lipid metabolism

Question 1

fatty acids are stored in what form

Answer 1

in the form of Triacylglycerols

Question 2

how are triacylglycerols formed

Answer 2

Condensation reaction combines 1 Glycerol and 3 Fatty Acids

• Hydroxyl group (-OH)
• Carboxyl group (-COOH)

Question 3

triacylglycerols act as the body’s what

Answer 3

major energy reserve

When hydrolysed, fatty acids released can yield 9kcal/g

Carbohydrates and amino acids only yield 4kcal/g

Question 4

how are Fatty Acids released from Triacylglycerols

Answer 4

Requires hydrolysis

• Process is initiated by Adipose triglyceride lipase and hormone sensitive lipase, which remove fatty acids from C1 and C3 of the glycerol backbone.​
• The remaining fatty acid at C2 is removed by monoacylglycerol lipase

Question 5

explain Adipose Triglyceride Lipase

Answer 5

504 amino acids in length with a weight of 55 kDa – so is a big protein

The rate limiting step of Triacylglycerol breakdown- so why its at the beginning
- Occurs on lipid droplets and in the cytosol

Produces either 2,3-diacylglycerol or 1,3-diacylglycerol
- Catalytic activity is strongly enhanced by CGI-58

Question 6

explain hormone sensitive lipase

Answer 6

786 amino acids with a molecular weight of 85.5 kDa
- Cytosolic

Hydrolysis occurs in 2 steps;
- Cleaves the covalent ester bond between glycerol and the fatty acid
- Water displaces the covalent intermediate

Cleaves the 1- and 3- ester bonds 3-4-fold faster than the 2- ester bonds

Question 7

explain Monoacylglycerol Lipase

Answer 7

303 amino acids in length, 33 kDa
- Cytosolic

Hydrolyses 1- and 2- diacylglycerols at the same rate
- Other monoacylglycerol lipases, such as ABHD6, do show a preference

Found in many tissues and is thought to be a ‘housekeeping’ enzyme for lipid metabolism

Question 8

why can glycerol not be metabolised by adipocytes
so what happens instead

Answer 8

because they do not have glycerol kinase.

Instead, glycerol is transported in the blood to the liver which can then subject it to one of 3 possible fates.
​

All require production of glycerol 3-phosphate by glycerol kinase

Question 9

what is the fate of glycerol dependent upon

Answer 9

upon conversion of glycerol 3-phosphate to dihydroxyacetone phosphate by glycerol 3-phosphate dehydrogenase​

Question 10

what are the 2 possible fates of Dihydroxyacetone phosphate

Answer 10

Glucose or Pyruvate
as it is a glycolytic / gluconeogenic intermediate

Question 11

what can Glycerol 3-phosphate be used to produce

Answer 11

more triacylglycerides

The liver then stores this, but not in its raw state
- Instead, it will be stored as Very Low-Density Lipoprotein (VLDL)

Question 12

give a brief description of a lipoprotein

Answer 12

Lipoproteins are like the fat alternative for storing glycogen, its basically s membrane and stores fat

Question 13

what is the fat of free fatty acids

Answer 13

Free Fatty Acids (FFAs) move through the plasma membrane of the adipocyte into the bloodstream.​
- When in the blood, 99% of FFAs bind to albumin​​

Albumin has 7 binding sites for fatty acids – so concentrated the amount of fatty acids in one place

Only free FAs can enter cells​

Fatty acids are quite big

Question 14

Only free FAs can enter cells​ Depending upon their what
and enter by what

Answer 14

chain-length, FFAs can enter cells by either:​

• Passive diffusion (short (2-6 Cs) & medium chain (7-12 Cs)​
• Specialised FA transport proteins (Long chain (13-21+ Cs)​

Question 15

explain how fatty acids Cross the Plasma Membrane

Answer 15

The precise mechanisms of fatty acid transport remain an area of active research

1. small fatty acids can Cross by diffusion
2. CD36 (Fatty Acid Translocase) and/or FABPpm (Fatty Acid Binding Protein) increase the local concentration of fatty acids, increasing diffusion events
3. CD36 can act as a shuttle for fatty acids if bound to the cytosolic form of FABP
4. Fatty Acid Transport Protein (FATP) transport Fatty acids (10-15 carbons in length) through the membrane, where they activate Acyl-CoA synthetase (ACS1) to form Acyl-CoA (not acetyl-CoA) esters
5. Very Long Chain Fatty Acids (22+ Cs) are transported by FATPs (fatty acid transport proteins) and directly converted to Acy-CoA esters

Question 16

explain Fate of the Fatty Acids: Activation

Answer 16

Free Fatty Acids are toxic to cells – as amphipathic
- The COO- makes them amphipathic (hydrophilic and hydrophobic) and thus they would act like detergents if not neutralised
- CH3- (CH2)n- COO-
-This is achieved by Acyl CoA Synthase (also called Thiokinase – as sulfur of the coenzyme group)

The reaction is driven forward by a pyrophosphatase
- Seen before in Glycogen production
- Hydrolyses PPi into 2Pi

Once the Acyl-CoA has been formed it can follow one of two paths:
- Re-esterification into Triacylglycerols
- Metabolised more by Β-oxidation to the Citric Acid Cycle and Ketone Bodies

Question 17

explain the beta oxidation of fatty caids

Answer 17

The major oxidation pathway for fatty acids catabolism​

β-Oxidation takes place in the mitochondrial matrix​

Essentially, 2C fragments are removed, sequentially from the fatty acyl CoA producing acetyl CoA which can be oxidised in the citrate cycle.

Question 18

explain the β-oxidation of Fatty Acids: Transport into the mitochondria

Answer 18

This is the rate limiting step of FA oxidation as fatty acyl CoA molecules cannot diffuse into the mitochondrial

The passage of fatty acyl CoA from the cytosol into the mitochondrial matrix is mediated by a special transport system – The Carnitine Shuttle

• Carnitine replaces CoA in the fatty acyl CoA by carnitine palmitoyltransferase I (CPT I) to form an acyl carnitine.​
• Acyl carnitine is transported through the inner mitochondrial membrane by a translocase.​
• In the mitochondrial matrix, carnitine palmitoyltransferase II (CPT II) converts the acyl carnitine to fatty acyl CoA
• Fatty acyl CoA now undergoes β-Oxidation​
• Carnitine is transported back to the inner membrane space by the translocase for further use.

to summarise: CoA enzyme is broken off and then eventually added back on after enters mitochondrial matrix

Question 19

β-oxidation of Fatty Acids comprises of how many reactions ? and whats the net result

Answer 19

4

Net result is that fatty acid is shortened by two carbon atoms​

Question 20

Each β-Oxidation cycle produces what

Answer 20

1 acetyl CoA​

1 Fatty Acyl CoA (minus 2C)​

1 NADH​

1 FADH2

Question 21

explain step 1 of β-oxidation of Fatty Acids

Answer 21

acyl CoA dehydrogenase:
it is specific to chain length. Members of this enzyme family include long-chain, medium-chain, and short-chain acyl CoA dehydrogenases (LCAD), (MCAD), and (SCAD), respectively.

These enzymes catalyze the formation of a double bond between the alpha and beta carbons on acyl CoA molecules by removing two electrons to produce one molecule of FADH2, which eventually accounts for ~2 ATP molecules produced in the electron transport chain

The α-carbon refers to the first carbon atom that attaches to a functional group, such as a carbonyl. The second carbon atom is called the β-carbon

Question 22

explain step 2 of β-oxidation of Fatty Acids

Answer 22

Enoyl CoA hydratase,
performs a hydration step of the double bond between the alpha and beta carbons; this results in the addition of a hydroxyl (OH-) group to the beta carbon and a proton (H+) to the alpha carbon.

There is no energy production associated with this step

(basically u add water)

Question 23

explain step 3 of β-oxidation of Fatty Acids

Answer 23

3(β)-hydroxyl acyl CoA dehydrogenase; as the name implies, electrons and two protons are removed from the hydroxyl group, and the attached beta carbon to oxidize the beta carbon and produce a molecule of NADH.

Each molecule of NADH will result in the production of ~3 ATP molecules from the ETC

Question 24

explain step 4 of β-oxidation of Fatty Acids

Answer 24

3 (β)-keto thiolase

Cleavage of the bond between the alpha and beta carbon by CoA. ]

The reaction produces one molecule of acetyl CoA and a fatty acyl CoA that is two carbons shorter. The process may repeat until the even chain fatty acid has completely converted into acetyl CoA. (5 carbons in length)

Question 25

explain β-oxidation of unsaturated Fatty Acids

Answer 25

Unsaturated fatty acids, such as oleate (18:1) and linoleate (18:2), contain double bonds that must be isomerized (enoyl CoA isomerase) or reduced at the expense of an NADPH molecule (2,4-dienoyl CoA reductase)

Question 26

explain the β-oxidation of odd chain Fatty Acids

Answer 26

Odd-chain fatty acids undergo beta-oxidation in the same manner as even chain fatty acids Once a five-carbon chain remains, the final spiral of beta-oxidation will yield one molecule of acetyl CoA and one molecule of propionyl CoA. This three-carbon molecule can be enzymatically converted to succinyl CoA, forming a bridge between the TCA cycle and fatty acid oxidation.

Question 27

explain β-oxidation of Fatty Acids: High ATP yield

Answer 27

Produces more ATP so is why fats are important for energy

Question 28

what do ketone bodies include

Answer 28

Acetoacetate​ 3-hydroxybutyrate​ Acetone (not metabolisable)​

Question 29

where are ketone bodies synthesised

Answer 29

Synthesised in the mitochondria of liver cells, transported in the blood to peripheral tissues​

Question 30

when are ketone bodies produced

Answer 30

Produced during periods when the amount of acetyl CoA produced EXCEEDS the oxidative capacity of the liver: E.g. - During starvation​ - Increased FAs from adipose tissue​ - Oxidation in liver leads to an increased [Acetyl CoA]​ ([…] = concentration

Question 31

what do words in brackets [...] mean

Answer 31

concentration of word in brackets eg; [Acetyl CoA]​

Question 32

Acetyl CoAs can be joined together to form what

Answer 32

ketone bodies Fuel by skeletal muscle, cardiac muscle, kidney and about 70% of the brain (if pushed)​ Act as a ‘substitute’ for glucose for those tissues that can adapt themselves during starvation.

Question 33

explain the process of ketone bodies

Answer 33

Fatty acids are brought into the mitochondria via CPT-I and then broken down into acetyl CoA via beta-oxidation. Two acetyl-CoA molecules are converted into acetoacetyl-CoA via the enzyme thiolase; this is also known as acetyl coenzyme A acetyltransferase (ACAT). Acetoacetyl-CoA is converted to HMG-CoA via the enzyme HMG-CoA synthase. HMG-CoA lyase then converts HMG-CoA to acetoacetate. - HMG = β-Hydroxy β-methylglutaryl- Acetoacetate can be converted to either acetone through non-enzymatic decarboxylation or to 3(β)-hydroxybutyrate via 3(β)-hydroxybutyrate dehydrogenase.

Question 34

Liver cannot reconvert acetoacetate into what

Answer 34

acetoacetyl CoA and hence cannot use ketone bodies as a fuel.​

Question 35

all othe cells in body with mitochondria apart from liver can oxidise what

Answer 35

acetoacetate and 3-hydroxybutyrate​ 3-hydroxybutyrate is oxidised to acetoacetate by 3-hydroxybutyrate dehydrogenase​ Acetoacetate is converted back to acetyl-CoA via the enzyme beta-ketoacyl-CoA transferase and thiolase.

Question 36

in ketone bodies Acetone does not convert back to acetyl-CoA so what happens to it

Answer 36

it is either excreted through urine or exhaled.

Question 37

A large proportion of the FAs used by the body are supplied by what

Answer 37

the diet

Question 38

Excess amounts of -CHO and protein obtained from the diet can be converted to what and why

Answer 38

to fatty acids which in turn can be incorporated into TAGs.​

Question 39

In humans FA synthesis occurs primarily in what

Answer 39

liver and lactating mammary glands.​

Question 40

fatty acid synthesis process incorporates what

Answer 40

incorporates carbons from acetyl CoA into the growing fatty acid chain, utilising ATP and reduced nicotinamide adenine dinucleotide phosphate (NADPH).​ NADPH is produced in a pathway called the Pentose Phosphate Pathway.

Question 41

explain Fatty Acid Synthesis: Production of Cytosolic Acetyl-CoA

Answer 41

First step of FA synthesis is the transfer of acetyl units from mitochondrial acetyl CoA to the cytosol, forming cytosolic acetyl CoA CoA moiety cannot cross the mitochondrial membrane, but acetyl can This is achieved by the condensation of oxaloacetate and acetyl CoA to form citrate (TCA cycle) and when mitochondrial [citrate] is high Citrate spills out from the mitochondrial matrix and into the cytosol via a transporter, the mitochondrial citrate transporter (MCT) Cytosolic citrate is then cleaved by ATP citrate lyase to produce acetyl CoA.

Question 42

explain the Fatty Acid Synthesis: Carboxylation of Acetyl-CoA to form Malonyl-CoA

Answer 42

The energy for the carbon-to-carbon condensations in FA synthesis is supplied by the process of carboxylation and then decarboxylation of acetyl groups in the cytosol.​ The carboxylation of acetyl CoA to form malonyl CoA is catalysed by acetyl CoA carboxylase and requires ATP.

Question 43

the remaining series of reactions of FA synthesis is catalysed by what

Answer 43

a multienzyme complex, fatty acid synthase (FAS)

Question 44

explain the series of reactions catalysed by fatty acid synthase

Answer 44

Initiated by the transfer of the acetyl moiety of the starter substrate acetyl-CoA to the acyl carrier protein (ACP) catalyzed by the malonyl-CoA-/acetyl-CoA-ACP transacylase (MAT). The β-ketoacyl synthase (KS) catalyzes the decarboxylative condensation of the acyl intermediate with malonyl-ACP to a b-ketoacyl-ACP intermediate acetoacetyl-ACP in the first cycle The β-carbon is processed by nicotinamide adenine dinucleotide phosphate (NADPH)–dependent reduction through β-ketoacyl reductase (KR). Β-hydroxyacyl-ACP is dehydrated by a dehydratase (DH) to a b-enoyl intermediate, which is reduced by the NADPH-dependent β-enoyl reductase (ER) to yield a four-carbon acyl substrate for further cyclic elongation with two-carbon units derived from malonyl-CoA until a substrate length of C16 to C18 is reached. The product is released from the ACP by the thioesterase (TE)

Question 45

Mono-, Di- and Triacylglycerols  consist of what

Answer 45

one, two or three molecules of FA esterified to a molecule of glycerol.​

Question 46

FAs are esterified through what

Answer 46

their carboxyl groups, resulting in a loss of negative charge and formation of “neutral fat”.

Question 47

stored fatty acids on the glycerol molecule are usually .....the same type

Answer 47

not the same type Carbon 1 - Normally saturated​ Carbon 2 - Unsaturated​ (double bonds) Carbon 3 - Either

Question 48

fatty acids are stored as what

Answer 48

components of triacylglycerols

Question 49

how are triacylglycerols stored

Answer 49

VLDLs Inside the cell Because they are only ‘slightly soluble in water and cannot form stable micelles themselves, they coalesce within adipocytes to form oily droplets

Question 50

Glycerol phosphate is the initial acceptor of FAs during the synthesis of what

Answer 50

of TAG

Question 51

explain the Synthesis of Glycerol-3-phosphate

Answer 51

Route of synthesis is different in liver and adipose tissue - No glycerol kinase in adipose cells thus, inly one route to glycerol phosphate synthesis.​ - Adipose cells can ONLY take up glucose in the presence of insulin

Question 52

explain Synthesis TAG from glycerol phosphate and fatty acyl CoA

Answer 52

Pathway involves: 4 reactions These include the sequential addition of 2 fatty acids from fatty acyl CoA, the removal of phosphate and the addition of a third fatty acid.