Metabolism part 2

Question 1

What are fats also known as and where are these found?

Answer 1

Triacyl glycerol
Adipocytes, lipid droplets, nucleus, mitochondria

Question 2

Outline lipolysis

Answer 2

TAG is broken down by lipase to form 3 x fatty acids and glycerol

Question 3

How are fatty acids activated?

Answer 3

Occurs on the mitochondrial matrix
1. 1. Reacts with ATP - ATP hydrolysed to form AMP and Ppi AMP combines with fatty acid to form an acyl adenylate - Ppi hydrolysed to form 2x Pi which releases energy
2. Sulfhydryl group of CoA attacks the acyl adenylate and replaces the AMP to form acyl CoA (thioester bond)
3. Fatty acid is now readily transferrable

Question 4

How are fatty acids oxidised?

Answer 4

Using carnitine
1. CoA is removed and the acyl group is combined with carnitine to form acyl carnitine
2. Acyl carnitine passes into the mitochondria via translocase
3. CPT2 reverses CPT1 to reform acyl CoA

Question 5

What is a Zwitterion?

Answer 5

Contains positively and negatively charged regions e.g. carnitine

Question 6

How are fats oxidised?

Answer 6

1. Oxidised by FAD (FAD reduced) fatty acid now has a double bond on the second carbon
2. Hydration (add water)
3. Oxidised by NAD (NADH + H+) to form beta ketoacyl CoA
4. Thiolysis by CoA to form Acetyl CoA and Fatty acyl CoA
   Cycle continues until fatty acid degraded
   Products - Acetyl CoA, NADH, FADH

Question 7

What is the difference between saturated and unsaturated fatty acids?

Answer 7

Saturated = no double bond
Unsaturated = double bond

Question 8

What is the difference between trans and cis unsaturated fatty acids?

Answer 8

Cis - the hydrogen is on the same side
Trans - hydrogen is on different side

Question 9

How are unsaturated fatty acids oxidised?

Answer 9

Palmitoleoyl CoA is a 16C fatty acyl CoA (cis)
The first 3 cycles occur normally
Then an isomerase converts the cis fatty acid into a trans fatty acid to be used as a substrate
(A polyunsaturated fatty acid will need a reductase aswell)

Question 10

How do peroxisomes oxidise TAGs?

Answer 10

Oxidation is identical but H2 is passed from FADH2 to O2 to form H202

Question 11

Why are ketones produced?

Answer 11

Low glycolytic flux so low pyruvate levels so low oxaloacetate and low anapleurosis so Acetyl CoA is diverted to produce ketones which can be used as fuel in tissues e.g. the heart

Question 12

Outline ketogenesis

Answer 12

1. Reverse of last step of beta oxidation to form Acetoacetyl CoA
2. Hydrolysis of thioester on Acetyl Co A drives reactions
3. Forming Acetoacetate this can be reduced by NADH some acetoacetate is decarboxylated to acetone

Question 13

What is fatty acid synthesis?

Answer 13

The elongation of a fatty acid chain by sequential addition of 2C units derived from Acetyl CoA

Question 14

Outline the steps in fatty acid synthesis

Answer 14

1. Acetyl CoA is carboxylated catalysed by ACC to form Malonyl CoA and biotin transfer CO2 to Acetyl CoA
2. CoA groups are replaced by ACP to form Acetyl ACP and Malonyl ACP
3. Acetyl ACP condenses with Malonyl ACP to form a 3-ketoacyl ACP
4. This unfavourable reaction is overcome by the decarboxylation of Malonyl ACP
5. The 4C product is reduced and dehydrated to form Butyryl ACP
6. Butyryl ACP condenses with another Malonyl ACP and the cycle continues
7. Formation of a C16 is detected by a thioesterase that cleaves ACP, generating palmitate.

Question 15

Where does fatty acid synthesis occur?

Answer 15

In fatty acid synthase which is a complex made of multiple enzymes and the ACP is used to shuttle the fatty acid chain between the enzymes

Question 16

How are fatty acids elongated?

Answer 16

Longer fatty acids generated on the cytosolic membrane of the ER - malonyl CoA is the donor
Double bonds are also added but we don’t have the enzymes to add past C9 so others e.g. linoleic are essential fatty acids from the diet

Question 17

How is FAS regulated?

Answer 17

AMPK senses energy levels of the cell
When energy levels is low (low ATP) AMPK phosphorylates ACC which inactivates it

Question 18

How do we get 14 NADPH here?

Answer 18

Acetyl CoA transfer
1. Acetyl CoA is formed in the mitochondria but FA-synth occurs in the cytosol
2. To escape the mitochondria Acetyl CoA is converted to Citrate (TCA cycle)
3. Citrate is transported into cytosol
4. ATP citrate lyase (reductive TCA cycle enzyme) then converts citrate to Oxaloacetate, regenerating Acetyl CoA
5. Oxaloacetate is converted to malate which is converted to pyruvate NADPH as a biproduct

Question 19

What is the Pentose Phosphate Pathway?

Answer 19

Crucial source of NADPH which can be used in the biosynthesis of fatty acids, cholesterol and nucleotides

Question 20

What is the oxidative phase of the PPP?

Answer 20

1. Glucose 6 phosphate is converted to ribulose 5 phosphate forming 2 NADPH
2. It is then Isomerised to form ribose 5 phosphate used in the production of DNA, RNA, FAD, CoA

Question 21

What is the non oxidative pathway of the PPP?

Answer 21

NADPH is needed more than DNA so
Ribose 5 phosphate is converted to metabolic intermediates
2x F6Phosphate 1x Glyceraldehyde 3 phosphate

Question 22

How is the PPP regulated?

Answer 22

Dependent on the concn of NADP+
And it is regulated by the cells requirement for ATP

Question 23

Outline the process of transamination

Answer 23

1. The amino acid group is transferred to a keto acid typically alpha Ketoglutarate which is catalysed by aminotransferase
2. Oxaloacetate can accept amino acid group to form aspartate required for urea formation and can be trans aminated to form glutamate

Question 24

Outline the process of deamination

Answer 24

Glutamate has amine group removed via redox reaction catalysed glutamate dehydrogenase to form alpha ketoglutarate and ammonia which is toxic

Question 25

Outline the urea cycle

Answer 25

Bicarbonate is phosphorylated to form carboxy phosphate
This is then converted to carbamoyl phosphate which combines with ornithine to form citrulline which then combines with aspartate to form argininosuccinate
A number of reactions occur and urea is formed

Question 26

What is the fate of the carbon skeleton?

Answer 26

Degraded to form 7 different intermediates e.g. Acetyl CoA

Question 27

Outline the Glucose Alanine cycle

Answer 27

Long exercise - run out of fuel
Muscle doesn’t have the urea cycle and a build up of glutamate causes glutamate to combine with pyruvate to generate alanine which leaves the muscle and is taken up by the liver and it is trans aminated