Session 2 ILO's - Energy Production (Carbohydrates 1 and 2)

Question 1

Describe the general structure of carbohydrates (4)

Answer 1

• General formula (CH2O)n (i.e. hydrated carbons)
• May contain aldehyde -CHO (aldose) or keto (-C=O) (ketose) groups
• Contain multiple OH groups
• Can be:

Monosaccharides (single sugar unit, 3-9 carbons, ie triode sugar = 3, pentose sugar = 5, hexose sugar =6)

Disaccharides (2 sugar units)

Oligosaccharides (3-12 sugars, ie dextrins) or

Polysaccharides (10-1000’s of units)

Question 2

Describe briefly how carbohydrates are digested and absorbed.

DIGESTION

Answer 2

• Breakdown occurs extracellularly in the GI tract in stage 1 of carbohydrate metabolism
• Large molecules get broken down into building blocks/monosaccharide molecules by:

1) Salivary amylase (starch and glycogen get broken down into smaller dextrin, oligosaccharide molecules)

2) Pancreatic amylase continues to work on the dextrins to break them down into monosaccharides.

3) In the intestine, we have a number of disaccharidases - enzymes that are specifically looking for disaccharides to complete the digestion into monosaccharides:

• lactase(cleaves lactose)
• sucrase(cleaves sucrose)
• Pancreatic amylase(Finishes off the glucose a1-4 bonds, the linear chain bonds)
• isomaltase(cleaves the a1-6 bonds, the branching bonds)

Question 3

Describe briefly how carbohydrates are digested and absorbed.

ABSORBTION

Answer 3

Question 4

Briefly describe by which processes sugars are absorbed

Answer 4

Question 5

Which 2 Glucose transporters do you need to know, and where are they found?

Answer 5

GLUT2 - Kidney, Liver, Pancreatic beta cells, small intestine

GLUT4 - Adipose tissue, Striated muscle (target tissues for insulin/) GLUT 4 is insulin regulated

(Just be aware that there are a number of glucose transporters)

Question 6

Name and describe the functions of the main 7 dietary carbohydrates

Answer 6

Question 7

Describe the general functions of carbohydrates.

Answer 7

• Mainly supply energy
• Dietary carbs have many different functions (see on later flashcard)
• Glucose is a carb some tissues have an absolute requirement for (see on later flashcard)

Question 8

Describe the glucose dependancy of some tissues

Which cells have an absolute requirement for glucose?

Answer 8

• Red blood cells
• Neutrophils
• Innermost cells of kidney medulla
• Lens of the eye

This is because these cells are unable to carry out glycolysis and cannot perform stage 3 or 4 of metabolism (they can only metabolise glucose)

Question 9

Describe the glucose dependancy of some tissues

Describe the brain’s usage of glucose

Answer 9

The CNS (brain) prefers glucose as fuel, but can use ketone bodies for some energy requirements in times of starvation, but needs time to adapt)

Question 10

Explain why cellulose is not digested in the human gastrointestinal tract.

Answer 10

• Between it’s glucose units, cellulose has beta 1,4 glycosidic bonds
• The human body does not have the enzymes required to break down the beta 1,4 glycoside bonds present in dietary fibres, ie no cellulase

Question 11

Explain the biochemical basis of the clinical condition of lactose intolerance.

Answer 11

• Failure to digest lactose due to lactase deficiency
• Thus, if lactose is ingested, it will persist in the colon, where bacteria would break it down
• The presence of lactose in the lumen of the colon increases the osmotic pressure of the contents and will draw water into the lumen, causing diahhorea.

(AKA lactose remains as an osmolite and it attracts water in the GI tract and can lead to the symptoms of lactose intolerance e.g. cramping, diarrhoea etc)

• Colonic bacteria can produce hydrogen, carbon dioxide and methane gas from lactose, causing feelings off bloating and discomfort.

Question 12

What are the 3 types of lactose intolerance?

Answer 12

• Primary lactase deficiency
• Secondary lactase deficiency
• Congenital lactase deficiency

Question 13

Describe Primary lactase deficiency

Answer 13

• Absence of lactase persistence allele.
• Highest prevalence in Northwest Europe * Only occurs in adults

Question 14

Describe Secondary lactase deficiency

Answer 14

1) Caused by injury to small intestine:
* Gastroenteritis
* Coeliac disease
* Crohn’s disease
* Ulcerative colitis

2) Occurs in both infants & adults

3)Generally reversible

Question 15

Describe Congenital lactase deficiency

Answer 15

• Extremely rare, autosomal recessive defect in lactase gene.
• Cannot digest breast milk.

Question 16

Which 2 monosaccharides make up lactose?

Answer 16

• Glucose and Galactose

Question 17

Give 6 Symptoms of Lactose intolerance

Answer 17

• Bloating/cramps
• Flatulence (Farting)
• Diarrhoea
• Vomiting
• Rumbling stomach

Question 18

Name 5 foods that contain lactose

Answer 18

• Milk
• Cream
• Yoghurt
• Cheese
• Many processed
  foods

Question 19

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

Answer 19

• Very common inherited defect where the enzyme that converts glucose-6-phosphate into pentose sugar phosphates is deficient.
• Therefore less NADPH is produced, so we aren’t able to prevent the formation of inappropriate disulphide bonds.
• If inappropriate disulphide bonds form, proteins aggregate and form heinz bodies and this leads to haemolysis = anaemia!
• Also, less NADPH means inappropriate disulphide bonds form so proteins in the lens of the eye get depleted.
• This structure damage leads to cataracts.

Symptoms:
- Jaundice due to haemolysis (as there is a build up of bilirubin)
- Cataracts

Question 20

Describe the key features of glycolysis (10)

Answer 20

• Central pathway of carbohydrate metabolism
• Occurs in all tissues (cytosolic)
• The process is exergonic (with a negative delta G value) and oxidative
• The starting material, end-products and intermediates are C6 or C3
• There is no loss of CO2
• Glucose (6 Carbon sugar) is oxidised to pyruvate (x2 3carbon sugars)
• NAD+ is reduced to NADH (2 NADH produced per glucose)
• With one additional enzyme, (PDH), it is the only pathway that can operate anaerobically
• Irreversible pathway
• 2 moles of ATP are required to activate the process and 4 moles of ATP are produced by the process, giving a net yield of 2 moles of ATP

Question 21

Give 4 functions of glycolysis

Answer 21

• Oxidation of glucose
• NADH production (2 per glucose)
• Synthesis of ATP from ADP (net= 2 ATP per glucose)
• Produces C6 and C3 intermediates for other uses

Question 22

Name the 3 important enzymes involved in glycolysis

Answer 22

• Hexokinase (Enzyme 1)
• Phosphofructokinase-1 (the most important enzyme in the regulation of glycolysis (Enzyme 3)
• Pyruvate Kinase (Enzyme 10)

Question 23

Describe how key metabolites may be derived from glycolysis

Answer 23

In the pathway:

• 1,3-biphosphoglycerate may be converted to 2,3-biphosphoglycerate by bisphosphoglycerate mutate which is important in oxygen transport
• DHAP can be converted to glycerol phosphate by glycerol-3-phosphase dehydrogenase which is important in lipid synthesis, triglyceride&phospholipid biosynthesis etc.

Question 24

Explain how sugars other than glucose are metabolised

Answer 24

• Fructose is converted to fructose-1-p by fructokinase (investment of ATP) and aldolase converts this into glyceraldehyde and triose kinase converts this to glyceraldehyde-3-p which then feeds into glycolysis
• Galactose is converted to galactose-1-p by galactokinase and can be converted into glucose-1-p by uridyl transferase and then glucose-6-p to be entered into glycolysis. However, galactose-1-p can also be converted into UDP galactose by UDP-galactose-epimerase which can be converted to UDP-glucose and then glycogen

Question 25

Explain the key role of lactate dehydrogenase in glucose metabolism

Answer 25

In poor O2 tissues, lactate dehydrogenate can metabolise pyruvate and NADH to produce NAD+ required for glycolysis of glucose It can also work to remove lactate in well oxygenate tissues by converting lactate and NAD+ into pyruvate and NADH

Question 26

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

Answer 26

- Pathway needs a continuous supply of NAD+ and normally NAD+ is regenerate in stage 4 of metabolism. However, in cells that don't have stage 3/4, then lactate dehydrogenase can regenerate NAD+ along with lactate (from metabolising pyruvate) The lactate can then be metabolised in the liver and kidney or heart back into pyruvate for energy (or gluconeogenesis in the liver only)

Question 27

Explain the biochemical basis of the clinical conditions of galactosaemia

Answer 27

- A deficiency in the following enzymes can because galactosaemia: galactokinase, uridyl transferase or UDP-galactose epimerase. - A deficiency in any enzyme saturates the other 2 enzymes and causes greater formation of galactose (less breakdown of galactose) and therefore galactose is converted to galactitol by aldose reductase (NADPH->NADP+) which can increase osmotic pressure as well as depleting NADPH which protects against oxidative damage - Depleting NADPH affects protein structure = cataracts and anaemia due to haemolysis

Question 28

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

Answer 28

2 main reasons: 1. Process uses NADP+ instead of NAD+, NADPH produced is important in providing reducing equivalents in biosynthesis (also protects again oxidative damage & maintain S-H bonds in proteins) 2. Pentose sugar phosphates are important in providing sugar in the production of nucleotides and ATP (DNA, RNA, coenzymes)

Question 29

Describe the key features of the control of glycolysis Describe the regulation of phosphofructokinase-1 and pyruvate kinase

Answer 29

COME BACK TO THIS Phosphofructokinase-1 (the enzyme that catylyses step 3) is the key regulatory enzyme Subject to regulation by: 1. Allosteric regulaltion (muscle): - Phosphofructokinase-1 has an allosteric site for both ATP and AMP - Phosphofructokinase-1is inhibited (turned down) by high ATP and citrate and stimulated by high AMP and high F2,6 and BP (low energy signal) (so high ATP tells the cell that we don't need more catabolism going on and high AMP tells the cell that we need more catabolism going on) 2. Hormonal regulation (liver): Phosphofructokinase-1 is stimulated by insulin and inhibited by glucagon Pyruvate kinase (enzyme 10) is also regulated in the same way!!

Question 30

Describe the key features of the control of glycolysis Describe the regulation of hexokinse

Answer 30

Hexokinase (enzyme 1) can also be regulated by allosteric inhibition by glucose-6-phosphate

Question 31

Describe the metabolic regulation of glycolysis

Answer 31

Question 32

What is allosteric regulation?

Answer 32

Modulating protein activity by the binding of an effector ligand at a site that is not the active site. Can inhibit or activate.

Question 33

CARDS AFTER THESE ARE EXTRAS, NOT PART OF ILO'S BUT FOR UNDRSTANDING

Question 34

How many sugar units are found in: a) Monosaccharides b) Disaccharides c) Oligosaccharides d) Polysaccharides

Answer 34

Question 35

Describe the different principals of the regulation of metabolic pathways

Answer 35

1.a. Allosteric regulation (ratio of ATP:ADP, NAD+/NADH etc) 1.b. Phosphorylation/dephosphorylation - hormone receptor binding activates either protein kinases or phosphotases 2. Product inhibition (last product inhibits first step) 3. Committing step (if inhibited, product can be diverted into other pathways)

Question 36

Explain the key role of pyruvate dehydrogenase in glucose metabolism - what factors activate the enzyme and which factors inactivate the enzyme?

Answer 36

Pyruvate dehydrogenase converts pyruvate into acetyl CoA and releasing CO2 in an IRREVERSIBLE / UNIDIRECTIONAL reaction (pyruvate can't enter directly into the cycle) If Pyruvate dehydrogenase was deficient, pyruvate couldn't enter the Krebs cycle and it would build up and be diverted out to lactate dehydrogenase Activated by: pyruvate, NAD+, ADP, insulin Inhibited by: acetyl-CoA, NADH, ATP, citrate

Question 37

Describe the roles of the Krebs cycle in metabolism (overview)

Answer 37

Acetyl CoA is oxidised to produce 2 x CO2 and some energy produced by substrate level phosphorylation Also produces precursors for biosynthesis: Per cycle (x 2 for per glucose): 3 x NADH 1 x FADH2 1 x GTP These can then be used to drive the electron transport chain to produce much more ATP

Question 38

Explain how the TCA cycle is regulated

Answer 38

Regulation occurs at 2 enzymes 1. Isocitrate dehydrogenase 2. Alpha-ketoglutarate dehydrogenase Both enzymes activated by low energy compounds e.g. AMP, NAD+ Both enzymes inhibited by high energy compounds e.g. ATP, NADH

Question 39

Describe the key features of oxidative phosphorylation

Answer 39

Occurs in the mitochondria NADH & FADH2 are re-oxidised O2 is required (reduced to H2O - gains oxygen) Lots of ATP is produced

Question 40

Explain the processes of electron transport and ATP synthesis and how they are coupled

Answer 40

Electrons are transferred through a series of carrier molecules along to O2, with release of energy 30% of energy is used to move H+ across the membrane, whilst 70% is released as heat (maintain core body temp) Movement of the H+ across the membrane, creates a proton motive force and this drives the movement of H+ back across through the ATP synthase - which drives the conversion of ADP to ATP = energy production!!! :)

Question 41

Describe how, when and why uncoupling of these processes occurs in some tissues

Answer 41

Uncoupling increase the permeability of mitochondrial inner membrane to protons, which dissipates the proton motive force and hence there is less of a driving force for ATP synthesis Uncoupling occurs in brown adipose tissue (which contains UCP1) because it allows for extra heat generation and maintains heat in newborn infants Noradrenaline is released in response to cold, which actives lipases to release fatty acids which active UCP1 = heat generation

Question 42

Compare the processes of oxidative phosphorylation and substrate level phosphorylation

Answer 42

Substrate level phosphorylation (SLP) requires soluble enzymes, whereas Oxidative Phosphorylation (OP) requires membrane associated complexes SLP occurs directly through formation of high energy bonds, whereas OP uses p.m.f to indirectly generate energy coupling SLP can occur to an extent anaerobically, whereas OP cannot occur anaerobically SLP minorly contributes to ATP synthesis in cells requiring large amounts of energy, whereas OP is the major ATP generator!

Question 43

Make sure to add on self assessment answers

Question 44

Describe the beginning of digestion

Answer 44

Broken down - GI Tract Absorbed - Small intestine - Carbohydrates are broken down into building blocks, extracellularly in the GI tract by 'glycosidase enzymes' - This process of digestion starts almost straight away - Salivary amylase starts to cleave starch and glycogen into smaller oligosaccharides like dextrins - Once the chyme reaches the stomach, digestion by amylase is temporarily halted, because the acidic environment inactivates the enzyme amylase - In the pancreas, pancreatic amylase breaks the dextrins down further into monosaccarides - In the small intestine, disc