Energy Production: Carbohydrate

Question 1

What is the general structure of carbohydrates?

Answer 1

(CH2O)n is the general formula.

May contain aldehyde (CHO) or keto (C=O) group and many hydroxyl (OH) groups

Question 2

What are monosaccharides, disaccharides, oligosaccharides, and polysaccharides?

Answer 2

Monosaccharide = single sugar units with 3-9 Cs. E.g. glucose.

Disaccharide = 2 units e.g. lactose.

Oligosaccharide = 3-12 units e.g. destins.

Polysaccharide = 10-1000s units e.g. glycogen, starch, cellulose.

Question 3

What are the three main dietary monosaccharides?

Answer 3

Glucose
Fructose
Galactose

Question 4

How are carbohydrates digested and absorbed?

Answer 4

Starch and glycogen are broken down into dextins facilitated by amylase in the GI tract. The enters the stomach where amylase is denatured.

In the pancreas, the dextins are broken down further by pancreatic amylase.

In the small intestine, disaccharides are attached to the brush order membranes of epithelial cells. Lactase, glycoamylase and sucrose/isomaltase are broken down into the monosaccharides: glucose, fructose and galactose.

Question 5

Explain the biochemical basis of lactose intolerance

Answer 5

The activity of lactase is high in infants, but decreases in childhood in most populations except Northern Europeans. This is primary lactase deficiency.

Where lactase is not present, lactose will
persist into the colon where bacteria break it down. Lactose in the colon increases osmotic pressure drawing water into causing diarrhoea. Colonic bacteria produce H, CO2 and CH4 causing bloating and discomfort.

Secondary lactase deficiency is caused by injury to small intestine.

Question 6

Explain why cellulose is not digested in the human intestinal tract

Answer 6

We lack appropriate enzymes to break it down. Cellulose is fibre.

Question 7

Describe the glucose-dependency of some tissues

Answer 7

Red blood cells, lens of the eye and the innermost cells of kidney medulla require glucose as they lack mitochondria. They depend on glucose metabolism for their energy as they are in an anaerobic environment.

Therefore, it is important that blood glucose stays at a stable level for passive transport of glucose into cells.

Question 8

Describe the key functions of glycolysis.

Answer 8

Glucose is oxidised.

2NADH is synthesised per glucose.

Net 2ATP is synthesised from ADP per glucose- phosphorylation.

Provides biosynthetic precursors for fatty acids, amino acids and nucleotides.

Question 9

How are monosaccharides absorbed?

Answer 9

Sodium-glucose transporter-1 are active transporters (low to high conc) that transport glucose into epithelial cells. They depend on the Na+ gradient, so it is secondary active transport.

Glucose transporter-2 (GLUT2) facilitates glucose diffusion from epithelial cells to the blood. This is passive transport (high to low conc).

Transport proteins GLUT1 - GLUT6 facilitate diffusion (high to low conc).

Question 10

Describe the key features of glycolysis

Answer 10

It is the central pathway of carbohydrate catabolism.

It occurs in all tissues (cytosolic).

Glucose is oxidised to pyruvate and NAD+ is reduced to NADH.

It is exergonic (-Ve delta G) and oxidative.

Irreversible.

C2 —> 2C3. No loss of CO2.

It is the only pathway that can operate anaerobically with the addition of LDH (an additional enzyme).

Question 11

Describe how glucose 6-phosphate may be derived from glycolysis.

Answer 11

Is produced by step 1 of glycolysis. Catalysed by hexokinase.
Glucose + ATP —> Glucose 6-phosphate + ADP.
This is isomerised to fructose 6-phosphate which can be cleaved into C3 units by further phosphorylation using ATP.
This makes step 1 irreversible as it makes glucose -vely charged so can’t cross the plasma membrane.

Question 12

What is the overall equation for aerobic glycolysis?

Answer 12

Glucose + 2Pi + 2ADP + 2NAD+ —> 2pyruvate + 2ATP + 2NADH + 2H+ + 2H2O

Question 13

Describe how fructose 1,6-bisphosphate may be derived from glycolysis.

Answer 13

Derived in step 3. Fructose-6-phosphate is converted to fructose 1,6-bisphosphate through conversion of ATP back to ADP. Phosphofructikinase is the enzyme that facilitates this step.
This stage is known as the committing phase to glycolysis as it is irreversible and can’t be diverted to different pathways now.

Question 14

Describe how pyruvate may be derived from glycolysis.

Answer 14

This is the product of step 10. Catalysed by pyruvate kinase which facilitates the transfer of a phosphate group from PEP to ADP. This yields 1 molecule of pyruvate and 1 molecule of ATP.
This step has a large -ve delta G and is irreversible.

Question 15

Explained how fructose is metabolised.

Answer 15

Fructose is metabolised in the liver by fructokinase to fructose 1-phosphate. This is then metabolised by alsolase to glyceraldehyde-3-phosphate. Then glycolysis occurs.

Question 16

What happens if someone is missing fructokinase?

Answer 16

Called essential fructosuria - build up of fructose in the blood. Gets cleared by the kidney into the urine. This isn’t too much of a problem.

Question 17

What happes if someone is missing aldolase?

Answer 17

Called fructose intolerance - fructose-1-P accumulates in the liver which leads to liver damage and possible death and inorganic P is sequestered.

Question 18

Explain how galactose is metabolised.

Answer 18

Galactokinase metabolises it into galactose-1P in the liver.
Then one of two pathways:
1. Uridyl transferase metabolises it into glucose 1P, then glucose 6P and glycolysis occurs.
2. UDP-galactose epimerise metabolises it and it follows the Glycogen pathway. This then follows pathway 1 and is converted into glucose.

Deficiency in any of the three enzymes above cause galactosaemia (1 in 30,000).

Question 19

Explain why lactic acid (lactate) production is important in anaerobic glycolysis.

Answer 19

NAD+ is required for glycolysis. This is the oxidised form that requires oxygen to regenerate. The conversion of pyruvate into lactate is another route through which NAD+ is regenerated.

Some cells - in the heart, liver and kidney utiles lactate as a fuel.

Question 20

How is lactate formed?

Answer 20

In cells without mitochondria and when there is inadequate oxygen, pyruvate is reduced to lactate by the enzyme lactate dehydrogenase (LDH).
2pyruvate + 2NADH + 2H+ —> 2lactate + 2NAD+.
This produces NAD+ which is required for glycolysis.

Question 21

What is the process of anaerobic glycolysis?

Answer 21

This is the process of converting peruvate into lactate with the enzyme lactate hydrogenase. This regenerates NAD+ from NADH which is essential for glycolysis.

Question 22

Explain how the blood concentration of lactate is controlled.

Answer 22

Without exercise, 40-50g is produced in 24hrs. With excessive exercise, can be up to 30g in 5 mins.

Lactate is transported to the heart, liver and kidneys. The heart converts it back to pyruvate. The liver and kidneys convert it to glucose. Under these conditions, the rate of lactate production equals rate of utilisation and plasma conc is constant.

Question 23

Explain why the pentose phosphate pathway is an important metabolic pathway in some tissues.

Answer 23

1. This pathway creates Ribose-5-phosphate which is required for forming nucleotides, DNA, and RNA.
2. This is the main pathway for creating NADPH which is required for reducing power for biosynthesis, maintenance of GSH levels and detoxification reactions.

(Glucose is converted to glucose-6-phosphate and then follows this pathway).

Question 24

Describe the clinical condition of glucose 6-phosphate dehydrogenase deficiency and explain the biochemical basis of the signs and symptoms.

Answer 24

This is seen in around 7% of the population - very common.

Reduced activity of glucose-6-phosphate results in low levels of NADPH. This is required to make glutathione (GSH) which protects the cells against oxidative damage. Therefore, damage occurs to red blood cells which are particularly affected as the pentose phosphate pathway is the only source of NADPH in these cells. Haemoglobin forms Heinz bodies and haemolytic anaemia occurs.

Question 25

What is the rate limiting enzyme for the pentose phosphate pathway?

Answer 25

Glucose-6-phosphate dehydrogenase

Question 26

What are the roles of the TCA (tricarboxylic acid) cycle in metabolism?

Answer 26

• Produces some energy as ATP/GTP.
• Produces important intermediates for biosynthesis.
  Generates reducing power (NADH, FAD2H from oxidation of acetyl-CoA.

Acetyl group has 6 high-energy bonds. TCA cycle saves these as reducing power while also generating precursors for biosynthesis.

Question 27

How is the TCA cycle regulated?

Answer 27

• Regulated by the ATP/ADP ratio.
• Isocitrate dehydrogenase is the enzyme that controls the rate-limiting step. This is activated by greater conc of ADP and inhibited by greater conc of ATP.
• Alpha-ketoglutarate dehydrogenase is the enzyme that catalyses the step to produce succinylcholine-CoA. Also produces NADH and ATP.

Question 28

Where does the TCA cycle take place?

Answer 28

Mitochondria

Question 29

Glucose catabolism releases a lot of energy, where does it go?

Answer 29

2ATP net from glycolysis.
2GTP = 2ATP net from TCA cycle.
Chemical bond energy in NADH and FAD2H. High energy electrons in these are transferred to O2 which releases large amounts of energy to drive ATP synthesis.

Question 30

What is oxidative phosphorylation and which 2 processes are involved?

Answer 30

The last stage of catabolism generating ATP from Electron transfer and ATP synthase.

Electrons in NADH and FAD2H are transferred to O2 through a series of carrier molecules. Energy is released in steps.
The free energy released in electron transport is used to drive ATP synthesis.

Question 31

Why is the inner mitochondrial membrane folded?

Answer 31

To increase surface area. These folds form cisternae.

Question 32

Explain the process of electron transport.

Answer 32

Proton translocating complexes pump protons against the conc gradient and against the elctrical gradient into the mitochondrial matrix. This creates a transmembrane electrochemical potential which is used for ATP synthesis.
The electrochemical gradient is called a proton motive force (pmf).
The protons are pumped to O2 in the inner mitochondrial membrane to form H2O.
30% of energy is used to move H+ across, the rest is lost as heat.

Question 33

What is the role of ATP synthase (ATPase)

Answer 33

The only way protons can return across the mitochondrial matrix. It drive ATP synthesis.

Question 34

What is oxidative phosphorylation?

Answer 34

Electron transport coupled to ATP synthesis.
Electrons transferred from NADH and FAD2H to O2. Energy released generates a pmf.
Energy from the dissipation (spreading out) of the pmf is coupled to the synthesis of ATP from ADP.

Question 35

Is lack of galactokinase or uridyl transferase more severe and why?

Answer 35

Galactokinase deficiency - accumulation of galactose in tissues causes glaucoma and cataracts.

Uridyl transferase deficiency - accusation of galactose and galactose - 1 - P in tissues causes damage to liver, kidneys and brain. This is more severe.

Question 36

Why can glucokinase deficiency lead to cataracts?

Answer 36

The cristallin protein in the lens of the eye is damaged by lack of NADPH. This no longer prevents oxidative damage.

Question 37

At which point does the pentose phosphate pathway occur and why?

What is the rate limiting enzyme in this pathway?

Answer 37

Glucose —> Glucose-6-P —> Pentose phosphate pathway.

When NADPH drops.
2NADPH per glucose-6-P.
Forms Ribose-5-P that’s important for nucleotides, DNA, RNA, and coenzymes.

Glucose-6-P dehydrogenase.

Question 38

What are the consequences of glucose-6–P dehydrogenase deficiency?

Answer 38

Low levels of NADPH.

This is required for reduction of glutathione to prevent cells from oxidative damage.

RBC are particularly affected because the pentose phosphate pathway is their only source of NADPH.

Haemoglobin becomes cross-linked from oxidative damage and forms Heinz bodies.

This leads to haemolytic anaemia.