to produce O2 and return enzyme to resting state.

Catalase is a fun enzyme to assay

• Mr. Bubble of the enzyme world

Staphylococcus aureus

Plants don’t store much fat, but seeds often do.

Ω-Oxidation•ER of vertebrates•Medium chain FAs

-oxidation

Herbivores consume a lot of chlorophyll. Chlorophylls have a long hydrophobic tail. Those tails are split off as part of digestion to form phytanates.

-oxidation(Peroxisomes)

Phytanates have methyl groupsCan’t do -oxidation

Dietary phytanates•Dairy•Fish•Animal fats

Refsum’s Disease

• Phytanoyl CoA Hydroxylase deficiency• Can also digest phytanic acid by -oxidation, but

only ~10 mg/day• Typical diet contains 50 mg• Builds up in myelin sheath• Also screws up vitamin A metabolism• Demyelinating neuropathy, cerebellar ataxia,

deafness, anosmia, cranial nerve degeneration

Refsum’s sign

Ketone Body Generation• During fasting or carbohydrate starvation,

oxaloacetate in the liver is used for gluconeogenesis.

• Acetyl-CoA then doesn’t enter Krebs cycle. • Acetyl-CoA converted in mitochondria to

ketone bodies, • Ketone bodies are transported in the blood to

other cells• Converted back to acetyl-CoA for catabolism

in Krebs cycle, to generate ATP.

-oxidation in reverse

Diabetic Ketoacidosis• Primarily in Type 1 (insulin-dependent)• Low insulin = low glucose transport into

cells• Liver thinks it’s starving• Ketone body production ramps up• Blood pH drops into danger zone