02: LIPID METABOLISM Fatty Acid Synthesis and Fatty Acid Breakdown

Dietary lipids are broken down by lipases

• ______ lipase

• ______ lipase

• _______ lipase

Lingual, Gastric, Pancreatic

Emulsification of lipids by ______ occurs in the _____

• micelles

bile salts, duodenum

Bile salts are ________

synthesized in the liver and stored in the gall bladder

Anabolism of fatty acids: requires _____

acetyl-CoA and malonyl-CoA, NADPH

Anabolism of fatty acids: takes place in ____ in animals, _____ in plants

cytosol, chloroplast

Catabolism of fatty acids: produces _______

acetyl-CoA, NADH and FADH2

Catabolism of fatty acids

• takes place in the _______

mitochondria

Acyl Carrier Protein (ACP) Serves as a Shuttle in Fatty Acid Synthesis

• Contains a covalently attached prosthetic group ______

• flexible arm to hold the acyl chain while carrying intermediates from one enzyme subunit to the next

4’-phosphopantetheine

Acyl Carrier Protein (ACP) Serves as a Shuttle in Fatty Acid Synthesis

• Delivers _______ (in the first step) or _____ (in all the next steps) to the FAS

acetate, malonate

Prep Step: Chain Transfer and Charging ACP and FAS I with Acyl Groups

• Two thiols must be charged with the correct acyl groups before the condensation reaction can begin.

  • thiol from _________ in ACP

  • thiol from _____ in FAS I

4-phosphopantetheine, Cysteine

Prep Step: Chain Transfer and Charging ACP and FAS I with Acyl Groups

• The acetate fragment of acetyl-CoA is transferred to ACP.

  • catalyzed by ___________

• ACP passes this acetate to the Cys-SH of the KS domain of FAS I

  • B-ketoacyl-ACP synthase (KS) domain (region)

• ACP –SH group is recharged with malonyl from malonyl-CoA.

  • catalyzed by _______

Acetyl CoA-ACP acetyltransferase, Malonyl CoA-ACP transacylase

Step 1 of FA synthesis: _____

Condensation

Step 1 of FA synthesis:

• attaches acetyl group (or longer fatty acyl chain) to malonyl group

  • Occurs with simultaneous release of CO2

• This activates malonyl group to attack the acetyl carbonyl

  • Creates _____, which is a B-keto intermediate

  • Catalyzed by _______

• Decarboxylation of malonyl-CoA makes the reaction energetically favorable

acetoacetyl-ACP, B-ketoacyl-ACP synthase

Step 2 of FA synthesis: _____

Reduction

Step 2 of FA Synthesis:

• NADPH reduces the B-keto intermediate to an alcohol.

• Ketone carbon (B-carbonyl) reduced to form an alcohol

  • _____

• NADPH is e− donor

• Reaction is catalyzed by ________

B-hydroxybutyryl-ACP, B-ketoacyl-ACP reductase (KR)

Step 3 of FA Synthesis: _____

Dehydration

Step 3 of FA Synthesis:

• -OH group from C-2 and -H from neighboring CH2 are eliminated, creating double bond (trans-alkene).

• -OH and -H removed from C-2 and C-3 of B-hydroxybutyryl-ACP to form _______

• catalyzed by ______

trans-(delta)2-butenoyl-ACP, B-hydroxyacyl-ACP dehydratase (DH)

Step 4 of FA Synthesis: _____

Reduction

Step 4 of FA Synthesis

• NADPH reduces double bond to yield saturated alkane

  NADPH is the e- donor to reduce double bond of trans-(delta)2-butenoyl-ACP to form ______

• catalyzed by ________

butyryl-ACP, enoyl-ACP reductase (ER)

Final Result of FA Synthesis: Increases chain length of the acyl group of the fatty acid by _____ carbons

2

Fatty Acid Storage

• Fatty acids are converted to triacylglycerol (TAGs)

• TAGs are stored in _____

  • Ready for mobilization when energy is needed

adipose tissue

Fatty Acid Storage

• Very little of TAG are stored in the liver

  Instead they are exported from the liver

  Packaged with ______

  Different lipoprotein particles (Chylomicrons, VLDL, LDL, HDL)

cholesterol esters and phospholipids

Fatty Acid Transport into Mitochondria

• Fats are degraded into fatty acids and glycerol in the ______

• Fatty acids are transported to other tissues for fuel through the blood.

• B-oxidation of fatty acids occurs in _____.

cytoplasm of adipocytes, mitochondria

Fatty Acid Transport into Mitochondria

• Small fatty acids (< 12 carbons) diffuse freely across mitochondrial membranes.

• Larger fatty acids (most free fatty acids) are transported via ______

acyl-carnitine/carnitine transporter

Catabolism of lipids

• Hydrolysis of TAG is catalyzed first by _______, and then by other lipases

• Hydrolysis releases free fatty acids (and glycerol)

hormone-sensitive lipase (HSL)

hormone-sensitive lipase (HSL)

• When insulin is low, epinephrine is high

• Epinephrine binds to the receptor

  • activates adenylyl cyclase

  • Causes rise in cellular cAMP levels

• cAMP activates a protein kinase

• Active Protein Kinase phosphorylatesHSL

phosphorylated HSL = active, Dephosphorylated HSL = Inactive

hormone-sensitive lipase (HSL)

• Activated HSL causes hydrolysis of TAGs

• Hydrolysis of TAGs produces _____

free fatty acids

To enter matrix, fatty acids need to be in ____ form

acyl CoA

Stages of Fatty Acid Oxidation: Stage 1

• consists of oxidative conversion of two-carbon units into ______ with concomitant generation of NADH and FADH2.

• involves oxidation of B-carbon to thioester of fatty acyl-CoA

acetyl-CoA via B oxidation

Stages of Fatty Acid Oxidation: Stage 2

• involves oxidation of acetyl-CoA into CO2 via _____

  • generation of GTP, NADH and FADH2

TCA cycle

Stages of Fatty Acid Oxidation: Stage 3

• generates ATP from NADH and FADH2 via the ______

e- transport chain

is a transport system that facilitates the transfer of long-chain fatty acids into the mitochondria for oxidation

• Catabolic reactions of fatty acids: Energy release!

• Location: Mitochondrial matrix

carnitine shuttle

A sequence of four reactions acting on the fatty acid-acyl CoA

B-oxidation

Outcome of B-Oxidation: Shortening of the fatty acid chain, 2C units a time

• The final thiolytic cleavage releases _______

two units of acetyl CoA

Fatty Acid Catabolism for Energy

• For palmitic acid (C16)

• Repeating the previous four-step process six more times (seven total) results in ______

  • FADH2 is formed in each cycle (seven total).

  • NADH is formed in each cycle (seven total).

  • Last step produces two molecules of Acetyl CoA

    • From acetoacetyl CoA

• Acetyl-CoA enters TCA cycle

  • This makes more GTP, NADH, and FADH2.

• Electrons from all FADH2 and NADH enter electron transport chain to produce ATP.

8 molecules of acetyl CoA

How much ATP is generated from Fatty Acid Catabolism

131 ATP

Synthesis of ketone bodies is termed _____

• During a fasting state, liver has elevated levels of fatty acid

• Liver produces acetyl CoA by fatty acid breakdown

  • Process: β-oxidation

ketogenesis

Ketogenesis

• Elevated Acetyl CoA levels cause upregulation of ______

pyruvate carboxylase

Ketogenesis

• In which pathway, have you seen this enzyme?

  _______

  Gluconeogenesis!

  • Acetyl CoA enters the ketogenic pathway, instead of coupling with OAA

  • This protects OAA from entering gluconeogenesis (or Krebs)

  Management of cellular resources

Pyruvate to OAA

Ketone bodies

• Entry of acetyl-CoA into citric acid cycle requires ______.

  When ______ is depleted, acetyl-CoA is converted into ketone bodies

  • This occurs when there is low carb intake, or diabetes

OAA

Ketone Bodies

• Three forms of ketone bodies can leave the liver, and are delivered by the blood to various tissues: _______

• Cardiac muscle, skeletal muscle, renal cortex, and brain can use ketone bodies as a fuel source

  • Acetone is not really used; it is eliminated in breathing

Acetone, Acetoacetate, B-hydroxybutyrate

The ____ Is the Source of Ketone Bodies

Liver

TAGs are hydrolyzed to yield free fatty acids through lipase action influenced by ____

epinephrine

_______ enables the movement of fatty acid acyl CoA to the matrix for B-oxidation

Carnitine shuttle

Under ____ or untreated _____ conditions, acetyl-CoA formed in the liver is converted to ketone bodies that serve as fuels for other tissues

• Once the ketone bodies reach the tissues, they get re-converted to acetyl CoA

  • This acetyl CoA can produce energy within the tissue

fasting, diabetic