Session 6 ILOs - Energy production (carbohydrates)

Question 1

Describe the general structures and functions of carbohydrates and how they are digested and absorbed

Answer 1

General formula (CH2O)n and contains either a 'keto' or 'aldehyde' group
Can be monosaccharides, disaccharides, oligosaccharides or polysaccharides
Breakdown occurs extracellularly in the GI tract by salivary amylase, pancreatic amylase or disaccharidases (lactase, sucrase, pancreatic amylase or isomaltase)

Question 2

Explain the biochemical basis of lactose intolerance

Answer 2

Failure to digest lactose due to lactase deficiency
This means that 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.

Question 3

Describe the glucose-dependancy of some tissues (listing the tissues)

Answer 3

Some cells are unable to carry out glycolysis

RBCs, neutrophils, innermost cells of the kidney medulla and the lens of the eye

Question 4

Describe the glucose-dependancy of some tissues (listing the tissues)

Answer 4

Some cells can only metabolise glucose (they have an absolute requirement for glucose) because they cannot perform stage 3 or stage 4 of metabolism
RBCs, neutrophils, innermost cells of the kidney medulla and the lens of the eye

Question 5

Describe the key features of glycolysis

Answer 5

An irreversible pathway that occurs in the cytosol of all tissues (intracellular)
2 ATP are invested to convert glucose into pyruvate with an outcome of 4 ATP (net 2 ATP per glucose) & 2 NADH produced per glucose

Question 6

Describe how key metabolites may be derived from glycolysis

Answer 6

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 7

Explain the key role of lactate dehydrogenase in glucose metabolism

Answer 7

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 8

Explain why lactic acid (lactate) production is important in anaerobic glycolysis and explain how the blood concentration of lactate is controlled

Answer 8

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 9

Explain how sugars other than glucose are metabolised

Answer 9

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 10

Explain the biochemical basis of galactosaemia

Answer 10

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

Question 11

Explain the importance of the pentose phosphate pathway

Answer 11

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 12

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

Answer 12

Very common inherited defect where the enzyme that converts glucose-6-p into pentose sugar phosphates is deficient. Therefore less NADPH is produced, so S-H bonds aren’t maintain (S-S bonds can form) which affects the structure of proteins
If disulphide bonds form, proteins aggregate and form heinz bodies and haemolysis = anaemia!

Question 13

Describe the different principals of the regulation of metabolic pathways

Answer 13

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

Question 14

Describe the key features of glycolysis and its control

Answer 14

Phosphofructokinase-1 (enzyme 3) is the key regulatory enzyme
Subject to regulation by:
1. Allosteric regualtion (increased activity from AMP & also fructose-2,6-bisphosphate, decreased activity from ATP & citrate, H+ and phosphoenolpyruvate)
2. Hormonal regulation (increased activity from insulin, decreased activity from glucagon)
Pyruvate dehydrogenase (enzyme 10) is also regulated in the same way!!

Hexokinase (enzyme 1) can also be regulated by allosteric inhibition by glucose-6-p
Enzyme 5 can also be inhibited by high NADH or low NAD (allosteric)

Question 15

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

Answer 15

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 16

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

Answer 16

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 17

Explain how the TCA cycle is regulated

Answer 17

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 18

Describe the key features of oxidative phosphorylation

Answer 18

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

Question 19

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

Answer 19

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 20

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

Answer 20

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 21

Compare the processes of oxidative phosphorylation and substrate level phosphorylation

Answer 21

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!