Lecture 25-Fatty Acid Synthesis and Regulation of Fatty Acid Metabolism

Intermediates in fatty acid synthesis are covalently linked to the sulfhydryl groups of an ____, whereas intermediates in fatty acid breakdown are covalently attached to the sulfhydryl group of ___

acyl carrier protein (ACP)

coenzyme A

The enzymes of fatty acid synthesis in higher organisms are joined in a single polypeptide chain called ___. In contrast, the degradative enzymes are ___

fatty acid synthase

not linked covalently

The growing fatty acid chain is elongated by the sequential addition of 2-carbon units derived from acetyl CoA. The activated donor of two-carbon units in the elongation step is ____.The elongation reaction is driven by the release of CO 2 .Degradation releases 2-carbon units as ____

malonyl ACP

acetyl CoA

The reductant in fatty acid synthesis is ____, whereas the oxidants in fatty acid degradation are____

NADPH

NAD+ and FAD

The isomeric form of the hydroxyacyl intermediate in degradation is the ____, while the____is used in synthesis

L isoform

D isoform

where does fatty acid synthesis occur?

Fatty acid synthesis takes place in the cytoplasm

where does fatty acid oxidation occur?

takes place primarily in the mitochondrial matrix

what is the rate determining step of fatty acid synthesis is catalyzed by

acetyl-CoA carboxylase

Outline the steps of how to synthesize two specific saturated fatty acids, 16:0 and 17:0 using fatty acid synthesis

16:0 (palmitate): made from 1 acetyl-CoA primer + 7 malonyl-CoA → 7 cycles

17:0 (margarate): made from 1 propionyl-CoA primer + 7 malonyl-CoA → 7 cycles

Both occur in the cytosol via the fatty acid synthase complex, consuming ATP and NADPH.

what are the 6 enzymes in fatty acid sythesis?

Acetyl CoA carboxylase

Acetyl transacylase

Malonyl transacylase

β-Ketoacyl synthase

β-Ketoacyl reductase

β-Hydroxyacyl dehydratase

Enoyl reductase

Acetyl CoA carboxylase

products

reactants (including cofactors & coenzymes)

name the enzyme (activity) that catalyzes the step

classify the enzymatic step into the 6 enzyme classes

determine if the step is reversible or irreversible

Acetyl-ACP + CoA

Acetyl-CoA + ACP

Acetyl transacylase (AT)

Transferase

Reversible

Acetyl transacylase

products

reactants (including cofactors & coenzymes)

name the enzyme (activity) that catalyzes the step

classify the enzymatic step into the 6 enzyme classes

determine if the step is reversible or irreversible

Reactants: Acetyl-CoA, Acyl Carrier Protein (ACP) (contains a prosthetic group — 4'-phosphopantetheine)

Cofactors/coenzymes: None required

Products: Acetyl-ACP, Coenzyme A (CoA-SH)

enzyme: Acetyl-CoA-ACP transacylase

Transferase

Reversible

Malonyl transacylase

products

reactants (including cofactors & coenzymes)

name the enzyme (activity) that catalyzes the step

classify the enzymatic step into the 6 enzyme classes

determine if the step is reversible or irreversible

Malonyl-ACP + CoA

Malonyl-CoA + ACP

Malonyl transacylase (MT)

Transferase

Reversible

β-Ketoacyl synthase

products

reactants (including cofactors & coenzymes)

name the enzyme (activity) that catalyzes the step

classify the enzymatic step into the 6 enzyme classes

determine if the step is reversible or irreversible

β-Ketoacyl-ACP (acetoacetyl-ACP for 1st round) + ACP + CO₂

Acetyl-ACP (or acyl-ACP) + Malonyl-ACP → CO₂ release

β-Ketoacyl-ACP synthase (KS)

Lyase

Irreversible

β-Ketoacyl reductase

products

reactants (including cofactors & coenzymes)

name the enzyme (activity) that catalyzes the step

classify the enzymatic step into the 6 enzyme classes

determine if the step is reversible or irreversible

β-Hydroxyacyl-ACP + NADP⁺

β-Ketoacyl-ACP + NADPH + H⁺

β-Ketoacyl-ACP reductase (KR)

Oxidoreductase

Reversible

β-Hydroxyacyl dehydratase

products

reactants (including cofactors & coenzymes)

name the enzyme (activity) that catalyzes the step

classify the enzymatic step into the 6 enzyme classes

determine if the step is reversible or irreversible

Reactants: β-Hydroxyacyl-ACP (also called 3-hydroxyacyl-ACP; e.g., β-hydroxybutyryl-ACP in the first round)

Cofactors/Coenzymes:None required

2. Products: Enoyl-ACP (specifically trans-Δ²-enoyl-ACP), H₂O (water)

enzyme: β-Hydroxyacyl-ACP Dehydratase (HAD)

Lyase

reversible

Enoyl reductase

products

reactants (including cofactors & coenzymes)

name the enzyme (activity) that catalyzes the step

classify the enzymatic step into the 6 enzyme classes

determine if the step is reversible or irreversible

Saturated Acyl-ACP + NADP⁺

trans-2-Enoyl-ACP + NADPH + H⁺

Enoyl-ACP reductase (ER)

Oxidoreductase

Reversible

Compare the energy requirements of the synthesis of palmitate, a 16-carbon saturated fatty acid, with the amount of ATP equivalents that can be generated by the complete oxidation of palmitate by fatty acid oxidation

the stoichiometry for the synthesis of palmitate is

Acetyl CoA + 7 malonyl CoA + 14 NADPH + 20 H + →palmitate + 7 CO 2 + 14 NADP + + 8 CoA + 6 H 2O

• The synthesis of the required malonyl CoA is

7 Acetyl CoA + 7 CO 2 + 7 ATP →7 malonyl CoA + 7 ADP + 7 P i + 14 H +

• The overall stoichiometry for the synthesis of palmitate is

8 Acetyl CoA + 7 ATP + 14 NADPH + 6 H + →palmitate + 14 NADP + + 8 CoA + 6 H 2O + 7 ADP + 7 P i

Fatty acid synthesis is very expensive interms of energy requirement

Describe how the substrates for fatty acid synthesis, namely acetyl groups and NADPH, are obtained.

Acetyl groups for fatty acid synthesis originate from acetyl-CoA, which is transported from the mitochondria to the cytosol via the citrate shuttle. NADPH is generated from two main sources: the pentose phosphate pathway and the malic enzyme acting on malate produced from the citrate shuttle.

The transport of acetyl groups to the cytosol involves exporting citrate, while transporting pyruvate into the mitochondrion

NADPH is produced by the oxidative phase of the pentose phosphate pathway and bymalic enzyme, also an important anapleuretic reaction for the citric acid cycle

Describe, in general, the processes by which elongation and unsaturation of fatty acids occurs after palmitate is synthesized and explain why we need to obtain some fatty acids from our diet

Common starting fatty acids for unsaturated fatty acids in mammals are derived from palmitoleate, oleate, linoleate, linolenate

Mammals lack the enzymes to introduce double bonds at carbon atoms beyond C-9 in the fatty acid chain. Linoleate and linolenate are essential fatty acids because mammals cannot synthesize them. Therefore, they must be supplied in the diet.

Eicosanoid hormones are derived frompolyunsaturated fatty acids

Explain how fatty acid synthesis and degradation are reciprocally regulated.

Fatty acid metabolism is stringently controlled so that synthesis and degradation are highly responsive to physiological needs. Fatty acid synthesis is maximal when carbohydrates and energy are plentiful

Acetyl CoA carboxylase is regulated byconditions in the cell - hormonal control

Acetyl CoA carboxylase (ACC) is subject to regulation on several levels. For example,acetyl CoA carboxylase is inhibited when phosphorylated by AMP-dependent kinase(AMPK). It is reversed by protein phosphatase 2A. Insulin activates the phosphatase,and glucagon activates the kinase.

Acetyl CoA carboxylase is regulated byconditions in the cell - local control

Acetyl CoA carboxylase is also subject to local regulation. For example, palmitoyl CoA,the end product of fatty acid synthase, inhibits acetyl CoA carboxylase by causingdepolymerization of the enzyme

Acetyl CoA carboxylase also plays a role in the regulation of fatty acid degradation

Acetyl CoA carboxylase inhibits fatty acid degradation because its product, malonyl CoA, prevents the entry of fatty acyl CoA into the mitochondria by inhibiting carnitine acyl transferase 1.