Fatty Acid Synthesis

T/F: lipids are water-insoluble organic molecules

true

Lipids exist in the following forms:

• triacylglycerols (TAG)

  • triglycerides (TG)

  • TAGS are esters of glycerol

• fatty acids (not coupled to ester)

• phospholipids

• steroids

  • cholesterol

• glycolipids

What are the functions of lipids?

• energy storage

• constituents of membranes

• anchors for membrane proteins

• cofactors for enzymes

• signaling molecules

• pigments

• detergents

• transporters

• antioxidants

Describe the process of lipid digestion, the enzymes involved and their location of action

• ingested lipids are in the form of TAGS

• remainders include:

  • unesterified “free” fatty acids

  • cholesterol and its esters

  • phospholipids

Dietary lipids are broken down by ____

lipases

• lingual lipase

• gastric lipase

• pancreatic lipase

Where does emulsification of lipids by bile salts occur? How?

• duodenum

  • micelles

  • bile salts are synthesized in the liver + stored in gallbladder

Describe the anabolism of fatty acids

• requires acetly-CoA and malonyl-CoA

• requires NADPH

• cytosol → animals

• chloroplast → plants

Describe the catabolism of fatty acids

• produces acetyl-CoA

• produces NADH + FADH2

• mitochondria

What is the role of Fatty Acid Synthase I (FAS I)?

• rxns occur in the ACP (acyl carrier protein) domain of FAS I

• uses 2 enzyme bound thiol groups as activating groups

• catalyzes a repeating 4 step cycle that elongates the fatty acyl chain

  • chain grows by 2 carbons after 1 turn of the cycle

  • uses NADPH as the electron donor

T/F: ACP serves as a shuttle in fatty acid synthesis

true

What are the roles of ACP?

• contains a covalently attached prosthetic group → 4’-phosphopantetheine

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

• delivers acetate or malonate to the FAS

• shuttles the growing chain within the active site of FAS I during the 4-step rxn

Describe the prep step

• 1. Chain transfer and charging ACP and FAS I w/ Acyl Groups

  • 2 thiols charged w/ the correct acyl groups BEFORE the condensation rxn

  2. acetate fragment of acetyl CoA is transferred to ACP

  • catalyzed by Acetyl CoA-ACP acetyltransferase

  3. ACP passes acetate to the Cys-SH of KS domain of FAS I

  4. ACP-SH group is recharged w/ malonyl from malonyl-CoA

  • catalyzed by Malonyl CoA-ACP transacylase

What are the correct acyl groups for each thiol in ACP and FAS I?

• thiol from 4-phosphopantetheine in ACP

• thiol from Cys in FAS I

What is the KS domain?

beta-ketoacyl-ACP synthase

Describe step 1 of FA synthesis: Condensation

• condensation rxn attaches acetyl group to malonyl group

  • occurs w/ simultaneous release of CO2

  • activates malonyl group to attack the acetyl carbonyl

    • creates acetoacetyl-ACP (beta-keto intermediate)

• catalyzed by beta-ketoacyl-ACP synthase

• decarboxylation of malonyl-CoA makes the rxn energetically favorable

What enzyme catalyzes the condensation reaction?

beta-ketoacyl-ACP synthase

Describe step 2 of FA synthesis: Reduction

• Reduction #1

• NADPH reduces the beta-keto intermediate to an alc

• ketone carbon reduced to form an alcohol → beta-hydroxybutyryl-ACP

• NADPH is an e- doner

• catalyzed by beta-ketoacyl-ACP reductase (KR)

What is the enzyme that catalyzes reduction #1?

beta-ketoacyl-ACP reductase

What is the product of reduction #1?

beta-hydroxybutyryl-ACP

Describe step 3 of FA synthesis

• dehydration:

  • OH group and H are eliminated → double bond created

• OH and H removed from beta-hydroxybutyryl-ACP

  → trans-delta²-butenoyl-ACP

• catalyzed by beta-hydroxyacyl-ACP dehydratase (DH)

What is the product of the dehydration reaction? What enzyme catalyzes this step?

• product: trans-delta²-butenoyl-ACP

• enzyme: beta-hydroxyacyl-ACP dehydratase (DH)

Describe step 4 of FA synthesis

• Reduction #2

  • NADPH reduces double bond to yield saturated alkene

• NADPH is the e-donor to reduce double bond of trans-delta²-butenoyl-ACP

  → butyryl-ACP

• catalyzed by enoyl-ACP reductase (ER)

What is the final result of FA synthesis?

• increases chain length of the acyl group of the FA by 2 carbons

What is the product of reduction #2? What enzyme catalyzes this step?

• product: butryl-ACP

• enzyme: enoyl-ACP reductase (ER)

Which molecule is formed in the end of the first elongation cycle of fatty acid synthesis?

butyryl ACP

Which enzyme determines fatty acid chain length in fatty acid synthesis?

thioesterase

What catalyzes the formation of 3-hydroxybutyryl ACP from acetoacetyl ACP in fatty acid synthesis?

beta-ketoacyl reductase

What catalyzes the formation of malonyl ACP in fatty acid synthesis?

malonyl transacylase

What catalyzes the formation of acetyl ACP in fatty acid synthesis?

acetyl transacylase

How many energy-rich molecules are needed for one elongation cycle of fatty acid synthesis?

2 NADH

The carrier of an acyl chain in fatty acid synthase during the process of fatty acid synthesis is:

ACP domain

What catalyzes the formation of malonyl CoA in fatty acid synthesis?

acetyl CoA carboxylase

What is the reaction product of the condensation of acetyl ACP and malonyl ACP in fatty acid synthesis?

acetoacetyl ACP