BIOL 1510 Chapter 21

Is fatty acid synthesis the reverse of breakdown?

No!

Committed step of fatty acid synthesis

Malonyl-CoA synthesis

What enzyme synthesizes malonyl-CoA?

Acetyl-CoA Carboxylase; from CO2 and acetyl-CoA

Necessary cofactor for Acetyl-CoA Carboxylase

Biotin

Two domains of Acetyl-CoA Carboxylase

transcarboxylase domain, biotin carboxylase domain

Steps of malonyl-CoA synthesis

1. CO2 binds to biotin carboxylase domain

2. Biotin arm swings from biotin carboxylase to transcarboxylase domain

3. acetyl-CoA binds to transcarboxylase domain

4. Malonyl-CoA is formed

Enzyme responsible for fatty acid synthesis

Fatty acid synthase

Steps of fatty acid synthesis

1. acetyl group from Acetyl-CoA is attached onto fatty acid synthase (FAS)

2. Acyl-carrier protein (ACP) binds malonyl CoA and associates with FAS

3. Condensation occurs between malonyl-CoA and acetyl group, making a carbonyl and giving off CO2

4. Carbonyl is reduced to OH

5. OH dehydrated to double bond via the removal of water

6. double bond is reduced to a single bond

Structure of acyl-carrier protein (ACP)

Long chain with thiol (SH) group on the end

Where on ACP is malonyl-CoA attached to?

thiol group

What is needed for the reduction steps of fatty acid synthesis?

NADPH→oxidized to NADP+

How many carbon units are added per elongation cycle?

Add two carbons at a time

What fatty acid product is given off after fatty acid synthesis?

palmitate→C16 fatty acid

Structure of mammalian vs bacterial FAS

Mammalian: all one polypeptide

Bacterial: multiple polypeptides that associate into a complex

Overall reaction of fatty acid synthesis

8 Acetyl-CoA + 7 ATP + 14 NADPH + 15 H+ → palmitate + 8 CoA + 8 H2O + 7 ADP + 7 Pi + 14 NADP+

Sources of NADPH in animals

• generated from the PPP

• generated from the decarboxylation of malate to pyruvate

Enzyme that does decarboxylation of malate to pyruvate

malic enzyme→gives off CO2 and NADPH

Where is NADPH made in animals and how does that connect to site of fatty acid synthesis?

NAPDH is made in the cytosol, and fatty acid synthesis also occurs in the cytosol

How is NADPH made in plants?

Made by the light reactions of photosynthesis

Where is NADPH made in plants and how does that connect to site of fatty acid synthesis?

NADPH made in chloroplasts and fatty acid synthesis also occurs in chloroplasts

Compartmentalization problem of fatty acid synthesis

Need acetyl groups but acetyl-CoA is in the mitochondria!

What is the source of most acetyl groups?

The decarboxylation of pyruvate to Acetyl-CoA by PDH complex

Ways to get acetyl groups from the mitochondria into the cytosol

1. condense OAA with acetyl-CoA to get citrate

2. some citrate enters TCA cycle

3. some citrate is exported out to the cytosol via a citrate transporter

4. citrate lyase cleaves citrate to OAA and Acetyl-CoA (goes off to fatty acid synthesis)

Fate of OAA after cleavage from citrate

1. converted to malate

2. malate can be converted to pyruvate via malic enzyme, producing NADPH

3. pyruvate enters mitochondria via pyruvate transporter

4. malate can enter mitochondria directly through malate aspartate shuttle

Places of fatty acid synthesis regulation

• acetyl-CoA carboxylase (commited step)

• at the level of transport

Things that stimulate Acetyl-CoA carboxylase

citrate→signifies a lot of acetyl-CoA

Things that inhibit Acetyl-CoA carboxylase

• palmitoyl-coA (negative feedback that signifies that there is already a lot of fatty acids)

• glucagon, epinephrine, and high [AMP] (signify low energy state)

How glucagon, epinephrine, and high [AMP] negatively regulate acetyl-CoA carboxylase

trigger phosphorylation of the enzyme through GPCR mechanisms, which inactivates it

How malonyl-CoA affects fatty acid synthesis

high malonyl-CoA prevents transport of fatty acids into the mitochondria, thereby preventing fatty acid oxidation. As a result, fatty acid synthesis is favored

How do you make unsaturated fatty acids?

Use mixed function oxidase

oxidase action of mixed function oxidase

Uses O2 as the electron acceptor, but the oxygen atoms don’t end up in the product and are instead given off as water

“Mixed function” action of mixed function oxidase

oxidizes multiple things→NADPH to NADP+ AND fatty acyl-CoA

What types of fatty acids can be synthesized and unsaturated in animals?

C16 and C18 fatty acids

How do animals get C18 fatty acids?

Separate enzyme elongates palmitate

Where are fatty acid modifications done?

Smooth ER

Essential fatty acids

Those fatty acids that we cannot synthesize and therefore must get from diet

What types of fatty acids can we NOT make?

Linoleate and alpha-linoleate→precursors to omega-3 and omega-6 fatty acids

Function of omega-6 fatty acids

precursors to arachidonic acid, which is a precursor for eicosanoids, which are lipids that do important signaling functions (TXA2, prostaglandins)

What kind of omega3/omega6 ratio do you want?

High ratio

Precursor for triacylglycerols and phospholipids

phosphatidic acid

Synthesis of phosphatidic acid

Acyl-CoA synthetase adds 2 CoA-SH groups onto glycerol-3-phosphate in an ATP dependent manner to give phosphatidic acid

Synthesis of glycerol-3-phosphate

• comes from DHAP (intermediate of glycolysis)

• comes from glycerol (product of triglyceride product)

Enzyme that converts DHAP to glycerol-3-phosphate

glycerol-3-phosphate dehydrogenase

What is created as a byproduct of turning DHAP into glycerol-3-phosphate

NADH is turned into NAD+

Turning phosphatidic acid into triacylglycerols

phosphatidic acid→diacylglycerol→triacylglycerol

What enzyme turns diacylglycerols into triacylglycerols?

Acyl transferase→adds fatty acyl group and CoA-SH is given off as byproduct

What does acyl transferase need?

ATP

How is phosphatidic acid turned into glycerophospholipids?

Addition of a head group

Two strategies for attaching a head group to phosphatidic acid to make glycerophospholipids

• attach a CDP nucleotide to the phosphatidic acid, then attach the head group (bacteria)

• attach CDP nucleotide to the head group, then attach to phosphatidic acid (other species)

Purpose of attaching a nucleotide group

• gives more binding energy

• nucleotides are good leaving groups

Regulation of triacylglycerol synthesis

• done when you have high energy

• insulin stimulates triacylglycerol synthesis

Glyceroneogenesis

• an abbreviated version of gluconeogenesis

• makes glycerol for triacylglycerol synthesis

Is acetyl-CoA carboxylase a reversible or irreversible enzyme?

Irreversible

What number carbon of malonyl-CoA is decarboxylated in the first step of fatty acid synthesis?

C1→the CO2 that was added to acetyl-CoA

Does FAS require ATP?

Yes

Two fates of synthesize fatty acids

1. get incorporated into triacylglycerol

2. become membrane phospholipids

When would you want to turn fatty acids into triacylglycerol vs phospholipids?

During high energy state→triacylglycerol for storage

During periods of growth→membrane phospholipids