Where does galactose obtained from?
Hydrolysed from dietary lactose by the digestive enzyme lactase, giving glucose and galactose
Where is galactose metabolised?
Mainly in the liver
What is galactose used for in the body?
Required for the synthesis of glycolipids and glycoproteins
What is the epimerase reaction?
A reversible reaction enabling galactose to be synthesised from glucose via UDP-glucose
Where is fructose obtained from?
Dietary sucrose is hydrolysed by sucrase to release glucose and fructose
The overall reaction of galactose metabolism is...
Galactose + ATP → Glucose 6-phosphate + ADP
In what two pathways is glucose 6-phosphate metabolised in?
Pentose phosphate pathway
What are the two major functions of the pentose phosphate pathway?
- Produce NADPH in the cytoplasm
- Produce c5-sugar ribose for the synthesis of nucleotides
What is the rate limiting enzyme in the pentose phosphate pathway?
Glucose 6-phosphate dehydrogenase
What occurs during phase I of the pentose phosphate pathway?
Glucose 6-phosphate is oxidised and decarboxylated by the enzyme glucose 6-phosphate dehydrogenase in a reaction that requires NADP+
What occurs during phase II of the pentose phosphate pathway?
A complex series of reactions converts any unused C5-sugar phosphates to intermediates of glycolysis
What regulates the activity of glucose 6-phosphate dehydrogenase activity?
NADP+ : NADPH ratio
Glucose 6-phosphate dehydrogenase deficiency is an ________ gene defect
Outline what glucose 6-phosphate dehydrogenase deficiency means for the host
- Low levels of NADPH
- NADPH needed to recycle glutathione to its reduced "active" state
- Reduced glutathione protects the cell against oxidative damage; maintaining the structural integrity and functional activity of key proteins
- Haemolytic anaemia as proteins in RBCs cross link
- only pathway in RBCs to obtain NADPH
-Jaundice; bilirubin as product of red blood cell destruction and kidney/liver inability to excrete
Why are red blood cells particularly affected in an individual with a glucose 6-phosphate dehydrogenase deficiency?
- The pentose phosphate pathway is the only source of NADPH in RBCs
- RBCs role as oxygen carriers puts them at an increased risk of oxidative damage
How are Heinz bodies created and what do they cause?
- Haemoglobin and other proteins become cross-linked by disulphide bonds resulting from oxidative damage and form insoluble aggregates
- This leads to premature destruction of RBCs and causes haemolysis (haemolytic anaemia)
What occurs in the link reaction?
Pyruvate is decarboxylated to acetyl CoA by the pyruvate dehydrogenase complex
How many carbons are present in acetyl CoA?
Where does the PDH complex (link-reaction) take place?
Pyruvate is transported from the cytosol into the mitochondrial matrix where the link-reaction occurs
The PDH reaction cannot be reversed in the cell. There are two major consequences of this.....
1) The loss of CO2 from pyruvate is irreversible
2) Acetyl CoA cannot be converted to pyruvate and therefore cannot be converted to glucose by the process of gluconeogenesis
The PDH complex is subject to 3 control mechanisms, what are they?
1) The reaction is energy sensitive - ATP/NADH inhibit and ADP activates activity allosterically
2) Cofactor binding - FAD, thiamine phosphate and lipoic acid
3) Activated when high supply of glucose for catabolism - insulin activates the enzyme by promoting dephosphorylation
Where does the TCA (Krebs) cycle take place?
The TCA cycle is an _____ pathway
Oxidative (does not work in the absence of oxygen)
What are the genetic defects associated with the TCA cycle?
There are no known defects in the pathway as they would be lethal
Other than Acetyl CoA, what else is required for the TCA cycle?
NAD+ , FAD and oxaloacetate
There are two irreversible steps in the TCA cycle, what are they?
1) Isocitrate dehydrogenase - allosterically inhibited by the high energy signals ATP and NADH and activated by the low energy signal ADP
2) α-ketogluterate dehydrogenase - allosterically inhibited by NADH, ATP and Succinyl-CoA
In addition to the major catabolic role of the TCA, it also has anabolic functions that include:
- C5 and C6 (α-ketogluterate, Succinate etc) intermediates used for the synthesis of non-essential amino acids
Succinate used for the synthesis of haem
- Citrate used in the synthesis of fatty acids
Where can oxaloacetate be sourced from
- Majority comes from the activity of the enzyme pyruvate carboxylase (that uses pyruvate) giving ADP and oxaloacetate
- Some comes from the breakdown of amino acids
Where does the electron transport chain occur?
The inner membrane of the mitochondria
What is oxidative phosphorylation?
When the electron transport chain is coupled with ATP synthesis
What is the terminal acceptor in the electron transport chain?
Where do the NADH and FAD2H in the electron transport chain come from?
From the TCA cycle - the C-H bonds are broken with an electron and hydrogen ion transferred to NAD+ and FAD+ (remember these two are required for the TCA cycle)
Outline the electron transport chain
- The carrier molecules that transfer electrons to molecular oxygen and organised into 4 highly specialised protein complexes on the inner mitochondrial membrane
- Electrons are transferred sequentially from NADH and FAD2H sequentially through these complexes to oxygen, releasing free energy on each transfer
- Complexes I, III and IV act as proton translocating complexes
- they use the free energy to move protons from the matrix to the intermembrane space
- This transforms the chemical bond energy of the electrons to electro-chemical potential difference of the protons - proton motive force
Which of the electrons, in NADH or FAD2H, has a higher energy? What does this mean in the context of the electron transport chain?
The electrons in NADH has more energy and as a consequence uses all 3 proton translocating complexes whilst FAD2H only uses 2
Outline ATP synthesis following the electron transport chain
- Protons pumped into the intermembrane space can normally only enter the mitochondrial matrix via the ATP synthase complex located on the inner membrane
- The greater the proton motive force, the more ATP produced
- The energy not conserved in this process is lost as heat energy
Compare and contrast oxidative phosphorylation (Electron transport chain + ATP synthesis) and substrate level phosphorylation (glycolysis + TCA cycle)
Oxidative Phosphorylation - Requires membrane associated complexes (inner membrane) - Energy coupling occurs indirectly through generation and subsequent utilisation of a proton gradient (P.M.F) - Cannot occur in the absence of oxygen - Major process for ATP synthesis in cells that require large amounts of energy
Substrate-level phosphorylation - Requires soluble enzymes (cytoplasmic and mitochondrial matrix) - Energy coupling occurs directly through hydrolysis of high energy bonds - Can occur to a limited extent in absence of oxygen - Minor process for ATP synthesis in cells that require large amounts of energy
What happens when ATP levels are high for oxidative phosphorylation
- ADP is low so ATP synthase stops (lack of substrate). This prevents transport of protons back to mitochondrial matrix
- The H+ concentration in the intermebrane space increases to a level that prevents more protons being pumped
- In the absence of proton pumping, electron transport stops
Name a substance that can increase the permeability of the mitochondrial inner membrane
Dinitrophenol or dinitrocresol
What is the function of uncoupling proteins and where are they found?
Functions to uncouple the electron transport chain and ATP synthesis. The free energy lost through electron transport is lost as HEAT. They are located in the inner membrane to allow the leak of protons across the membrane
How does the sympathetic nervous system help respond to cold?
- stimulates lipolysis releasing fatty acids to provide feel for oxidation in brown adipose tissue
- As a result of β-oxidation of fatty acids, NADH and FAD2H are created driving the electron transport chain and increasing the P.M.F
- However, noradrenaline also activates UCP1 allowing proton leak, with energy dissipating as heat
The electron transport chain is inhibited by...
- Anaerobic conditions
- Carbon monoxide
- Various poisons - cyanide,rotenone
Under these conditions NADH and FAD2H cannot be oxidised, thus there is no energy to drive the pumping of protons and p.m.f cannot be created
What causes jaundice?
The breakdown of RBCs produces bilirubin as byproduct that is yellow. The liver and kidneys are unable to remove this byproduct and this accumulates giving the yellow appearance of the skin
How would you treat a patient with galactosaemia?
Implement a low-lactose diet
List the end products of glycolysis under anaerobic and aerobic conditions in skeletal muscle
Aerobic - Pyruvate
Anaerobic - Lactate
List the end products of glycolysis under anaerobic and aerobic conditions in red blood cells
Aerobic - Lactate
Anaerobic - Lactate
What is the catabolic role of the TCA cycle?
- To oxidise the acetyl group of acetyl Co A to two molecules of carbon dioxide
- Produces reducing power in form of 6NADH and 2 FAD2H
- Produces free energy in form of 2 GTP
Explain why cyanide is toxic to cells
Blocks NADH and FAD2H oxidation . This prevents generation of the proton motive force and hence ATP synthesis. Without ATP generation, call structure and function is impaired leading to cell death
Why can acetyl-CoA not enter gluconeogenesis?
Conversion of pyruvate to acetyl-CoA by pyruvate is an irreversible reaction
Name three classes of lipids
1) Fatty acid derivatives
2) Hydroxy-methyl-glutaric acid derivatives
3) Fat soluble vitamins
Explain the process of non-shivering thermogenesis and where it would occur
- Uncoupling proteins such as UCP1 found in brown adipose tissue on the inner mitochondrial membrane
- In response to cold, noradrenaline released from medulla of adrenal gland and stimulates lipolysis releasing fatty acids which undergo beta oxidation
- NADH and FAD2H produced which drive electron transport chain and increase p.m.f
- Noradrenaline also activates UCP1 which increases permeability of inner membrane to protons
- move through without passing through ATP synthase
- The p.m.f dissipates as heat energy
Under what conditions are ketone bodies formed?
- Glucose low and acetyl-CoA high
- Inhibited by insulin
- Activate the reductase enzyme
- Stimulated by glucagon
- Activates the Hydroxy-methyl-glutaryl CoA lyase enzyme
What enzyme is involved in ketone body formation?
Hydroxy-methyl-glutaryl CoA Lyase
How are dietary triacylglycerols metabolised?
- Hydrolysed by pancreatic lipase in the small intestine to release glycerol and fatty acids
- Glycerol and fatty acids are metabolised differently to derive energy
How is glycerol metabolised to produce energy?
- Glycerol enters the bloodstream and is converted to glycerol 3-phosphate by glycerol kinase
- Through a series of reaction glycerol 3-phosphate is oxidised glyceraldehyde 3-phosphate, an intermediate in the glycolysis pathway
How are fatty acids metabolised to produce energy?
- Fatty acids travel in the bloodstream, bound to albumin, to the liver, heart muscles or skeletal muscle
- Fatty acids are activated by binding to coenzyme A to form fatty acyl-CoA
- The Carnitine shuttle then allows fatty acyl-CoA through to the mitochondrial matrix where it undergoes beta-oxidation
- This requires oxygen
- The process produces Acetyl CoA (for TCA cycle) and FAD2H and NADH (for electron transport chain)
- No ATP produced at this stage
Name the three ketone bodies produced in the body
Where are acetoacetone and β-hydroxybutyrate synthesised?
In the liver from acetyl CoA
Why are ketone bodies an important feel source?
- Can be used in all tissues containing mitochondria including the nervous system
- Water soluble
- Converted to acetyl-CoA and then used in the TCA cycle
Which class of metabolite can partially replace the use of glucose as a metabolic fuel in the brain during starvation?
- Ketone bodies (acetoacetate and beta-hydroxybutyrate) (however cannot be used on "short notice" as 10-14 days are needed to increase plasma ketone levels such they can provide energy for neural tissue. Even then, ketone bodies can only supply up to 50% of brains energy requirement)
Which cycle converts lactate produced in skeletal muscle back to glucose in the liver?
What maternal metabolic change occurs in the second half of pregnancy?
- Decreased maternal utilisation of glucose
Switches to the use of fatty acids to conserve glucose for the maternal brain and for supply for foetus to grow