Lecture Connections 21 | Lipid Biosynthesis

Catabolism and Anabolic of Fatty Acids Proceed via Different • Anabolism of fatty acids – requires malonyl-CoA and acetyl-CoA – reducing power from NADPH – location: cytosol in animals,

Lecture Connections

21 | Lipid Biosynthesis

© 2009 W H Freeman and Company

Lipids Fulfill a Variety of Biological Functions

• Storage of energy

• Constituents of cellular membranes

• Anchors for membrane proteins

• Cofactors for enzymes

Catabolism and Anabolic of Fatty

Acids Proceed via Different

• Anabolism of fatty acids

– requires malonyl-CoA and acetyl-CoA

– reducing power from NADPH

– location: cytosol in animals, chloroplast in plants

Lipid Catabolism and Anabolism in

Animal and Plant Cells

Overview of Fatty Acid Synthesis

• Fatty acids are built in several passes

processing one acetate unit at a time

• The acetate is coming from activated malonate

in the form of malonyl-CoA

• In each pass involves reduction of a carbonylcarbon to a methylene carbon

Overview of Fatty Acid Synthesis

Synthesis of Malonyl-CoA (1)

• The three-carbon precursor for fatty acid

synthesis is made from acetyl-CoA and CO2

• The reaction is catalyzed by acetyl-CoA

carboxylase (ACC)

• ACC is a bifunctional enzyme

– Biotin carboxylase

– Transcarboxylase

• ACC contains biotin, nature’s carrier of CO2

– Biotin shuttles between the two active sites

Synthesis of Malonyl-CoA (2)

• Bicarbonate reacts with the terminal phosphate

of ATP to give carbamoyl phosphate

• Biotin carries out a nucleophilic attack to

carbamoyl phosphate

• The product is a good donor of a carboxylate group

Synthesis of Malonyl-CoA (3)

• The arm swing moves carboxybiotin to the

transcarboxylase site

• Terminal methyl of acetyl-CoA probably

deprotonates to give a resonance-stabilized

carbanion

• The carbanion picks up the carboxylate moiety from biotin

Fatty Acid Synthesis

• Overall goal is to attach a two-carbon acetate unit from malonyl-CoA to a growing chain and then reduce it

• Reaction involves cycles of four enzyme-catalyzed steps

– Condensation of the growing chain with activated acetate

– Reduction of carbonyl to hydroxyl

– Dehydration of alcohol to trans-alkene

– Reduction of alkene to alkane

• The growing chain is initially attached to the enzyme via

Acyl Carrier Protein

• Contains a covalently attached prothetic group phospho-pantethiene

4’-• The acyl carrier protein delivers acetate (in the

first step) or malonate (in all the next steps) to the fatty acid synthase

• The acyl carrier protein shuttles the growing chain from one active site to another during the four-

step reaction

Charging the Acyl Carrier Protein

and Fatty Acid Synthase

• Two thiols participate in the fatty acid synthesis

– Thiol from 4-phosphopantethine in acyl carrier protein – Thiol from cysteine in fatty acid synthase

• Both thiols must be charged for the condensation reaction to occur

– In the first step, acetyl from acetyl-CoA is transferred to acyl carrier protein

– Acyl carrier protein passes this acetate to fatty acid

synthase

– Acyl carrier protein is then re-charged with malonyl

from malonyl-CoA

Assimilation of Two-Carbon Units Condensation and First Reduction

Assimilation of Two-Carbon Units Dehydration and Second Reduction

Fatty Acid Synthase in Animals

and Fungi is a Large Multifunctional Polypeptide

Enzymatic Activities in Fatty Acid

• Reduction of alkene to alkane

– enoyl-ACP reductase (ER)

• Chain transfer

– Malonyl/acetyl-CoA ACP transferase

Sequence of Events in Synthesis

of Fatty Acids

Regulation of Fatty Acid Synthesis

in Vertebrates

Insulin in Regulation of Fatty Acid

Synthesis

Synthesis of Unsaturated Fatty

Acids

• Animals can readily introduce one double bond to palmitate and stearate

• Vertebrates cannot introduce additional double

bonds between C10 and methyl-terminal

• We must obtain linoleate and -linolenate with

diet; these are essential fatty acids

• Plants, algae, and some insects synthesize

linoleate from oleate

Vertebrate Fatty Acyl Desaturase

is Non-Heme Iron-Containing

Mixed Function Oxidase

• O2 accepts four electrons from two substrates

• Two electrons come from saturated fatty acid

• Two electrons come from ferrous state of

Cytochrome b5

Oxidases, Monooxygenase,

Dioxygenase

• Molecular oxygen can serve as an electron acceptor

• Oxidases do not incorporate oxygen atoms into the

organic product

– Oxygen atoms usually end up in hydrogen peroxide

– Often use flavin as redox cofactors

• Monooxygenases incorporate one of the oxygen atoms into the product

– The other oxygen ends up in water

– Often use iron as redox cofactor

• Dioxygenases incorporate both oxygen atoms into the organic product

Dioxygenase Reaction: Example

• Converted to starch in the chloroplast

• Converted to sucrose for export

• Recycled to ribulose 1,5-bisphosphate

Iron Cofactors in Mono-oxygenases

• Single iron in the heme ring

– Found in the cytochrome P-450 family

– Heme ring stabilizes radicals intermediates

• Di-iron-oxo bridged clusters

– Well-studies in methane monooxygenase

– Not so well understood in fatty acid desaturases

– Co-substrate Cyt b5 reduces di-iron Fe3+ to di-iron Fe 2+

– O2 acquires two electrons from di-iron Fe 2+ and forms a Fe2(iV) O2intermediate that oxidizes the fatty acid

Phosphatidylcholine-bound Oleate

Acts as A Substrate for Plant

Desturases

Biosynthesis of Cholesterol

Formation of Mevalonate

• HMG-CoA reductase is a target for some cardiovascular drugs

Formation of Squalene

• Pyrophosphate is a good leaving group in these nucleophilic substitution reactions

Regulation of Cholesterol

Biosynthesis

• Insulin means “we have plenty of glucose”

• Glucagon means “we are out of glucose”

Chapter 21: Summary

• Malonyl-CoA is an important precursor for biosynthesis of

fatty acids

• Fatty acid synthesis is carried out by a large enzyme that

contains multiple catalytic activities needed for the

condensation, and subsequent reduction of acetate units

• Not all organisms can synthesize polyunsaturated fatty acids; these that can utilize mixed function oxidases as desaturase

• Cholesterol biosynthesis starts with synthesis of mevalonate from acetate; mevalonate yields two activated isoprenes;

series of isoprene condensation steps gives squalene;

oxidation and ring closure of squalene gives cholesterol

In this chapter, we learned that: