Session 6 - Energy Production: Carbohydrates

Question 1

Why can’t cellulose be used as a fuel source

Answer 1

There are no enzymes that break down the Beta 1-4 Links in cellulose. Only Alpha 1-4 enzymes present in the GI tract

Question 2

Define a catabolic pathway

Answer 2

the breakdown of larger molecules into smaller ones, releasing large amounts of free energy

Question 3

What is the convergence point for many catablic reaction of amino acids, glucose, fatty acids, Acohol?

Answer 3

Acetyl CoA

Question 4

Outline the 4 stages of catabolism

Answer 4

1) breakdown of fuel molecules to building block molecules
2) Degredation of buiding block materials into small number of organic precursors
3) Kreb cyle
4) Electron transport and oxidative phosphorylation

Question 5

At what stage does respiration turn mitochondrial?

Answer 5

Stage 3, Pyruvate is transported into the mitochondria

Question 6

What sugars make up: sucrose, Maltose and Lactose

Answer 6

Sucrose: Glucose and Fructose
Maltose: Glucose and Glucose
Lactose: Glucose and Galactose

Question 7

What cell have an absolute requirement for glucose? and what is the normal blood glucose level?

Answer 7

Blood glucose = 5mM

RBC, Neutrophils, Innermost cells of the kidney medulla, Lens of the eye

Question 8

Where are disaccharidases found in the GI tract?

Answer 8

They are found attached to the brush border membrane of epithelial cells of the small intestine

Question 9

What is the difference between primary, secondary and congenital lactose intolerance?

Answer 9

primary - absence or persistence allele
secondary - injury to small intestine
Congenital - Autosomal recessive defect in lactase gene. can’t digest breast milk

Question 10

How is glucose absorbed from through the intestines?

Answer 10

Active - SGLT1 utilises the Na+ gradient set up by active Na+/K+ ATPase pump
Passive - Glut 2 allows facilitated diffusion into blood

Question 11

what are the two main glucose transporters and why are they relevant?

Answer 11

GLUT 2 - pancreatic Beta cells

GLUT 4 - insulin regulated in striated muscle

Question 12

outline the main features of glycolysis

Answer 12

• Glucose is converted to pyruvate (x2)
• Synthesis of 2 ATP (2 invested 4 produces)
• 2 NADH produces
• Irreversible pathway

Question 13

Name 3 key enzymes of glycolysis and what they are involved in

Answer 13

1- Hexokinase
2- Phosphofructokinase
3- Pyruvate Kinase

Question 14

Why does glycolysis occur in so many stages?

Answer 14

1) it can be controlled
2) Efficient energy conservation
3) Versatility (interconnections between pathways)

Question 15

Name 2 important intermediates of glycolysis

Answer 15

glycerol phosphate - important to lipid biosynthesis

2,3 diphosphoglycerate (2,3 BPG) - Produced in RBC and decreases there affinity to O2

Question 16

RBS can’t perform oxidative phosphorylation. How are NADH and Reduced carriers oxidised?

Answer 16

Anearobic respiration ultilising Lactose Dehydrogenase. Which converts pyruvate to lactate. Lactate is then metabolised by liver to reform pyruvate

Question 17

What are the main causes of lactic acidosis?

Answer 17

1) over production of lactate
2) Lack of metabolism
3) Renal function hindered

Question 18

Where do galactose and fructose enter glycolysis?

Answer 18

Fructose - G-3-P

Galactose -> G-1-P -> G-6-P

Question 19

What causes “essential fructosuria” and fructose intolerance

Answer 19

Essential fructosuria - caused by missing fructose kinase and as such fructose can’t be used at all and is excreted in the urine
Fructose intolerance - Aldose B is non functioning meaning fructose 1-P accumulates in the liver and causes liver damage

Question 20

What are the 3 enzymes (or lack there of) that cause galctosaemia?

Answer 20

Galactokinase
Uridyl Transferase
UDP galactose epimerase

Question 21

Why is having non-fucntioning Uridyl Trandferase problematic?

Answer 21

This would lead to a build up of galactose 1-p which is damaging to the liver but also a build up of galactose (galactosaemia). This promotes the conversion of galactose to Galactitol which converts NADH to NADP+. The depletion of NADPH results in the formation of cataracts as there is inappropriate disulphide bridge formation

Question 22

What are pentose phosphates requires for?

Answer 22

they are required for the biosynthesis of nucleotides DNA, RNA and coenzymes

Question 23

What are the key factors in the pentose phosphate pathway?

Answer 23

Starts from Glucose-6-P
source of NADPH
C5-sugars important in synthesis
CO2 produced so reaction is unidirectional
Rate limiting enzyme is called - Glucose-6-Phosphate Dehydrogenase

Question 24

Why aren’t reversible steps sites of regulation?

Answer 24

If you inhibit these steps the reaction will still come to equilibrium. So inhibition would have no effect on levels of product

Question 25

What is product and feedback inhibition?

Answer 25

If sufficient product is available then that product will reduce the action of the enzyme as to slow the pathway down and not to build up intermediates

Question 26

What is a committing step?

Answer 26

This is a step that can’t be reversed and as such it is important that it is controlled so that it can be diverted down another pathway. Not wasted

Question 27

In catabolic pathways what are inhibiting and what are activating signals?

Answer 27

Inhibitory = high energy signals - ATP, NADH, FAD2H the product
Activating = low energy signals - ADP, AMP, NAD+, FAD, the reactant

Question 28

What is feed forward regulation?

Answer 28

Where an early pathway provides activating stimulation to an enzyme later in the pathway

Question 29

What is the role of pyruvate dehydrogenase?

Answer 29

Reacts pyruvate with CoA to form acetyl CoA which is fed into the TCA cycle

Question 30

Why is the conversion of pyruvate to acetyl CoA sensitive to Vitamin B1 deficiency?

Answer 30

Vitamin B1 is used to provide many of the factors such as FAD when the reaction takes place

Question 31

name 2 enzymes involved in the regulation of the TCA cycle

Answer 31

Isocitrate dehydrogenase
Alpha- ketoglutarate dehydrogenase
(both these stages involve the removal CO2)

Question 32

Describe the stages in Oxidative phosphorylation

Answer 32

1) electrons from NADH and FAD2H transfered to a series of electron transfer carrier molecules
2) Energy released pumps H+ into intramembranous space
3) H+ diffuse back down [H+] gradient (pmf) and pass through synthetase enzyme which synthesises ATP

Question 33

Explain the action of cyanide

Answer 33

CN- inhibits the electron transport chain by preventing O2 from accepting e-. As such there is no flow of electrons to generate the p.m.f and ultimately no ATP synthesis

Question 34

Explain the action of “uncouplers”

Answer 34

Uncouplers increase the permeability of the inner membrane to protons as such dissipating the p.m.f. Also lots of heat is emitted leading uncontrolled thermogenesis

Question 35

What is themogenin and how does it relate to brown adipose tissue?

Answer 35

Themogenin (UCP-1) is a naturally occurring uncoupling protein in brown adipose tissue. As such it is found in new-born infants to generate heat particularly around vital organs