Carbohydrate metabolism (lecture 3-5)

Question 1

What 4 main cell types have an absolute requirement for glucose as it’s fuel molecule?

Answer 1

Red blood cells, neutrophils, inner cells of kidney medulla, lens of the eye
The brain prefers glucose but during starvation can use ketone bodies

Question 2

Why can humans not digest cellulose?

Answer 2

We do not possess enzymes that can break down the B1-4 glycosidic bonds in cellulose. We can only breakdown alpha1-4 bonds (with amylase from the pancreas)

Question 3

What are the 3 different types of lactose intolerance?

Answer 3

1. Primary lactase deficiency - a sense of lactase persistence Allele in adults - levels of lactase present decline as you get older
2. Secondary lactase deficiency - Caused by injury to small intestine e.g. Crohn’s disease - any age. Generally reversible.
3. Congenital deficiency - autosomal recessive defect in gene so can’t digest breast milk

Question 4

How are monosaccharides absorbed into the blood stream?

Answer 4

1. Active transport - from low to high conc into intestinal epithelium by SGLT
2. Passive transport - from high to low gradient via GLUT2 into capillary

Question 5

How is glucose taken up by individual cells?

Answer 5

Facilitated diffusion - high to low
GLUT1: BBB, RBC, Fetal tissues
GLUT2: pancreatic beta cells, kidneys, liver, small intestine
GLUT3: placenta, neurones
GLUT4: adipose tissue & striated muscle
GLUT5: intestine, spermatazoa

Question 6

What are the 2 key enzymes involved glycolysis?

Answer 6

Hexokinase (glucokinase) in liver and phosphofructokinase-1 (rate limiting enzyme)

Question 7

Why does glucose get phosphorylated at the start of glycolysis?

Answer 7

To make glucose more reactive to encourage subsequent steps and also to make it anionic (negative charge) to prevent it going back across the plasma membrane via positive glucose transportersu - hexokinase involved in this

Question 8

What is the committing step in glycolysis?

Answer 8

The phosphorylation of fructose-6-P to Fructose-1,6-bisphosphate. Catalysed by phosphofructokinase-1.

Question 9

What is produced in red blood cells that regulates the haemaglobin 02 affinity promoting release?

Answer 9

2,3-
Catalysed by bisphosphoglycerate mutate.

Question 10

What dihydroxyacetone-P important for in glycolysis?

Answer 10

It forms glycerol phosphate when catalysed by glycerol 3-phosphate dehydrogenase which is important for triglyceride and phospholipid biosynthesis.
The reaction is reversible meaning glycerol phosphate can be turned into DHAP meaning it can enter glycolysis.

Question 11

What 3 things inhibit the action of phosphofructokinase?

Answer 11

High ATP, high citrate (stage 3 of metabolism), glucagon (promotes breakdown of glycogen to glucose In liver)

Question 12

What 3 things stimulate the action of phosphofructokinase-1?

Answer 12

High AMP
high Fructose-2,6-Bisphosphate which increases the enzymes affinity for F1-6-Bisphosphate
Insulin - to decrease blood glucose levels by breaking glucose down in glycolysis

Question 13

What inhibits the activity of hexokinase?

Answer 13

Glucose-6-Phosphate

Question 14

What activates the enzyme pyruvate kinase?

Answer 14

It is increased by a high insulin:low glucagon ratio

Question 15

How is NAD+ normally regenerated and when would you need to regenerate it using another method?

Answer 15

NAD+ normally regenerated when NADH is oxidised in stage 4 of carbohydrate metabolism.
- some cells don’t undergo stage 3 and 4 of metabolism e.g. RBCs
-sometimes the 02 supply needed for oxidative phosphorylation is insufficient e.g. skeletal muscles so needs to be regenerated differently

Regenerated via Lactate Dehydrogenase reducing pyruvate to form lactate.
NADH + H+&raquo_space;> NAD+ + Lactate

Question 16

What are the normal levels of lactate in the body?

Answer 16

Without strenuous exercise - 40-50g made in 24hrs

With strenuous exercise 30g in 5 mins, increasing x10 fold in 2-5mins
But back to normal levels in 1.5hrs

Plasma lactate concentration normally constant <1mM

Question 17

What enzyme causes essential fructosuria if it is missing?

Answer 17

Fructokinase which converts Fructose to Fructose-1P

Unable to metabolise the fructose so it builds up and gets excreted by the kidneys in urine

Question 18

Which enzyme causes liver damage in fructose metabolism if it is missing?

Answer 18

Aldolase enzyme - this causes an accumulation of Fructose 1-P because it is not able to be broken down into Glyceraldehyde or DHAP.

Build up of F1P accumulates in the liver and leads to liver damage and is sometimes fatal

Question 19

How do you treat fructosuria or fructose intolerance?

Answer 19

Remove fructose and sucrose (glucose and fructose disaccharide) from the diet

Question 20

Where does fructose metabolism join glycolysis?

Answer 20

As Glyceraldehyde-3-phosphate

Question 21

What are the 3 main enzymes involved in Galactose metabolism?

Answer 21

Galactokinase, Uridyltransferase, UDP-galactose epimerase

Question 22

Where does galactose metabolism join glycolysis?

Answer 22

Glucose-6-phosphate

Question 23

What enzyme catalyses the reaction of galactose to galactose-1-p?

Answer 23

Galactokinase

Question 24

What does uridyl transferase catalyse?

Answer 24

The transformation of galactose-1-P into glucose-1-p

Question 25

What enzyme helps convert Galactose-1P to UDP galactose?

Answer 25

UDP-galactose epimerase

Question 26

How is glycogen formed from galactose-1P?

Answer 26

UDP galactose epimerise enzyme converts it to UDP galactose, then conversion to UDP glucose then this forms glycogen u

Question 27

What are the 2 types of deficiencies associated with galactosaemia and what do they cause a build up of?

Answer 27

1. Galactokinase deficiency - accumulation of galactose (rare)
   2.uridyl transferase deficiency which causes a build up of both galactose and galactose 1-P

Question 28

What health problems occur with a build up of galactose in the body?

Answer 28

1. It enters other pathways, so it reacts with aldose reductase to form Galactitol which uses up NADPH because it is oxidised. This depletes the NADPH found in the lens and this is normally a protective mechanism against ROS so this leaves the eye susceptible to damage. Leads to a build up of crystalline protein from disulphide bridge formation and this causes cataracts.

Question 29

What health issues occur when galactose-1-p builds up in the body?

Answer 29

Can affect liver, brain and kidneys. May affect disruption of metabolism of glycogen and oxidative stress.

Question 30

What is the pentose phosphate pathway?

Answer 30

A reaction which starts from glucose-6P In glycolysis which is catalysed by glucose-6P dehydrogenase (rate limiting enzyme) eventually ending in the formation of Ribose-5P.

Question 31

Why is the pentose phosphate pathway important?

Answer 31

It is a source of NADPH which is used for fatty acid and steroid biosynthesis as it has reducing power. It also maintains GSH.

It also produce ribose which is needed for nucleotides and DNA/RNA.

Question 32

What is the medical importance of glucose-6-P dehydrogenase deficiency?

Answer 32

No pentose phosphate pathway. Causes depletion of NADPH which for example in red blood cells cause the formation of disulphide bonds and these proteins aggregate and form Heinz bodies which cause haemolysis.

Question 33

What is the function of pyruvate dehydrogenase enzyme complex?

Answer 33

It converts pyruvate into acetyl-coma so it can enter the TcA cycle. NAD+ is reduced to NADH at the same time.

Question 34

What vitamin is most important in the conversion of pyruvate to acetyl-CoA?

Answer 34

Vitamin B provides the coenzymes needed for pyruvate dehydrogenase complex to work so a deficiency effects this step of carbohydrate metabolism.

Question 35

What other biosynthetic molecules come from the TCA cycle?

Answer 35

Citrate - fatty acids
Succinate - haemaglobin and amino acids
Malate - amino acids
Oxaloacetate - glucose & amino acids

Question 36

Which electrons have the highest energy - those from NADH or FAD2H?

Answer 36

NADH - so it can use 3 proton translocating complexes. Fadh2 can use only 2.

Question 37

How many ATP molecules are generated from the oxidation of 2 moles of NADH and 2 moles of FAD2H?

Answer 37

NADH - 5 moles of ATP
FAD2H - 3 moles of ATP

Question 38

Give some examples of inhibitors of oxidative phosphorylation?

Answer 38

Cyanide - it can prevent acceptance of electrons by O2

CO - prevents O2 binding to PTC

Congenital defects in mitochondrial DNA proteins on some PTCs or the ATP synthase. This leads to a decrease in ATP synthesis due to a decrease in Ox Phos.

Question 39

What is an Ox Phos uncoupler and what does it do?

Answer 39

This is something that makes the mitochondrial membrane more permeable to H+ so they come back through the membrane and reducing the protein gradient and pmf. This means there is no drive for ATP synthesis and H+ enters mitochondrial membrane without generating ATP and energy made disapates as heat. Ox phos continues but just with no ATP made.

Examples - dinitrophenol, dinitrocresol, fatty acids

Question 40

What is the importance of brown adipose tissue in thermoregulation?

Answer 40

It contains thermogenin which is a uncoupling protein (UCP-1).

When it’s cold, noradrenaline activates lipase. This releases FA from TAG.
This is oxidised via ETC and they activate UCP1. This allows H+ to move back into mitochondrial membrane and pmf is dissipated. ET uncoupled from ATP synthesis.
The pmf energy released as heat to increase body temperature.