Energy Production- Carbohydrates

Question 1

Why can’t humans digest cellulose?

Answer 1

Don’t have the enzyme to break the B1-4 glycosidic linkages in cellulose

Question 2

3main disaccharides

Answer 2

• sucrose = glucose and fructose
• lactose = glucose and galactose
• maltose = glucose and glucose

Question 3

Minimum glucose requirement for a healthy adult is 180g/day, why?

Answer 3

• ~40g/day needed by tissues that can only use glucose such as RBCs, WBCs, kidney medulla and lens of eye
• ~180g/day required by CNS and this prefers glucose
• variable amounts needed by tissues for specialised functions

Question 4

What are the 3 limiting steps in glycolysis ?

Answer 4

Steps 1,3 and 10

Question 5

What are the enzymes at each of these steps?

Answer 5

Step 1 = glucokinase (liver) and hexokinase (skeletal muscle)
Step 3 = phosphofructokinase
Step 10 = pyruvate kinase

Question 6

What regulates PFK levels?

Answer 6

In the muscle high ATP:ADP ratio decreases it (allostery)

In the liver low insulin:glucagon ratio increases it

Question 7

Key features if glycolysis

Answer 7

• starting material, end products and intermediates all c6 or c3
• no loss of CO2
• some of the C3 intermediates are used by the cell for specific functions
• overall exergonic process
• substrate level phosphorylation
• net gain of 2 ATP

Question 8

2 important intermediates in gylcolysis

Answer 8

1) Dihydroxyactone phosphate - (DHAP) required for synthesis of TAGs in liver and adipose for storage.
2) 2,3-Bisphosphoglycerate -for regulation of O2 affinity in RBC haemoglobin. Present in RBCs at same level as haemoglobin

Question 9

How do cells like RBCs (no mitochondria) regenerate NAD+?

Answer 9

They use the enzyme lactate dehydrogenase in this reaction:

NADH + H+ + pyruvate NAD+ + lactate

Question 10

What then happens to the lactate ?

Answer 10

It is released into the blood, where it is then normally metabolised by the heart or liver back into pyruvate and NADH. These both have good oxygen supplies so can recycle the NADH

Question 11

What happens if lactate conc in the blood rises above 5mM?

Answer 11

Lactic acidosis - blood pH lowers and it is above the renal threshold

Question 12

How is pyruvate dehydrogenase regulated?

Answer 12

High insulin: glucagon ratio increases the amount

dephosphorylation

Question 13

Where does galactose metabolism occur ?

Answer 13

In the liver

Question 14

The 3 enzymes in galactose metabolism

Answer 14

Galactokinase
Galactose 1-phosphate uridyl transferase
Epimerase

Question 15

Symptoms of lactose intolerance

Answer 15

Abdominal bloating and cramps
Flatulence 
Diarrhoea 
Nausea
Vomiting

Question 16

What 2 enzymes can be absent in galactosaemia?

Answer 16

1) galactokinase - absence is rare and characterised by accumulation of galactose
2) transferase - absence is more common and serious as both galactose and galactose 1-P (toxic to liver) accumulate

Question 17

What happens when galactose builds up?

Answer 17

Galactose is converted to galactitol (and NADPH is converted to NADP+)

Question 18

What does this do?

Answer 18

This causes the eye lens to be depleted of NADPH which causes structural damage and causes the formation of cataracts due to cross linking of SH group in lens

Jaundice

Glaucoma - galactitol has a high osmotic potential > draws water into eye

Question 19

What happens when galactose 1-P accumulates?

Answer 19

It damages the liver, kidney and brain.

Question 20

What 2 enzymes are involved in fructose metabolism?

Answer 20

Fructokinase

Aldolase

Question 21

What happens if fructokinase is missing?

Answer 21

Essential Fructosuria

Question 22

What happens if aldolase is missing?

Answer 22

Fructose 1-phosphate accumulates in the liver causing damage

Question 23

The enzyme in the pentose phosphate pathway is…….

Answer 23

Glucose 6-phosphate dehydrogenase

Question 24

Functions of the PPP

Answer 24

• -Produce NADPH in the cytoplasm which has a biosynthetic reducing power (ie lipid synthesis-lots of activity in liver/adipose tissue) and maintain free -SH groups on proteins (stop oxidation to disulfide bonds)
• -Produce C5 sugar for nucleotides needed for nucleic acid synthesis

Question 25

What happens in glucose 6-phosphate dehydrogenase deficiency?

Answer 25

** In RBCs, reduced NADPH > disulfide bonds formed > aggregated proteins > Heinz bodies > haemolysis > bilirubin formation The liver usually conjugated bilirubin so can be released into small intestine but too much and released into blood stream **structural damage to lens of eye

Question 26

What is the role of pyruvate dehydrogenase ?

Answer 26

Pyruvate cannot enter the TCA cycle directly so PDH converts it to acetyl~CoA. This reaction is irreversible therefore has an irreversible loss of CO2 also cannot be converted back to pyruvate for use in gluconeogenesis

Question 27

What inhibits and activates PDH?

Answer 27

ATP and NADH inhibit ADP and NAD+ activate Enzyme also activated when theres lots of glucose to be catabolised

Question 28

Where does the TCA cycle occur ?

Answer 28

Mitochondria

Question 29

What does the TCA cycle require?

Answer 29

NAD+ FAD Oxaloacetate

Question 30

Main function of the TCA cycle

Answer 30

To break the C-C bond in acetate (acetyl~CoA) and oxidise the C atoms to CO2 The H+ and electron removed from acetate are transferred to NAD+ and FAD

Question 31

Does TCA cycle require oxygen

Answer 31

YES!!!!!!

Question 32

How is the TCA cycle regulated ?

Answer 32

By the ADP:ATP and NAD+:NADH ratios ADP activates NADH deactivates

Question 33

Does the TCA cycle produce any energy?

Answer 33

Yes it produces some a GTP | It also produces NAPH and FAD2H which are oxidised in oxidative phosphorylation releases energy

Question 34

Describe electron transport ....

Answer 34

* carrier molecules transferring electrons to oxygen are organised into a series of 4 highly specialised protein complexes spanning the inner mitochondrial membrane * electrons are transferred from NADH and FAD2H through and series of complexes to molecular oxygen with the release of free energy * 3 of these complexes also act as proton translocation complexes

Question 35

How is the pmf produced?

Answer 35

* free energy from electron transport used to move protons from inside to outside of inner mitochondrial membrane via p.t.complexes * membrane is impermeable to protons and as electron transport continues the conc of protons outside the IMM increases * The p.t.compexes therefore transform the chemical bond energy of the electrons into an electro-chemical potential difference of protons * NADH uses all 3 pumps and FAD2H only uses 2

Question 36

Does this process need oxygen?

Answer 36

Yesssss as its the terminal electron acceptor

Question 37

Explain how ATP synthesis occurs

Answer 37

* for ATP synthesis to occur 31kJ/mole is needed to drive the reaction * this energy is derived from the proton gradient * protons can only normally reenter the mitochondrial matrix via the ATP synthase complex * this drives the synthesis of ATP from ADP +Pi * greater the PMF the more ATP synthesised

Question 38

How much ATP do you get from NADH and FAD2H?

Answer 38

2 moles of NADH = 5 moles of ATP | 3 moles of FAD2H = 3 moles of ATP

Question 39

Differences between OP and SLP

Answer 39

*OP requires membrane associated compexes, SLP requires soluble enzymes * OP energy coupling occurs indirectly through generation and subsequent utilisation of pmf SLP energy coupling occurs directly through formation of high energy hydrolysis bond *OP cannot not occur in absence of O2 SLP can to a limited extent *OP major process for ATP synthesis in cell, SLP is a minor process

Question 40

What are uncoupling proteins?

Answer 40

Proteins with a function to uncouple ET and ATP synthesis to produce heat

Question 41

Tell me about UCP1

Answer 41

*expressed in brown adipose tissue *involved in non-shivering thermogenesis *in response to the cold noradrenaline in released by the sympathetic nervous system *stimulates lipolysis releasing fatty acids to provide fuel for oxidation in brown adipose tissue *NADH and FAD2H formed by B-oxidation of fatty acids *drive ET and increase pmf BUT noradrenaline also activates UCP1 allowing protons to cross inner mitochondrial membrane without passing through the ATP synthase complex * therefore the PMF is released as heat