16 Biochemistry _ Metabolism

LIPID METABOLISM

Lipid Metabolism

Major roles of lipids in cell structure and metabolism:triacylglycerols: major form of stored energy in mammalsphospholipids, glycolipids, cholesterol: components of cell membranescholesterol: precursor of steroid hormones and bile salts

A. Simple lipid - ester of fatty acids with various alcohols

1. Natural fats and oils (triglycerides)

2. Waxes

(a) True waxes: cetyl alcohol esters of fatty acids

(b) Cholesterol esters

(c) Vitamin A esters

(d) Vitamin D esters

B. Compound lipid - esters of fatty acids with alcohol plus other groups

1. Phospholipids and spingomyelin: contains phosphoric acid and often a nitrogenous base

2. Sphingolipids (also include glycolipids and cerebrosides): contains aminoalcohol spingosine, carbohydrate, N-base; glycolipids contains no phosphate

3. Sulfolipids : contains sulfate group

4. Lipoproteins : lipids attached to plasma/other proteins

5. Lipopolysaccharides: lipids attached to polysaccharides

Classification

CH3

OH

CH3

CH3

CH3 CH3

E E E E

Vitamin A

Vitamin D

Phospholipids

Nomenclature and Structure

Fats and oils: Vegetable oils are triglycerides that are liquid at room temp

due to their higher unsaturated or shorter-chain fatty acids

Triglycerides are most abundant natural lipids

Natural fats have D-configurationUsually R1 and R3 are saturated and R2 is unsaturatedNatural fats are mixture of two or more simple triglycerides

ADIPOSE TISSUE

•90% of adipose tissue is triglycerides

•It supplies energy.

•Insulation.

•Provides minor physical protection

•Cholestrol storage

Why Fatty Acids?

(For energy storage?) Two reasons:

The carbon in fatty acids (mostly CH2) is almost completely reduced (so its oxidation yields the most energy possible).

Fatty acids are not hydrated (as mono- and polysaccharides are), so they can pack more closely in storage tissues

Triglyceride storage

Stored in large quantities in cells Non-reactive with other cell components Segregated into lipid droplets Do not affect osmolarity of cytosol

Stored in anhydrous state Non-polar Provide ~ 6 times energy of hydrated glycogen

Triglycerides, cont…

To be used as fuel: insoluble in H2O Must emulsify before lipid digestion in

intestine Must be “carried” in blood (proteins)

Sources of fat Three cellular sources

Fat in diet Fat stored in cells Fat synthesized in one organ and

transported to another

Triglycerides, cont…

Fats obtained vary by organism Vertebrates

Fat in diet Fat in adipose tissue Convert excess carbohydrate to fat

in liver for export

Fat from Diet & Adipose Cells

Triacylglycerols either way

Triglycerides are also the major form of stored energy in the body

Hormones (glucagon, epinephrine, ACTH) trigger the release of fatty acids from adipose tissue

Humans, Triglycerides, cont…

Hibernating animals and migrating birds

Higher plants: Do not depend on fats for energy

Germinating seeds

Used for > half the energy in: Liver Heart Resting skeletal muscle

Lipids in the Blood

• Fatty Acids

• Bound to albumin

• Cholesterol, Triglycerides and Phospholipids

• Transported by lipoproteins

• Cholesterol can be free or esterified

• Triglycerides must be degraded to be absorbed by cells

Mobilization of Stored Fats

Action of second messenger hormones triggers mobilization of stored triglycerides

Free fatty acid released into bloodstream

Binds to serum albumin Carried to tissues

Released to diffuse into cells

The utilization of stored triacylglycerols requires three stages of processing

1. Hormone-sensitive lipase of adapose tissue liberates fatty acids, that are carried in the blood by serum albumin.

2. At the consuming tissues, fatty acids are activated and transported into the mitochondrion for degradation.

3. In the mitochondrion, fatty acids are broken down in a step-wise fashion to form acetyl-CoA, which is used in the TCA cycle.

Lipid digestion, cont…

Digestion Triglyceride digestion

takes place at lipid-water interfaces Rate is based on surface area at

interfaces Emulsification by bile

Lipid digestion, Bile, cont…

Acts as digestive detergent Converts dietary fats into mixed micelles Micelles contain bile salts and triglycerides

Bile Synthesized from cholesterol By liver Stored in gallbladder Released after ingestion of fat

Lipid digestion, cont…

Role of lipase Pancreatic lipase:

catalyzes hydrolysis of triglycerides At 1 and 3 positions sequentially Forms 1,2-diglycerol 2-glycerol Soap

Role of lipase, Lipid digestion, cont…

Phospholipase A: Pancreatic enzyme Degrades phospholipids Hydrolysis at C(2)

Absorption of Lipids

Absorption Molecules diffuse into cells of

intestine Facilitated by bile salts

Micelles transport non-polar lipids across aqueous boundary layer

Reconversion: in mucosa cells Lipid digestive products triglycerides Packaged into Chylomicrons

Lipoprotein aggregates Triglycerides, cholesterol, protein

Released into bloodstream: Via lymphatic system Lacteals, a lymphatic capillary that absorbs dietary

fats in the villi of the small intestine.

Absorption of Lipids

Lipid Uptake in Vertebrates

Role of lipase, cont…

Transport APOLIPOPROTEINS: lipid-binding

blood proteins Transport between organs

Form classes based on density

Transport, Role Chylomicrons, cont…

Chylomicrons and VLDL: Move triglycerides, cholesterol To Skeletal muscle & adipose LIPOPROTEIN LIPASE

an extracellular enzyme Activated by apo C II Hydrolyzes triglycerides

Chylomicron

Carry triglycerides, cholesterol to tissues via lymphatic system (lacteals)

Proteins on outside of Chylomicron Are apoproteins Act as cell surface receptors for

recognition (apo C-II)

Model for plasma triacylglycerol and cholesterol transport in humans.

Lipids are transported in lipoprotein complexes

Transport, Role of lipase, cont…

Deletes lipoprotein from triglyceride Forms IDL Then LDL

LDL: (130-159 mg/dl) removed from plasma at liver, adrenals, adipose

HDL: (above 35 mg/dl) moves cholesterol from tissues Sends to liver for excretion in bile salts

Ratio LDL:HDL = 2:1

Low-density lipoprotein (LDL)Intermediate density lipoprotein (IDL) High-density lipoprotein (HDL)

Atherosclerosis an artery wall thickens as a result of the accumulation of calcium and fatty materials such as cholesterol and triglyceride.

LipoproteinsProteins covalently linked to lipid found in the blood plasma

Plasma lipoproteins transport lipid through the bloodstream. On the basis of density, lipoproteins are classified into four major classes:

Chylomicrons: Large lipoproteins of extremely low density; transport dietary triglycerols and cholesteryl esters from intestine to the tissues (muscle/ adipose)

VLDL (0.95-1.006 g/cc) : synthesized in the liver ; transport lipids to tissues; converted to LDL with depletion of triglycerol, apoproteins and phospholipids

LDL (1.006 - 1.063 g/cc): carry cholesterol to tissues; engulfed by cells after binding to LDL receptors

HDL (1.063 - 1.210 g/cc ): produced in liver; scavenge cholesterol from cell membrane as cholesteryl ester which is transported to liver from where the excess cholesterol is disposed of as bile acids

LDL-receptor activity controls cholesterol homeostasis

Liver removes IDL from serum, therefore thenumber of LDL receptors plays a direct role.

high cholesterol diet leads to repression ofLDL receptor synthesis.

Transport

Remnants of Chylomicrons triglycerides; cholesterol and

apoproteins (apo E, apo B-48) To liver

Uptake by endocytosis Triggered by apolipoproteins

Glycerol Transported to liver or kidney Converted to dihydroxyacetone

phosphate

Uptake by cells Fatty acids taken in by:

muscle: oxidized for energy

adipose: stored as triglyceride

Uptake in Liver via Endocytosis

a cell-surface receptor that recognizes the apoprotein B100

Chylomicron Remnants, cont…

Triglycerides Oxidized for energy or Converted to ketone bodies

Excess fatty acid Convert to triglycerides Pack into VLDL To adipose for storage

Mobilization of Stored Fats

Mobilization of stored triglycerides Triggered by hormones

Stimulus is in blood glucose Hormones:

Epinephrine Glucagon

Mobilization of Stored Fats, Hormone Action, cont…

Activate adenylate cyclase in adipocyte plasma membrane

Increases cAMP Activates protein kinase Activates triglycerol lipase Catalyzes hydrolysis of ester bonds in

triglycerides

HYDROLYSIS

GLYCEROL

GLUCOSE

PYRUVATE

GLY

CO

LYS

IS

Conversion of glycerol to dihydroxyacetone phosphate

NOTE: Glycerol glycerol-3-phosphate dihydroxyacetone phosphate glycolysis

GLUCOSE TRIGLYCERIDES

PYRUVATE

GLYCEROL

ACETYL-CoAglycolysis

gluc

oneo

gene

sis

-ox

idat

ion

lipog

ensi

s

Acetyl coenzyme A or acetyl-CoA

acetyl-CoA is the thioester between coenzyme A (a thiol) and acetic acid (an acyl group carrier).

Fatty Acid (FA) Oxidation

FA activation: Free FA cannot enter Mitochondria Occurs in cytosol

Acyl-CoA synthetases (thiokinases) 3 isozymes mitochondrial membrane

FATTY ACIDS

•Activation of Fatty Acid

FA + CoA + ATP fatty acyl-CoA + AMP + PPi

Fatty acid oxidation, cont…

Transport across mitochondrial membrane: Formation of fatty acyl-carnitine

Carnitine acyl transferase I Outer face of inner membrane Fatty acyl from CoA carnitine CoA released cytoplasm

Transport of fatty acids into the mitochondrion

Transfer of acyl from Carnitine to intra-mitochondrial membrane

Carnitine acyltransferase II Inner face of inner membrane Regenerates fatty acyl-CoA Frees carnitine returns to cytosol

Acyl-carnitine/carnitine transporter

Carrier Fatty acyl-carnitine

matrix Facilitated diffusion

Cytoplasm: makes fatty acidsMitochondria: oxidative degradation

ß-oxidation

ß-oxidation: FA dismembered to fatty acyl-CoA Mitochondrial oxidation of FA

Stage One: ß-oxidation Remove 2-C chunks as acetyl-CoA Begins at carboxyl end of fatty acid chain

ß-Oxidation

Fatty acids are dismembered into Acetyl-CoA subunits

Each acetyl Co-A sends 4 H to NAD, FAD

1. Activation by addition Coenzyme-A

2. carbon oxidized from CH2 to C=O (ketone)

3. Molecule split into acetyl CoA and Fatty acid 2 carbons shorter

4. Another Coenzyme-A added to shortened Fatty acid

-OXIDATION

Beta Oxidation of Fatty Acids

The process begins with oxidation of the carbon that is "beta" to the carboxyl carbon, so the process is called"beta-oxidation"

Products: an acetyl-CoA and a fatty acid two carbons shorter

CoA activates Fatty Acids for oxidation

Acyl-CoA synthetase condenses fatty acids with CoA, with simultaneous hydrolysis of ATP to AMP and PPi

Formation of a CoA ester is expensive energetically Reaction just barely breaks even with ATP hydrolysis But subsequent hydrolysis of PPi drives the reaction

strongly forward

Carnitine as a Carrier

Short chain fatty acids are carried directly into the mitochondrial matrix

Long-chain fatty acids cannot be directly transported into the matrix

Long-chain fatty acids are converted to acyl carnitines and are then transported in the cell

Acyl-CoA esters are formed inside the inner membrane in

this way

Carnitine carries fatty acyl groups across the inner mitochondrial membrane

NADH & FADH

Electrons are passed to an electron transfer flavoprotein, and then to the electron transport chain

Summary of -Oxidation

Repetition of the cycle yields a succession of acetate units

Thus, palmitic acid yields eight acetyl-CoAs Complete -oxidation of one palmitic acid yields

106 molecules of ATP Large energy yield is consequence of the highly

reduced state of the carbon in fatty acids This makes fatty acids the fuel of choice for

migratory birds and many other animals

Each cycle produces 1 acetyl-CoA 16 carbon fatty acid would give 8 acetyl-CoA

Cycles required is # of carbons/2 – 1 16 carbon fatty acid would require 7 cycles

Amount of energy from Fatty Acid depends on length of carbon chain

The complete oxidation of palmitate yeilds ~106 molecules of ATP

Palmitoyl~CoA is C16-acyl CoA

There will be 7 turns of the cycle, with the final turn generating 2 acetyl~CoA.

palmitoyl~CoA + 7 FAD + 7 NAD+ + 7 CoA---> 8 acetyl~CoA + 7 FADH2 + 7 NADH + 7 H+

7 NADH X 2.5 ATP/NADH-----------------> 17.5 ATP

7 FADH2 X 1.5 ATP/FADH2--------------->10.5 ATP

8 acetyl~CoA X 10ATP/acetyl~CoA------->80 ATP

Total--------------------------------------------108 ATP

activation of palmitate--------------------> -2 ATP

Yeild 106 ATP

Unsaturated Fatty Acids

Consider monounsaturated fatty acids:

Oleic acid, palmitoleic acid

cis-3 acyl-CoA cannot be utilized by acyl-CoA dehydrogenase

Enoyl-CoA isomerase converts this to trans- 2 acyl CoA

-oxidation continues from this

point

Same, but only up to a point:

3 cycles of -oxidation enoyl-CoA isomerase 1 more round of -

oxidation trans- 2, cis- 4

structure is a problem 2,4-Dienoyl-CoA

reductase to the rescue

Polyunsaturated Fatty Acids

Peroxisomal -Oxidation

Peroxisomes - organelles that carry out flavin-dependent oxidations, regenerating oxidized flavins

by reaction with O2 to produce H2O2 Similar to mitochondrial -oxidation, but initial double

bond formation is by acyl-CoA oxidase Electrons go to O2 rather than e- transport Fewer ATPs result

Branched-Chain Fatty Acids

An alternative to -oxidation is required

Branched chain FAs with branches at odd-number carbons are not good substrates for -oxidation

-oxidation is an alternative Phytanic acid -oxidase

decarboxylates with oxidation at the alpha position

-oxidation occurs past the branch

Ketone Bodies

A special source of fuel and energy for certain tissues Some of the acetyl-CoA produced by fatty acid

oxidation in liver mitochondria is converted to acetone, acetoacetate and -hydroxybutyrate

These are called "ketone bodies" Source of fuel for brain, heart and muscle Major energy source for brain during starvation They are transportable forms of fatty acids

Ketone Bodies and Diabetes

"Starvation of cells in the midst of plenty" Glucose is abundant in blood, but uptake by

cells in muscle, liver, and adipose cells is low Cells, metabolically starved, turn to

gluconeogenesis and fat/protein catabolism In type I diabetics, oxaloacetate is low, due to

excess gluconeogenesis, so acetyl-CoA from fat/protein catabolism does not go to TCA, but rather to ketone body production

Acetone can be detected on breath of type I diabetics

Ketone Bodies

Acetyl-CoA from ß-oxidation Enters citric acid cycle Converted to KETONE BODIES

Water-soluble “equivalent” of FA

Ketone bodies, cont…

Ketogenesis Occurs in liver:

acetyl CoA ketone bodies Primary ketone bodies

Acetoacetate - out of liver D-ß-hydroxybutyrate - out of liver Acetone - exhaled


Function: fuel for peripheral tissues Heart, skeletal muscle Brain

Normal fuel is glucose In starvation: ketone bodies

Enzyme production adapts over time >40 days, provide 70% energy


Production and transportation Determined by availability of

oxaloacetate Combine with acetyl group enter TCA cycle


In starvation Oxaloacetate pulled from citric

acid cycle Used in gluconeogenesis

[oxaloacetate] decreases therefore less kreb’s cycles Production of ketone bodies is

favored

Ketosis

A 4-carbon acid (OAA, oxaloacetate) is needed to react with excess acetyl-CoA and form citrate

When OAA is not available excess acetyl - CoA in liver are condensed to form ketone bodies

OAA is limited during scarcity of glucose for glycolysis. In starvation and diabetes, glycogen is broken down. Fatty acids of fat depots are metabolized to supply ATP needs producing excess of the ketone bodies

In normal metabolic pathway, acetoacetate and -hydroxybutyrate are the ketone bodies which are converted to acetyl-CoA. However, during starvation and in uncontrolled diabetes, concentration of acetoactate is very high and supply of oxaloacetate (a TCA component) is insufficient, thus acetoacetate spontaneously decarboxylated to acetone - KETOSIS

Ketone Body Formation

Production and transportation, cont…

Overproduction: starvation, diabetes Moved from liver to other tissues

Allows fatty acid oxidation in liver In tissues:

ketone bodies converted back to acetyl-CoA


Formation of ketone bodies from acetyl-CoA occurs in the mitochondrial matrix

thiolase 2 Acetyl-CoA acetoacetyl-CoA Acetoacetyl-CoA + acetyl-CoA

ß -hydroxy- ß -methylglutaryl-CoA


Acetoacetate D- ß -hydroxybutyrate dehydrogenase D-ß-hydroxybutyrate

Acetoacetate decarboxylase

Acetone High in uncontrolled diabetes


Ketosis Pathological Acetoacetate produced too fast

for elimination Breath smells like acetone Blood pH decreases acidosis

LIPOGENESIS

Making triglycerides from glycerol and fatty acids

Anabolic process

Fatty acids made linking 2 carbon acetyl groups (from acetyl CoA) to growing chain

Most Fatty acids have an even number of carbons

ESSENTAIL FATTY ACIDS

•Those needed by the body, but not synthesized within the body in adequate amounts.

•Found in fish and some plants

Lipid Biosynthesis

“ Reverse” of lipid catabolism Know differences

Where in the cell does each occur? What are the e- transport

molecules?

Comparison of fatty acid biosynthesis and degradation

Cholesterol Biosynthesis

Sites of regulation of fatty acid metabolism

Sample question Atherosclerosis can cause blood A. thinning B. clotting C. thickening D. none of these

Sample question Ketosis is ascribed in part to:

A. Slowdown in fat metabolism B. An insufficient intermediates of TCA cycle C. An underproduction of acetyl-CoA D. An inhibition of glycogen synthesis

Sample question In the intestine, the dietary fats are hydrolysed by A.triacylglycerol lipase B.adenylate cyclase C.pancreatic lipase D.protein kinase

Sample question In eukaryotes fatty acid breakdown occurs in A. mitochondrial matrix B. cytosol C. cell membrane D. endoplasmic reticulum

Sample question Fatty acid synthesis takes place in A. mitochondria B. cell membrane C. cytosol D. endoplasmic reticulum

Sample question Chylomicrons are synthesized in A. blood B. liver C. intestine D. pancreas

Sample question VLDLs are synthesized in A. blood B. liver C. intestine D. pancreas

Sample question Cholestrol is the precursor of A. steroid hormones B. vitamin A C. bile salts D. both (a) and (c)

Education

16 Biochemistry _ Metabolism