3e © 2004 john wiley & sons,...

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Lipid Metabolism – Part I: (Chap. 25, sec.1-3)Glucose

Fats (palmitic acid)

C6H12O6 + 6 O2 6 CO2 + 6 H2O ∆Go’ = -2823 kJ/mol

1. Digestion and Absorption

2. Storage and Mobilization


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Table 25-1 Energy Content of Food Constituents.

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~4 Cal

~9 Cal

~4 Cal

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Table 27-1 Fuel Reserves for a Normal 70-kg Man.

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Lipids can be ingested, released from storage, or synthesized(liver).

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Lipid Uptake

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Figure 25-1 Mechanism of interfacial activation oftriacylglycerol lipase in complex with colipase.

Pancreatic (origin) lipase acts on mixed micelles (substrate). Lipid interaction exposes the active site to allow ester hydrolysis of triacyl-glycerides, generating free fatty acids.

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Fig. 25-2 Catalytic action of phospholipase A2.

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A2

Substrate here is phospholipid, not triacyl-glyceride.

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Figure 25-3a Substrate binding to phospholipase A2. (a) A hypothetical model of phospholipase A2 in complex with a micelle of lysophosphatidylethanolamine.

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Figure 25-4a (a) The X-ray structure of the 124-residue monomeric porcine phospholipase A2 (lavender) in complex with the tetrahedral intermediate mimic MJ33. (b) The catalytic mechanism of phospholipase A2.

D99H48

Mechanism resembles catalytic triad of serine proteases, with H2O playing the role of Ser. There is no covalent intermediate.

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Figure 25-5 X-Ray structure of rat intestinal fatty acid–binding protein.

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This cytoplasmic protein helps carry fatty acids through intestinal cells, where they are assembled into chylomicrons. The fatty acid carboxylate ion pairs with Arg.

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Glycerol-3-P

DHAP

Glycolysis

Glucagon, which signals a need to increase blood glucose, also calls for fatty acid release. It really signals “we need more energy rich molecules”.

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Figure 25-7 X-Ray structure of human serum albumin in complex with 7 molecules of palmitic acid.

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Solubility of free fatty acids ~ 10-6 M

In serum complex with albumin ~ 2 mM

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Lipid Metabolism – Part I: (Chap. 25, sec.1-3)Glucose

Fats (palmitic acid)


Triglycerides are broken down to glycerol and free fatty acids.

1. Metabolism of Glycerol

2. Fatty Acid Oxidation (Knoop 1904)


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Figure 25-6 Conversion of glycerol to the glycolytic intermediate dihydroxyacetone phosphate.

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Glycerol part: ( -ATP + NADH ) / DHAP glycolysis

This looks familiar!

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Figure 25-9 Mechanism of fatty acid activationcatalyzed by acyl-CoA synthetase.

Acyl~AMP

Step 1: Activation(cytosol)

CoA derivative

(fatty acyl CoA)

(many – vary by FA length)

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Figure 25-10 Acylation of carnitine catalyzed by carnitine palmitoyltransferase.

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Transesterification rxn

Keq ~ 1

Step 2: Transport

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Figure 25-11 Transport of fatty acids into the mitochondrion.

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Step 2: Transport

Transferase is inhibited by malonyl-CoA

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The Beta oxidation pathway.

Note the resemblance of first 3 steps to TCA steps:

Succ – fum -mal – OAA

β-hydroxyCoDHis inhibited by NADH

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thiolase

dehydrogenase

hydratase

dehydrogenase

β- Oxidation - more

(spiral)

(human mito β-oxid: Rxn steps 2,3,4 for long chain lipids is carried out by a multifunctional protein)

Humans have 4 different DHs for different FA chain length. MCAD deficiency correlated with SIDS.

Electron transfer flavoprotein

There are 3 of these, for short, medium, and long chain fatty acids

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Figure 25-13 Ribbon diagram of the active site region in a subunit of medium-chain acyl-CoA dehydrogenase from pig liver mitochondria in complex with octanoyl-CoA.

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Regulation of β-oxidation

Fatty acids are a crucial energy reserve and must be administered carefully.

Carnitine transferase is inhibited by malonyl-CoA, the feed stock for fatty acid synthesis. Once a fatty acid is in the mitochondria, it is burned up.

β-hydroxyacyl-CoA dehydrogenase is inhibited by NADH, slowing catabolism when energy levels are high.

Thiolase in inhibited by high concentrations of AcCoA

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5. Other fatty acid oxidation pathways:

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Fig 25-16 Structures of two common unsaturated fatty acids.

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(ω-6)

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Problem #1: cis ∆3 vs. trans ∆2

β

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β-oxidation of poly unsaturated fatty acids:

Linoleic acid

Again, specialty enzymes are brought in to keep intermediates within the basic β-oxidation pathway.

3,2- enol-CoA

isomerase

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biotin

B12

Odd # C FAFigure 25-18Conversion of propionyl-CoA to succinyl-CoA.

Ahh. This looks familiar

This rather convoluted path allows C-C bond formation at a C which is α to a carbonyl. You can’t just stick it on C3 of propionyl CoA, because there is no way to stabilize the carbanion character needed for the attack.

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Figure 25-19 The propionyl-CoA carboxylase reaction.

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Figure 25-20 The rearrangement catalyzed by methylmalonyl-CoA mutase. (only a couple of B12 dependent enzymes in mammals).

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Figure 25-21 Structure of 5′-deoxyadenosylcobalamin (coenzyme B12).

5,6-dimethylbenzimidazole

Only certain bacteria can synthesize B12; neither plants nor animals can do it. We get most of our B12 from meat (which got it from gut bacteria). A deficiency leads to “pernicious anemia”

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Figure 25-23 Proposed mechanism of methylmalonyl-CoA mutase.

Facile homolyticcleavage of Co-C bond is key to this cofactor’s utility.

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Figure 25-22a X-Ray structure of P. shermanii methylmalonyl-CoA mutase in complex with 2-carboxypropyl-CoA and AdoCbl. (a) The catalytically active α subunit. (b) The arrangement of AdoCbl and 2-carboxypropyl-CoA molecules.

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In animals:Long Chain FA are used. Import does not require carnitine. Shortened FAstransfer to mitochondia.

In yeast and plants: all β-ox is done inPeroxisomes &

Glyoxysomes

β-oxidation in peroxisomes.

First DH passes electrons from flavin to O2, producing heat.

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Lipid Metabolism

8. Ketone Bodies

The ketone bodies are a form of “soluble lipid”; they reach fairly high blood concentrations, 0.3 mM. The ketone bodies are readily converted to AcCoA and drop straight into the TCA cycle. They are used by heart and kidney, and the brain can adapt to use ketone bodies after several days of starvation.

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Ketone bodies

Note: We will see these first 2 steps later in steroid biosynthesis.

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Figure 25-26 The metabolic conversion of ketone bodies to acetyl-CoA.

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3-ketoacyl-CoA transferase is an alternative to the action of succinylCoA synthetase in TCA.

Each ketone body activation therefore costs a GTP.

The liver lacks the transferase since it only “wants” to make ketone bodies for other tissues.

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Conditions that accelerate gluconeogenesis, like starvation or diabetes, drain TCA intermediates and slow that cycle. Malonyl CoA is not formed and so carnitine transferase shuttles fatty acids through β-ox. AcCoAaccumulates and ketone bodies pour out of the liver. They are acidic and lead to acidosis and ketosis.

Recall, in mammals there can be no NET synthesis of glucose for fatty acids.

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