Session 3: Carbohydrate 3

Question 1

Why does the TCA cycle not function in absence of O2?

Answer 1

because it is tightly coupled to electron transport chain

Question 2

What is pyruvate dehydrogenase (PDH)?

Answer 2

Converts pyruvate into acetyl CoA which allows for entry of stage 3 of glycolysis (which requires acetyl CoA rather than pyruvate).
It is a key site regulation of regulation into TCA

Question 3

What are the roles of the TCA cycle (3)?

Answer 3

• Breaks C-C bond in acetyl CoA & oxidises the C-atoms to CO2, The H+ and e- removed from acetate are transferred to NAD+ and FAD
• ATP/GTP production in all tissues containing mitochondria
• Produces precursors for biosynthesis

Question 4

How is the TCA cycle regulated?

Answer 4

Regulated by energy availability: ATP/ADP ratio & NADH/NAD+ ratio
- Signals that feed information on rate of utilisation of energy
- Regulated by isocitrate dehydrogenase => allosterically inhibited by high energy signal NADH & activated by low energy signal ADP.

Question 5

Why does electron transport require oxygen?

Answer 5

oxygen acts as the terminal electron acceptor

Question 6

Why does NADH have more energy than FADH2?

Answer 6

FADH2 produces less ATP because it produces a larger proton gradient
NADH has more energetic electrons

Question 7

Which vitamins are fat-soluble?

Answer 7

A, D, E & K

Question 8

What are the classes of lipids?

Answer 8

• Fatty acid derivatives
• Hydroxy-methyl-glutaric acid derivatives (C6 compound)
• Vitamins

Question 9

How are fatty acids more efficient stores of energy than carbohydrates?

Answer 9

They are hydrophobic so can be stored in an anhydrous form so more fuel per gram of weight

Question 10

Why can fatty acids generate more ATP than carbohydrates?

Answer 10

they are more reduced

Question 11

Why does fatty acid & glycerol metabolism not occur in the brain and in RBCs?

Answer 11

Brain: fatty acids do not reach brain due to blood-brain barrier
RBCs: do not have mitochondria so cannot undergo the metabolism

Question 12

What is the main role of acetyl CoA?

Answer 12

it is a cofactor for a number of oxidative and biosynthetic reactions in intermediary metabolism

Question 13

What is the normal plasma concentration of ketone bodies?

Answer 13

< 1mM

Question 14

What is the plasma concentration of ketone bodies during starvation?

Answer 14

2-10 mM

Question 15

What is the plasma concentration of pathological ketosis?

Answer 15

> 10 mM

Question 16

What does ketone bodies’ water-soluble characteristic allow?

Answer 16

high plasma concentration & excretion in urine (ketouria)

Question 17

Where and how are ketone bodies synthesised?

Answer 17

in mitochondria of liver from acetyl CoA in excess by lypase & reductase enzymes

Question 18

How is ketone body synthesis regulated? (fed & starvation state)

Answer 18

lypase & reductase are controlled by insulin/glucagon ratio

In a fed state (high insulin/glucagon ratio) => lypase is inhibited & reductase is activated -> cholesterol synthesis

In starvation state (low insulin/glucagon ratio) => lypase is activated & reductase is inhibited -> ketone bodies

Question 19

Which tissues are ketone bodies used by?

Answer 19

peripheral tissues (muscles)

Question 20

Is the liver able to metabolise ketone bodies? What happens to them?

Answer 20

No, they are transported in the blood and used by many different cells

Question 21

What could be a reason for ketone bodies to be used as an alternative fuel?

Answer 21

way of sparring/preserving glucose eg for tissues that depend on glucose eg brain

Question 22

What could be a reason for ketone bodies to be used as an alternative fuel?

Answer 22

way of sparring/preserving glucose eg for tissues that depend on glucose eg brain

Question 23

How do dietary triglycerols generate ATP?

Answer 23

• hydrolysed by pancreatic lipases in SI = release glycerol & fatty acids (lipolysis, cytoplasm, requires bile salts & colipase)
• fatty acids oxidised by β-oxidation into acetyl CoA => used in TCA cycle = produce ATP.

Question 24

Describe the features of dietary triglycerols (6).

Answer 24

• Derived from glycerol => glycerol -> triglycerol = esterification
• Hydrophobic
• Stored in anhydrous form
• Stores in adipose tissue
• Used in prolonged exercise, starvation & pregnancy
• Storage/mobilisation under hormonal control

Question 25

How do uncoupling proteins (UCP) work? Give an example

Answer 25

they uncouple electron transport chain from ATP production to produce heat located in inner membrane & allow leak of protons across membrane = reduces pmf and stops synthesis of ATP. Example: brown adipose tissue contains UCP1 - infants breakdown brown adipose tissue to generate heat

Question 26

How do uncoupling substances work?

Answer 26

- They increase permeability of inner mitochondrial membrane to protons - Protons enter mitochondria without driving ATP synthetase. - Dissipates proton gradient (provide alternative route for H+ to penetrate back into inner membrane) - No phosphorylation of ADP so oxidative phosphorylation is inhibited but ET continues => processes uncoupled & pmf of energy = heat

Question 27

How & why does uncoupling occur in some tissues?

Answer 27

The mitochondrial concentration of ATP regulates oxidative phosphorylation and electron transport, when high [ATP]: o [ADP] is low & ATP synthesis stops due to lack of substrate o Inward flow of H+ stops and [H+] in intramitochondrial space increases o Prevents further H+ pumping/translocation = ET stops o Reverse occurs when [ATP] is low

Question 28

Where does electron chain transport occur?

Answer 28

inner membrane of mitochondria

Question 29

How does ATP synthesis work?

Answer 29

* Energy derived from proton gradient (pmf) produced by electron transport * Protons can only re-enter mitochondrial matrix via ATP synthase complex driving synthesis of ATP from ADP + Pi * Higher the pmf = more ATP synthesised (ATP/O ratio or P/O ratio) * Oxidation of 2 moles of NADH => synthesis of 5 moles of ATP (P/O = 2.5) * Oxidation of 2 moles of FADH2 => synthesis of 3 moles of ATP (P/O = 1.5)

Question 30

What is electron transport?

Answer 30

Electrons on NADH & FADH2 are transferred through a series of carrier molecules to oxygen => energy is released in steps & oxygen is required

Question 31

How does electron transport work?

Answer 31

* Protons are pumped across membrane * Proton translocating complexes use the free energy (derived from electron transport) to move H+ from the matrix of the mitochondria to the intermembrane space. * membrane = impermeable to protons and as electron transport proceeds the proton concentration on the outside of the inner membrane increases * proton translocating complexes transform the chemical bond energy of the electrons into an electro-chemical potential difference of protons = proton motive force (pmf) * greater the chemical bond energy of electrons = more protons can be translocated = greater the pmf * NADH produces more energy than FADH2 so used 3PTCs whilst FADH2 uses only 2 * Needs oxygen as it acts as the terminal electron acceptor

Question 32

What happens by the end of stage 3 of catabolism?

Answer 32

- All the C-C bonds have been broken and the C-atoms oxidised to CO2. - All the C-H bonds have been broken and the H-atoms (H+ + e-) transferred to NAD+ and FAD.

Question 33

What is oxidative phosphorylation and where does it occur?

Answer 33

ATP synthesis is coupled to the movement of electrons through the mitochondrial electron transport chain Final stage of catabolism Involves electron transport & ATP synthesis (oxidative phosphorylation) Occurs in inner mitochondrial membrane

Question 34

Compare & contrast oxidative phosphorylation (OP) and substrate-level phosphorylation (SLP)

Answer 34

- OP requires membrane-associated complexes (inner mitochondrial membrane) whilst SLP requires soluble enzymes (cytoplasmic & mitochondrial matrix) - OP = indirect energy coupling whilst SLP = direct energy coupling - OP cannot occur in absence of O2 whilst SLP can occur to limited extent in absence of O2 - OP is a major process for ATP synthesis in cells which require a lot of energy whilst SLP is a minor process for ATP synthesis in cells which require a lot of energy