Lecture 13: Fatty Acid Biosynthesis

Front 1

Describe the general structure of a fatty acid

Back 1

alkyl chain with terminal carboxyl group
saturated species=CH3-(CH2)n-COOH

Front 2

Describe the features of the double bonds found in the unsaturated fatty acids that occur common in man

Back 2

-up to six double bonds per chain
-almost always cis
-always separated by one CH2 group (no conjugated bonds)
-put a "kink" in the chain

Front 3

Expalin the two different naming systems for fatty acids

Back 3

1. numbered from the carboxyl end, length:number of doubles(position)
2.Numbered from the omega end (omega-position of bond from omega end)
note the omega end is farthest from the carboxyl group

Front 4

What is the structure of palmitic acid

Back 4

16:0

Front 5

What is the structure of palmitoleic acid

Back 5

16:1(9)

Front 6

What is the structure of stearic acic

Back 6

18:0

Front 7

What is the structure of oleic acid

Back 7

18:1(9)

Front 8

what is the structure of arachidonic acid

Back 8

20:4( 5,8,11,14)

Front 9

What are the essential fatty acids

Back 9

linoleic 18:2 (9,12) and a-linolenic 18:3(9,12,15)

technically all omega 3 and omgea 6 are essential but the longer ones can be made from the previous two

Front 10

How does the structure of a fatty acid affect its melting point?

Back 10

-longer chain=higer melting
-more double bonds=lower melting
this is important for controlding membrane fluidity

Front 11

what is the first FA synthesized (the one from which all other non-essential fatty acids are made)

Back 11

palmitic acid 16:0

Front 12

What is the source of all of the carbons for fatty acids

Back 12

acetyl CoA, the major source of which is PDH complex rxn which occurs in the matrix

Front 13

Where does FA biosynthesis occur? What problem does this present and how is it solved?

Back 13

FA synthesis occurs in the cytosol but the PDH rxn which supplies acetyl-CoA occurs in the matrix.Acetyl CoA is shuttled into the cytosol by converting it to citrate (tricaboxylate transporter)

Front 14

Explain how the tricaboxylate transporter works

Back 14

Purpose:Move acetyl CoA from PDH rxn in matrix to cytosol
1. Acetyl CoA combines with OAA to form citrate via citrate synthase
2.Citrate is exported to the cytosol
3. In the cytosol, citrate is converted back to OAA +acetyl CoA via ATP citrate lyase
4. acetyl coA goes on to FA biosyn, OAA gets recycled (malate=>pyruvate)

Front 15

Why is ATP required in the ATP citrate lyase rxn

Back 15

citrate=> OAA +acetyl-CoA requires the formation of a high energy thioester bond

Front 16

What is the commited step of FA biosynthesis (sequesters the acetyl CoA)

Back 16

acetyl-CoA carboxylase, converts acetyl-CoA to malonyl CoA

Front 17

describe the Acetyl-CoA carboxylase rxn. What are the substrates, products, and cofactors?

Back 17

This is the commited step of FA biosynthesis
acetyl CoA +bicarbonate=>malonyl CoA, bition is a cofactor and ATP is used.

Front 18

What 5 reactions occur in a cycle of fatty acid synthesis

Back 18

1. transacetylation
2. condensation
3. reduction
4. dehydration
5. reduction

Front 19

Describe the first step of FA biosynthesis via FAS

Back 19

Transacetylation
MalonylCoA and Acetyl CoA are linked to ACP via a phosphopantethine group. This forms acetyl-ACP and malonyl-ACP
(catalyzed by acetylCO:ACP transacylase and malonlCoA:ACP transacylase)

Front 20

Describe the second step of FA biosynthesis after the transaceylation to ACP

Back 20

acetyl-ACP (2 carbons) and malonyl-ACP (3 carbons) condense to form 4 carbon acetoacetyl-ACP. CO2 is lost driving the reaction forward

Front 21

Describe the third step of FA biosynthesis after the condensation to form acetylacetyl-ACP

Back 21

The first reduction
The C3 (from the ACP end) is reduced from a ketone to an alcohol

Front 22

Describe the 4th step of FA biosynthesis after the reduction of C3 from a ketone to an alcohol

Back 22

The C3 alcohol is dehydrated producing a TRANS double bond between C2 and C3 (the carbon closest to ACP is C1)

Front 23

Describe the 5th step of FA biosynthesis after the dehydration reaction

Back 23

Second Reduction
The double bond between C2 and C3 is reduced to a single bond

Front 24

Each two carbon unit added to an FA chain comes in as....

Back 24

malonyl-ACP
(every acetyl CoA unit must first be activated by Acetyl-CoA carboxylase)

Front 25

Elongation of FA biosynthesis continues for a total of 7 rounds to form palmitic acid 16:0. How is the chain reaction terminated once this occurs

Back 25

C16 acyl-ACP is a substrate for thioesterase which hydrolyzes the molecue to form palmitate and ACP

Front 26

What is the net reaction of the formation of palmitate

Back 26

8 acetyl CoA+ 7 ATP +14 NAPDH + 14H+=> palmitate + 8CoASH +7ADP+7Pi+14 NADP +6H2O

Front 27

In the synthesis of palmitate (16:0), 8 acetyl CoA are used but only 7 ATP are consumed. Why is this

Back 27

The ATP is consumed during the activation step of acetyl-CoA=>malonyl-CoA. During the very fist addition to the chain, 1 of the reactants is just acetyl CoA so not ATP is needed.

Front 28

In the synthesis of palmitate 16:0, 8 acteyl CoA and 7 ATP's are used but only 6 waters are formed? Give that 7 hydrations occur why is 1 water missing?

Back 28

thioesterase consumes a water when hydrolyzing the C16-acyl-ACP

Front 29

Describe the "formula" to figure out how many acetyl-CoA's, ATP's, waters, and NADPH's are used based on the number of carbons in the chain

Back 29

acetyl CoA=#carbons/2
ATP= (#carbons/2)-1
NADPH= ((#carbons/2)-1)x2
H2O= (#carbons/2)-2
ex:palmitate 16:0
8 acetyl CoA, 7 ATP, 14 NADPH, 6 H2O

Front 30

What are two advantages of using a muti enzyme complex for FA synthesis

Back 30

1. direct transfer of intermediates between sites=more efficient
2. coordiante transcriptional regulation (1 mRNA, many enzymes)

Front 31

where do FA elongation reactions occur

Back 31

cytosolic face of endoplasmic reticulum and inside the mitochondria

Front 32

Compare the mitochondrial and ER fa elongation reactions in terms of
-carbon donor
-reducing agent

Back 32

mitochondrial=acetyl CoA, NADPH and NADH,elongates shorter FAs
ER= malonyl CoA, NADPH only, produces sterate (18:0) almost exclusivley

Front 33

Where are unsaturated fatty acids made? What is the commited step

Back 33

Made in the ER, commited steop is the introduction of the first double bond by steraroyl-CoA desaturase between C9 and 10 of palmitic or stearic acid (producing palmitoleic and oleic respectivley

Front 34

What enzyme catalyzes desaturation events in the ER? What is unique about where the bond is added

Back 34

Stearoyl-CoA desaturase adds a cis double bond between C9 and 10 of palmitic or stearic acid. There must be at least six carbons on the omgea end of the alkyl chain for this enzyme to work

Front 35

Why are there always three carbons between double bonds in a polyunsaturated fatty acid

Back 35

A two carbon elongation accompanies the desaturation even

Front 36

Why specifically are linoleic acid and a-linolenic acids esstential. What are they used for

Back 36

linoleic (18:2(9,12)) and a-linolenic (18:3(9,12,15)) are essential because they contain double bonds within six carbons of the omega end. Steroyl-CoA desaturase cannot desaturate unless there are at least 6 carbons from the omega end. These essential acid are staring points for other polyunsaturated acids suc as arachidonic which is a precursor of ecosanoid hormones.

Front 37

What is the basic structure of triglycerides?

Back 37

three molecules of fatty acids esterified through their carboxyl groups to a molecule of glycerol

Front 38

What is the first step of TG synthesis

Back 38

activation
FA+ATP+CoASH=>FA-CoA +AMP +PPi+ H2O
via aceyl-CoA synthetase

Front 39

Where does the backbone of a TG come from

Back 39

reduction of DHAP to glycerol-3-phosphate

Front 40

Describe the formation of a TG after the FA's have been activated by acyl-CoA synthetase

Back 40

The activated FAs are esterified to C1 and C2 hydroxls of glycero phosphate to yield phosphatidic acid, this is catalyzed by glycerol phosphate acyltransferase. Usually C1 is saturated and C2 is unsaturaed. The phsopahte group is then removed and a third activated FA is added.

Front 41

How is FA biosynthesis regulated in the short term

Back 41

Citrate allosterically stimulates inactive (phosphorylated) acetyl CoA carboxylase (commited step of synthesis) but hsa little effect on the active (dephosphorylated) enzyme.
The overall effect is a partially active caboxylase.

Front 42

Describe how FA biosynthesis is regulated in the "intermediate term"

Back 42

Phosphorylation events
-acetyl CoA carboxylase (acetyl CoA=>malonyl CoA) is active when DEphosphorylated and inactive when phosphorylated

Front 43

What enzyme regulates the phosphorylation of acetyl CoA carboxylase converting it from the active to the inactive form

Back 43

AMP-activated protein kinase (AMPK)

Front 44

What enzyme regulates the dephosphorylation of acetyl CoA carboxylase converting the inactive enzyme into the active form

Back 44

Protein Phosphatase 2A converts the inactive acetyl CoA carboxylase into the activated (dephosphorylated) form

Front 45

How is the activity AMPK regulated? What affect does this have on acetyl-CoA carboxylase

Back 45

AMPK is the "fuel guage" because it is activated by AMP and inhibited by ATP. When AMP is high the cell is low in energy. Low AMP activates AMPK which adds a phosphate group to acetyl CoA carboxylase. Phosphorylated acetyl CoA carboxylase is inactive. This makes sense because we don't want to make FA's when the cell is low in energy

Front 46

How is the actvity of protein phosphatase 2A regulated? What affect does this have on acetyl-CoA carboxylase

Back 46

Protein phosphatase 2A is under hormonal control. It is inactive when phosphorylated by PKA. PKA is on when glucagon is present which indicates a fasting state. When protein phosphatase is inactive, acetyl CoA carboxylase remains phosphorylated and inactive. This is good because when we are fasting we don't want to make FAs

Front 47

Describe the relative activites of AMPK and Protein phosphatase 2A in the Fed state

Back 47

In the fed state AMP is low and ATP is high. AMPK is inactive.
PKA is not on (because no glucagon) so protein phsophatase is active.
The combined affects lead to a dephosphorylated and active acetyl CoA carboxylase

Front 48

How is FA biosynthesis regulated in the long term

Back 48

regulated by changes in the rates of protein synthesis and degradation that alter the number of enzyme molecules

Front 49

What is the source of NADPH for FA biosynthesis

Back 49

PPP

Front 50

What effects does fasting have on acetyl CoA carboxylase activity? What are the mechanistic bases for these effects?

Back 50

Decreased activity. Initially, this is due to AMPK-mediated phosphorylation (also reinforced by PKA phosphorylation of phosphatase 2A, which inactivates this phosphatase, thus preventing it from reversing the effect of AMPK). Later, a decrease in gene expression results in a further decrease in activity based on fewer molecules of enzyme.

Front 51

Provide a metabolic explanation for why citrate is a logical short-term regulator of acetyl CoA carboxylase? HINT: consider isocitrate dehydrogenase regulation.

Back 51

Citrate is made in the TCA cycle. The NAD+-linked isocitrate dehydrogenase, often considered the key regulatory enzyme of the TCA cycle, is stimulated by ADP and NAD+ and inhibited by ATP and NADH. Thus, under the high-energy conditions that are conducive for fatty acid synthesis, the activity of this enzyme, and consequently of the TCA cycle, is inhibited. This causes the build-up of isocitrate, which is converted back to citrate by the reversible reactions catalyzed by aconitase. This citrate is then transported out of the mitochondria into the cytosol, where it can activate acetyl CoA carboxylase.

(Once in the cytosol, citrate is cleaved by ATP citrate lyase into acetyl CoA and OAA to begin FA biosynthesis)