Fuel Digestion, Absorption and Storage

Question 1

Name the 3 major fuel stores.

Answer 1

• Glycogen – stored in muscle and in the liver
• Triacylglycerol in adipose (liver and muscle)
• Protein in muscle and all other cells, only used as a fuel in significant quantities in starvation, usually conserved until triacylglycerol in adipose tissue is depleted.

Question 2

Can animals convert fatty acids to carbohydrates?

Answer 2

Animal cannot convert most fatty acids in carbohydrates, only when odd chain fatty acids are broken down into propionyl-CoA.

Question 3

How are carbohydrates digested, absorbed and used?

Answer 3

Carbohydrates can be broken down into monosaccharides.

Can be absorbed directly into the blood as glucose. Glucose can then be taken up by a variety of different tissues.
Can be:
- Immediately oxidised to CO2 and water
- Used to synthesise glycogen
- Synthesised to acetyl-CoA and then used to synthesise fats, stored as triacylglycerol

Question 4

How are proteins digested and used?

Answer 4

Proteins are broken down into amino acids. These can be:

• Directly used for protein synthesis
• Synthesis of glucose at the liver and stored as glycogen
• Can be broken down to acetyl-CoA and synthesise fatty acids, stored as triacylglycerol
• Can be synthesised into fatty acids that can be oxidised to produce CO2 and water

Question 5

How are fats digested and used?

Answer 5

Fat can be broken down into fatty acids and glycerol, which can be stored as triacylglycerol by chylomicrons.
Can be broken down into fatty acids by lipases and be:
- Stored as triacylglycerol
- Oxidised to CO2 and water

Question 6

Describe the structure of glycogen.

Answer 6

Linear chain of glucose molecules joined by 1-4 and 1-6 alpha glycosidic linkages.

In cells, forms spherical molecules with a glycogenin in the centre. The highly branched nature means that there are many free end for glucose to be rapidly synthesised and added on, meaning it can be very quickly hydrolysed.

Question 7

What is the function of glycogenin?

Answer 7

Is a self-glycosylating protein – it can covalently modify itself by the addition of glucose units. These glucose units acts as a primer upon which the glycogen molecule can grow. This is essential, as glycogen synthase can only add 1 glucose at a time.

Question 8

Describe the synthesis of glycogen synthesis.

Answer 8

UDP = activated form of glucose that acts as the glucose donor

1. Glucose converted to glucose 6-phosphate by hexokinase, requiring hydrolysis of an ATP molecule.
2. Glucose 6-phopshte converted to glucose 1-phopshate by phosphoglucomutase.
3. Glucose 1-phosphate is converted to UDP glucose by uridine transferase, which uses UTP (uridine triphosphate) as a donor for the UDP group.
4. UDP glucose transfers UDP units to glycogen to elongate the strand in glycogen synthesis.
5. Branching enzyme forms alpha 1-6 glycosidic bonds.

Question 9

Describe glycogen breakdown.

Answer 9

1. Debranching enzyme breaks down the alpha 1-6 glycosidic bonds in glycogen chain.
2. Glycogen strand is converted to a glycogen strand with one less unit and glucose 1-phosphate by glycogen phosphorylase, which uses an inorganic phosphate.
3. Glucose 1-phosphate is converted to glucose 6-phosphate by phosphoglucomutase.
4. In muscle, glucose 6-phosphate can only be used for glycolysis.
5. In liver cells, glucose 6-phosphate can be converted to glucose by glucose 6-phosphatase. Glucose can then be exported by the liver.

Question 10

How is glycogen metabolism controlled?

Answer 10

Key regulatory enzymes = glycogen synthase and glycogen phosphorylase. Are co-ordinately regulated to prevent futile cycling, meaning when one is turned on, the other is turned off.

Hormonal control:
• Insulin activates glycogen synthase and inhibits glycogen phosphorylase. Released in the fed state when glucose is abundant.
• Adrenaline and glucagon inhibit glycogen synthase and activate glycogen phosphorylase

Allosteric control but control differs in the liver and muscle due to the presence of different isoenzymes.
• Liver: when glucose and glucose 6-phosphate levels rise, glycogen synthase is activated and phosphorylase is inhibited, promoting glycogen synthesis in the fed state.
• Muscle: glycogen phosphorylase is allosterically activated by AMP and calcium ions. Calcium ion concentrations rise on contraction when ATP is required. AMP stimulated when ATP levels drop in the cell.

Question 11

Name 3 glycogen storage diseases.

Answer 11

• Type 1 glycogen storage disease is more prevalent in Maltese dogs, due to a defect in or lack of liver glucose 6-phosphase activity.
• Type III glycogen storage disease is due to a defect in debranching enzyme. Seen in Alsatians.
• Type V seen in Chevrolet cattle and is due to a muscle phosphorylase deficiency.

Question 12

How can glycogen storage diseases manifest?

Answer 12

• Diseases that affect the liver manifest as hypoglycaemia

* Diseases that affect muscles manifest as muscle weakness

Question 13

What are the 2 types of tissue that fat is stored as triacylglycerol in and their function?

Answer 13

• White adipose tissue – the main lipid storage. The large spherical white adipocytes have a single lipid droplet that occupies most of the cytoplasm. Main functions are to store triacylglycerol and mobilise it during fasting or starvation.
• Brown adipose tissue – non-shivering thermogenesis. Cells contain many small lipid droplets and many mitochondria (which lead to their brown colour)

Question 14

What is the function of thermogenin and where is it found?

Answer 14

Inner mitochondrial membrane in brown adipose mitochondria contains large amounts of uncoupler protein 1/thermogenin.

• The protein acts as a proton channel and can dissipate the proton gradient which is usually used for protein synthesis in oxidative phosphorylation. The energy dissipated by the gradient is lost as heat instead of being used for protein synthesis.
• Thermogenin is a gated channel that opens in response to noradrenaline stimulation.

Question 15

Describe the process of mobilisation in white adipose tissue.

Answer 15

1. Mobilisation of triacylglycerol by hormone sensitive lipase in lipolysis to produces 3 fatty acids and a glycerol, which both leave the cell. This is activated by glucagon and adrenaline.
2. Glycerol converted to glucose in the liver and fatty acids are bound to albumin in the circulation.
3. Fats can be synthesised from glucose which enters the cell via a glucose transporter. Activated by insulin.
4. Glucose rapidly metabolised by glycolysis, providing acetyl-CoA for fatty acid synthesis and dihydroxyacetone phosphate for glycerol 3-phospahet synthesis, which is required for triacylglycerol synthesis and is activated by insulin.
5. Acetyl-CoA can be converted to acyl-CoA by lipogenesis pathway. Activated fatty by insulin.
6. Acyl-CoA and glycerol 3-phosphate are used by glycerol 3-phosphate acyltransferase to from triacylglycerol in esterification.
7. Adipocytes can also uptake fatty acids produced from lipoproteins in the circulation by lipoprotein lipase. These can enter the esterification process as well for triacylglycerol storage.

Question 16

Outline the key steps of lipogenesis/fatty acid synthesis.

Answer 16

• Export of acetyl-CoA from mitochondria to the cytosol via the citrate shuttle.
• Carboxylation of acetyl-CoA to from malonyl-CoA.
• Formation of the fatty acid by fatty acid synthase
• Further modification of fatty acid and esterification

Question 17

What does citrate shuffle allow? Describe the process of citrate shuffle.

Answer 17

Allows acetyl-CoA in the mitochondrial matrix to leave and be used in fat synthesis in the cytosol.

1. Acetyl-CoA converted to citrate by citrate synthase.
2. Citrate is transported out and into the cytosol by a transporter in the inner mitochondrial membrane.
3. In the cytosol, it is acted on by ATP citrate lyase and is converted into oxaloacetate and acetyl-CoA, utilising ATP hydrolysis.
4. Acetyl-CoA sent back into the mitochondria. Oxaloacetate is converted to malate by the cytoplasmic form of malate dehydrogenase, oxidising an NAD molecule.
5. Malate is decarboxylated by malic enzyme to form pyruvate, reducing an NADP. NADPH used in fat synthesis.
6. Pyruvate can enter the mitochondria via a specific transporter protein in the inner mitochondrial membrane.
7. Here, pyruvate is properly carboxylated to re-form oxaloacetate, utilising ATP hydrolysis.

Question 18

What is the role of malonyl-CoA?

Answer 18

Malonyl-CoA is the intermediate that provide most of the carbon atoms required for the synthesis of fatty acids in the reactions that are catalysed by fatty acid synthase.

Question 19

Describe the synthesis of palmitate.

Answer 19

1. Fatty acid synthase binds 1 acetyl-CoA to 1 malonyl-CoA.
2. The malonyl group binds to a domain called the acyl carrier protein/ACP domain.
3. Malonyl-CoA then acts as a donor for all subsequent carbon atoms in the synthesis of palmitate.
4. Palmitate can be further elongated and desaturated in the smooth endoplasmic reticulum.

The overall reaction requires 14 NADPH molecules.

Question 20

Describe the action of fatty acid synthase to produce 16 C palmitate.

Answer 20

1. Acetyl-CoA donates an acetyl group and malonyl-CoA donates a malonyl group.
2. These both bind to the fatty acid synthase enzyme.
3. These groups then condense to form an acetoacetyl group and a CO2 molecule is lost.
4. Acetoacetyl group undergoes reduction, dehydration and a further reduction to form a butanyl group, still attached to the fatty acid synthase enzyme.
5. Butanyl group can be further elongated with 2 carbon additions from addition melanoma CoA molecule.
6. This continues until 16 carbon palmitate is produced.

Question 21

How is fatty acid synthesis controlled?

Answer 21

Acetyl-CoA carboxylase: is subject to allosteric and hormonal control. This controls the production of malonyl CoA.

Pyruvate: pyruvate supply and activity of pyruvate dehydrogenase.

Hormonal and nutrient control of gene expression affects the expression of enzymes:
• Acetyl-CoA carboxylase
• Fatty acid synthase
• Pentose phosphate pathway enzymes

Question 22

What is the function and source of cholesterol?

Answer 22

Cholesterol is used in cell membranes, steroid hormones, bile acids, vitamin D.

Sourced either from diet or endogenous.

Excess is secreted in bile.

Question 23

Describe cholesterol synthesis and its control.

Answer 23

Endogenous synthesis: synthesised from acetyl-CoA, occurring in most tissues, especially the liver.

Rate limiting enzyme is HMG-CoA reductase.

Short term control:
• Allosterically inhibited by cholesterol
• Inhibited by glucagon, activated by insulin

Long term control: expression of the enzyme is inhibited by cholesterol.

Question 24

What is produced when pancreatic lipase acts on the micelles produced from fat emulsification?

Answer 24

Triacylglycerol, which can produce fatty acids, diacylglycerol and monoglycerols

Cholesterol esters, which can produce cholesterol and fatty acids

Phospholipids, which can produce lysophospholipids and fatty acids

Question 25

Describe lipid absorption.

Answer 25

1. Enterocytes uptake mixed micelles and unpackage them.
2. Lipid components are solubilised by binding to fatty acid binding protein, which helps to stabilise the hydrophobic molecules.
3. These complexes move to the SER, where triacylglycerol is re-synthesised.
4. These move through the Golgi apparatus and are packaged with proteins, cholesterol and phospholipids into chylomicrons.
5. Chylomicrons are secreted into the lacteals before they enter the lymphatic system, so do not enter the blood directly.
6. Medium or short chain fatty acids can actually enter the cell by simple diffusion.
7. Very small fatty acids dop in fact enter the blood directly.

Question 26

State the classes of lipoproteins in order of increasing density.

Answer 26

• Chylomicrons CM
• Very low density lipoproteins VLDL
• Intermediate density lipoproteins IDL
• Low density lipoproteins LDL
• High density lipoproteins HDL

Question 27

Briefly describe the composition of the lipoprotein classes.

Answer 27

Chylomicrons have 90% triglyceride. Triglyceride decreases as density increases.

Cholesterol and phospholipid content generally increase with density, except LDL being greatest in cholesterol and a lower phospholipid content than the trend.

HDL has greatest protein content at 50%. Protein increases as density increases.

Question 28

What are the functions of each class of lipoprotein?

Answer 28

• CMs are involved in the transport of lipids derived from the diet – exogenous.
• VLD, IDL and LDL transport endogenously synthesised triacylglycerol and cholesterol from the liver and other tissues – endogenous.
• HDL cats as a reservoir for a number of apoproteins and a role in cholesterol transport.

Question 29

State 4 roles of apoproteins.

Answer 29

• Structural
• Ligands for receptors
• Activators of enzymes
• Some are loosely associated with lipoproteins and can be transferred between them

Question 30

Describe lipoprotein metabolism.

Answer 30

1. Chylomicrons are acted on by lipoprotein lipase, which breaks down the triacylglycerol core, hydrolysing triacylglycerol.
2. This liberates free fatty acids, which are taken up by tissues.
3. This process repeats and the chylomicrons gets smaller as its triacylglycerol core is hydrolysed.
4. Eventually forms a chylomicron remnant, which is then taken up by the liver.
5. Lipids that are synthesised in the liver are packaged and exported as VLDL particles.
6. These enter the circulation and are acted upon by lipoprotein lipase and their triacylglycerol content is diminished. They are transformed from VLDL to IDL to LDL.
7. LDL can be taken up by peripheral tissues, which helps deliver cholesterol to these peripheral tissues.
8. Or some are recycled back to the liver in endogenous pathway.

Question 31

Describe the pathway of exogenous lipid transport.

Answer 31

1. The nascent (first enter the blood and contain B48 and A proteins) chylomicrons enter the circulation at the thoracic duct, the site of turbulent blood flow that ensures that the chylomicrons are mixed well with blood.
2. Chylomicrons pick up additional apoproteins CII and E from HDL to become fully mature.
3. As the chylomicron move through the tissue capillary bed, they encounter lipoprotein lipase, which is expressed on the surface of blood vessel endothelial cells.
4. Lipoprotein lipase is activated by the lipoprotein CII and it hydrolyses the triacylglycerol within the particle, forming free fatty acids and glycerol, which can be take up by cells, such as muscle and adipose. In muscle, free fatty acids will be used for oxidation. In adipose tissue, the free fatty acids will be stored as triacylglycerol molecules.
5. Chylomicrons are diminished in size and are now called chylomicron remnants because the triacylglycerol core has been hydrolysed.
6. Chylomicron remnants can interact with HDL and there will be some movement of lipoproteins back to the HDL particles.
7. The remnants can then be taken up by hepatocytes via receptor mediated endocytosis.

Question 32

Describe the pathway of endogenous lipid transport.

Answer 32

1. Liver packages newly synthesised triacylglycerol as VLDL particles with cholesterol and apoprotein B100 to form nascent VLDL particles.
2. In the circulation, they can interact with HDL and pick up apoprotein CII and apoprotein E to form the mature VLDL particle.
3. Mature VLDL particles will interact with LPL on the surface of the endothelial cells in the capillary bed. Lead to breakdown of the triacyclglycerol core, leading to a reduction in size of the VLDL particle until it is VLDL remnant.
4. The VLDL remnant can interact with HDL particles. There is some movement of apoproteins back into the HDL particle.
5. This movement will lead to the formation of IDL particles.
6. IDL particles will continue to interact with HDL in the circulation until they become LDL particles.
7. LDL particles can be taken up by peripheral tissues through interaction with LDL receptors in receptor mediated endocytosis.
8. The cholesterol that from a large part of the LDL can be esterified the cholesterol acyl transferase to form cholesterol esters.
9. IDL and LDL returned to the liver and recycled by receptor mediated endocytosis.

Question 33

What is HDL and reverse cholesterol transport?

Answer 33

Mops up cholesterol from peripheral tissues and transfers it to other lipoprotein particles back to the liver for secretion.

1. Picks up cholesterol from peripheral tissue and esterifies cholesterol to form cholesterol esters by lecithin acyl transferase.
2. HDL converts cholesterol into a spherical, mature HDL particle. This can either interact with and transfer cholesterol to other particles via cholesterol ester transfer protein, or it can be taken up by the liver.

Question 34

Define vitamin and state the 2 classes.

Answer 34

Complex organic substances required in relatively small quantities in the diet to maintain health. They are generally required as enzyme cofactors or as antioxidants.

• Water soluble vitamins – the B group and vitamin C
• Hydrophobic/fat soluble vitamins – vitamins A, D, E and K

Question 35

Name and describe the 3 forms of fat soluble vitamin A.

Answer 35

Retinoic acid acts as a steroid hormones promoting epithelial cell growth. Deficiencies manifest in Xeropthalmia, a thickening and dryness of the skin and mouth. Seen in dry eyes due to the impairment of tear duct epithelial function, as well as keratinisation of the cornea.

Ret8inal is a component of the visual pigment, rhodopsin. Deficiencies manifest in night blindness.

Beta-carotene is an antioxidant.

Question 36

Name and describe the 3 other fat soluble vitamins.

Answer 36

• Vitamin D plays an essential role in calcium homeostasis and deficiencies manifest themselves as rickets.
• Vitamin E is an antioxidant and deficiencies are rare.
• Vitamin K is an important cofactor in the maturation of clotting factors. Deficiencies are rare as K is produced by gut bacteria, but when they occur, they manifest as bleeding disorders.

Question 37

When may there be deficiencies in fat soluble vitamins?

Answer 37

Absorption of fat soluble vitamins relies upon normal fat digestion and absorption, so severer intestinal diseases may cause deficiencies.

Question 38

What do the B vitamins have in common?

Answer 38

All co-enzymes in metabolic pathways.

Question 39

What is the function of vitamin B12 and what do deficiencies manifest as?

Answer 39

Carrier of methyl groups.

Deficiencies are largely due to defective absorption of this vitamin. Manifest as anaemia and failure to thrive. Sometimes seen in animals on cobalt deficient pastures or GI diseased companion animals.

Question 39

What is the function of vitamin B12 and what do deficiencies manifest as?

Answer 39

Carrier of methyl groups.

Deficiencies are largely due to defective absorption of this vitamin. Manifest as anaemia and failure to thrive. Sometimes seen in animals on cobalt deficient pastures or GI diseased companion animals.

Question 40

How is vitamin B12 absorbed?

Answer 40

1. Gastric parietal cells synthesise and secrete intrinsic factor.
2. Vitamin B12 in the stomach binds to IF to form a B12:IF complex.
3. B12:IF complex is absorbed by enterocytes in small intestine.
4. B12 is released into the blood stream.

Question 41

What are the 2 roles of vitamin C?

Answer 41

Antioxidant and as a cofactor for collagen maturation:

During collagen maturation, required for the hydroxylation of lysine and proline main acid side chains. These hydroxylated amino acids are essential for stabilisation of the collagen triple helix.

Question 42

What do deficiencies in vitamin C manifest as?

Answer 42

Deficiencies manifest as poor connective tissue formation and poor wound healing. These rarely occur, as most animals, except monkeys ad guinea pigs, can produce their own vitamin C. gives rise to scurvy in these species, having symptoms of lethargy, painful joints and tooth loss.

Question 43

What is the effect of free radicals on red blood cells?

Answer 43

• Oxidation of iron in haemoglobin to form methaemoglobin and damage to membranes.
• Methaemoglobins precipitate to form Heinz bodies, which can cause RBC rupture and haemolysis.

Cats are particularly susceptible and oxidative stress which can be caused by onions and paracetamol.

Question 44

What are 3 enzymes that combat free radicals?

Answer 44

Superoxide dismutase: 2O2*- + 2H+ –> H2O2 + O2

Catalase: H2O2 –> H2O + 1/2O2

Glutathione peroxidase: contains selenium and removes hydrogen peroxide and other reactive oxygen species via the oxidation of glutathione and reduction of NADP. Consists of glycine-cystine-glutamic acid via disulphide bonds.

Question 45

Name and describe the main pathway of NADPH production.

Answer 45

1. Glucose 6-phosphate is converted to 6-phosphogluconolactone by glucose 6-phosphate dehydrogenase. This reduced an NADP in the process to form NADPH.
2. 6-phosphogluconolactone is converted to ribose 5-phosphate.

Pathway is entirely cytosolic.
Particularly important in RBCs, which can’t synthesise NADPH any other way.

Glucose 6-phophate dehydrogenase is the rate determining step and is controlled by feedback of NADPH – as levels of NADPH rise, its activity decreases.

Question 46

How do antioxidant vitamins detoxify lipid peroxides?

Answer 46

1. Reactive oxygen species produce lipid peroxides.
2. Vitamin E can pick up these unpaired electrons and give them to the water soluble vitamin C.
3. Vitamin C is able to pass on the free radical to glutathione, which is oxidised and the free radical is entirely detoxified.
4. Regeneration of glutathione to the reduced form by glutathione reductase, which requires NADPH to be oxidised.

Question 47

Name some trace elements that are required for health in small quantities.

Answer 47

• Iodine – required for the production of thyroid hormone
• Cobalt – a constituent of vitamin B12
• Selenium – cofactor for glutathione peroxidase
• Copper – cofactor for a number of enzymes such as superoxide dismutase, component to the electron transport chain, cofactor required for collagen synthesis

Question 48

Give an example of the 2 forms of nomenclature that can be used to name fatty acids.

Answer 48

Omega nomenclature: the carbon after the acid group is alpha, the next is beta, then gamma, etc. the last methyl group at the end of the chain is always omega carbon. This system is useful for identifying the position of the double bond in unsaturated fatty acids.

Linoleic acid has 18 carbons and 2 double bonds:

• In numerical system, it is 18:2 [delta]9,12.
• In omega system, count back to each double bond form the omega carbon: 18:2 [delta]-6,9.

Question 49

What are the priorities in fasting and starvation?

Answer 49

To mobilise fuel sources such as triacylglycerol and glycogen to provide substrates for cellular respiration.

Protein breakdown in minimised as much as possible to preserve function.

It is a priority to maintain a supply of glucose, as some tissues are obligate glucose uses, such as red blood cells. Other tissues are preferential glucose users, such as the CNS.

Question 50

What changes occur after 24 hours after the last meal?

Answer 50

• Triacylglycerol breakdown in adipose tissue to liberate fatty acids and glycerol
• Glycerol can be taken up by the liver and converted into glucose by gluconeogenesis
• The liver is also breaking down its own glycogen to form glucose
• Glucose released by the liver prevents hypoglycaemia in the fasted state and allows preferential glucose utilisers, such as the brain, to continue using glucose oxidation to fulfil their ATP needs
• Very little protein is broken down at this stage

Question 51

Which tissues can oxidise fatty acids?

Answer 51

The CNS cannot use fatty acids, as these are transported in the blood bound to albumin. The albumin-fatty acid complex cannot cross the blood brain barrier.

Tissues like muscles can oxidise fatty acids and mobilise their own glycogen stores. This glycogen cannot contribute to blood glucose, instead the glucose 6-phopshate that is produced by the glycogen mobilisation is oxidised by the cells themselves.

Question 52

What changes when the time period since the last meal increases?

Answer 52

• Liver starts to take up and metabolise some of the increasing amounts of fatty acids that are being mobilised from adipose tissue.
• These fatty caids are oxidised to acetyl-CoA, which is converted to molecules called ketone bodies – important fuel molecules during starvation, rapidly used by tissues including the brain and muscles.
• In late starvation, there are significant amounts for protein being broken down to amino acids.
• Amino acids can be used by the liver to synthesise glucose to maintain blood glucose levels.

Question 53

Describe the fed state in hepatocytes.

Answer 53

• Glucose can enter the cell via the GLUT-2 transporter
• Converted to glucose 6-phosphate by glucokinase, the liver isoenzyme of hexokinase in the first step of glycolysis
• Much of the glucose 6-phosphate will be used for glycogen synthesis while some may be oxidised to pyruvate then acetyl-CoA and used for fat synthesis.

Question 54

Describe the starved state in hepatocytes.

Answer 54

• Glycogen is broken down to glucose 6-phopshate and converted to glucose by glucose 6-phopshatase, only present in the liver and kidney.
• The direction in which glucose is transported is entirely dependent on the glucose concentration gradient.
• In the starved state, pyruvate can be converted to glucose 6-phospahte by gluconeogenesis.

Question 55

What 3 essentially irreversible steps does glycolysis have? What does this mean?

Answer 55

1. Phosphoenolpyruvate + ADP –> pyruvate + ATP by pyruvate kinase
2. Fructose 6-phosphate + ATP –> fructose 1,6-bisphosphate + ADP by phosphofructokinase
3. Glucose + ATP –> glucose 6-phosphate + ADP by glucokinase

Alternative enzymes and energy inputs are required to by-pass these reactions. This means that glucose cannot be made by reversing glycolysis.

Question 56

Describe the process of gluconeogenesis.

Answer 56

1. Lactate converted to pyruvate by lactate dehydrogenase or from the carbon skeleton of alanine.
2. Pyruvate is carboxylated to oxaloacetate by pyruvate carboxylase.
3. Oxaloacetate is then decarboxylated to from phosphoenolpyruvate by phosphoenolpyruvate carboxykinase.
4. These 2 steps must overcome the irreversible glycolytic step that is catalysed by pyruvate kinase.
5. Propionyl-CoA is synthesised from odd chain fatty acids and comes into the gluconeogenesis pathway here and forms phosphoenolpyruvate.
6. Gluconeogenesis uses the same reversible steps as glycolysis to produce 1,3-bisphosphoglycerate.
7. 1,3-bisphosphoglycerate is converted to fructose 1,6-bisphopshate.
8. Fructose 1,6-bisphosphate can be converted to fructose 6-phosphate via fructose 1,6-bisphopahtase-1.
9. Fructose 6-phosphate can be converted to glucose 6-phosphate by glucose 6-phosphatase.
10. Phosphate is removed to produce glucose.

Question 57

Describe the short term mechanisms that control gluconeogenesis.

Answer 57

• Carboxylation of pyruvate to oxaloacetate is allosterically inhibited by ADP, when ATP is low.
• Phosphoenolpyruvate carboxykinase is allosterically inhibited by ADP.
• Pyruvate kinase is allosterically inhibited by alanine. Levels of alanine rise in hepatocytes when proteins are being broken own in starvation
• Pyruvate carboxylase is allosterically activated by acetyl-CoA as an intermediate which rises during starvation as rates of fatty acid oxidation rises.
• Fructose 1,6-phosphatase is allosterically activated by citrate levels, which is high in cells when ATP is abundant. This is also inhibited by AMP, which rises as ATP levels drop. It is also sensitive to the levels of fructose 2,6-bisphosphate, which is determined by the rate of its synthesis (inhibited by glucagon and activated by insulin).

Question 58

Can glucose be synthesised from fats?

Answer 58

No:

• Fatty acids are oxidised to acetyl-CoA (and CO2), which cannot be converted to pyruvate because pyruvate dehydrogenase is irreversible.
• The only exception to this is odd chain length fatty acids which are oxidised to acetyl-CoA and propionyl-CoA, which can be converted to phosphoenolpyruvate. This is particularly important in ruminants.

Question 59

Describe glucose homeostasis in ruminants.

Answer 59

1. Cellulose is fermented in the rumen by the ruminants bacteria and produce short chain fatty acids acetate, propionate and butyrate.
2. These are absorbed into the bloodstream, of the ruminant.
3. Propionate can act as a substrate for gluconeogenesis and can produce glucose.

This can be seen to some extent in non-ruminants, occurring in the large intestine when complex carbohydrates not broken down in the small intestine are fermented by gut bacteria.

Question 60

What is the cori cycle?

Answer 60

The process by which lactate produced by tissues such as RBCs or anaerobically respiring muscles is taken up into the blood and then taken up into the liver. It can then be used to generate glucose, which the liver exports to other tissues.

Question 61

What is the alanine cycle?

Answer 61

The metabolism of amino acids in muscles generates the toxic compound ammonium.

• This combines with pyruvate to form alanine, which leaves the cells and is taken up by the liver.
• The amino group in alanine is removed, leaving behind the carbon skeleton, which is pyruvate.

This is an example of ammonium being safely transported to the liver to be detoxified to form urea.

Question 62

Name the 3 ketone bodies.

Answer 62

Acetoacetate
Beta-hydroxybutyrate
Acetone

Question 63

Describe ketogenesis.

Answer 63

1. Beta-oxidation of fatty acids and ketogenic amino acids produce acetyl-CoA.
2. The first ketone body produced is called acetoacetate, which can spontaneously breakdown into acetone or reduction to form beta-hydroxybutyrate.

Rate of ketone body formations controlled by the availability of oxaloacetate.

Question 64

Describe ketoacidosis.

Answer 64

Hepatocytes cannot use ketone bodies are must transport them to other cells for oxidation. They are oxidised by the mitochondria of tissues, such as the brain, muscle and heart.

In starvation, liver oxaloacetate is used in gluconeogenesis so acetyl-CoA cannot enter the citrate cycle and it builds up in the cell.

Normally the rate of ketone body formation = the rate of ketone body oxidation. If synthesis exceeds oxidation, there can be a build up of ketone bodies in ketoacidosis , lowing blood pH.

Question 65

What is ‘glucose-sparring’ effect and what is the mechanism for this?

Answer 65

Oxidation of fatty acids and ketone bodies has a ‘glucose-sparing’ effect in tissues such as muscles, meaning these oxidations decrease the rate at which glucose is being utilised in these tissues. This helps preserve glucose for other tissues during fasting/starvation for obligate glucose utilisers.

1. Increase in lipolysis liberates fatty acids from adipose.
2. Fatty acid oxidation in muscle increases acetyl-CoA levels.
3. Increasing acetyl-CoA inhibits pyruvate dehydrogenase.
4. Increasing Acetyl-CoA also increases citrate, which inhibits phosphofructokinase.

Question 66

What is excess amino acids converted to?

Answer 66

Excess amino acids cannot be stored so their metabolism takes preference over that of excess glucose and fat. Ammonia, the immediate natural product of amino acid breakdown, is toxic.

• Mammalian liver converts ammonium to urea and glutamine
• Birds and reptiles produce uric acid
• Fish secrete ammonium directly

Question 67

Describe ammonium metabolism.

Answer 67

1. Muscles and other tissues metabolise proteins to amino acids, which are further broken down into alanine and glutamine.
2. These are exported and broken down in the liver.
3. Bacteria in the gut also produce ammonia through their metabolism of nitrogenous compounds.
4. This ammonia is directly absorbed into the bloodstream.
5. Intestinal cells use glutamine to produce ammonium.
6. This ammonia and the ammonia produced from but bacteria enter the foetal circulation.
7. The foetal circulation is also rich in amino acids absorbed from the diet. The liver will metabolise some of these amino acids along with glutamine and alanine it receives from other tissues, generates ammonia.

Question 68

What are the effects of hyperammonaemia?

Answer 68

Hyperammonaemia can cause tremors, seizures and blurred vision. In much higher concentrations, it may lead to irreversible brain damage, coma and death.
Any disease that compromises liver function can lead up hyperammonaemia.

Question 69

Describe enzyme action in hepatocytes on amino acids.

Answer 69

• Transfers the amino group to alpha-ketoglutarate, generating the amino acid glutamate and the carbon skeleton of the original amino acid, alpha keto acid.
• Glutamate produced by the aminotransferase reactions can be deaminated by glutamate dehydrogenase to produce ammonia and alpha-ketoglutarate.
• This ammonia is used to synthesise urea.
• The liver also uptakes glutamine from the circulation. This is deaminated by glutaminase to generate ammonium and glutamate.

Question 70

What is a test for liver damage?

Answer 70

Liver damage leads to the release of amino transferases into the circulation. The presence of 2 alanine amino transferase and aspartate aminotransferase are used as liver function tests.

Question 71

State the 2 main fates of glutamate in the liver.

Answer 71

• Oxidative deamination via GDH to produce ammonium
• Amino transfer via aspartate aminotransferase to produce asparate

Ammonium and aspartate enter the urea cycle and provide the Nitrogen in urea synthesis.

Question 72

Describe the urea cycle.

Answer 72

1. Ammonia produced by glutamate dehydrogenase and glutaminase.
2. Ammonium is converted to carbamoyl phosphate by carbamoyl phosphate synthase. This is an irreversible reaction that consumes ATP.
3. Carbamoyl phosphate combines with ornithine to produce citrulline.
4. Citrulline combines with aspartate to form arginosuccinate, which breaks down to arginine.
5. Arginine is then converted back to ornithine with the generation of urea.

Question 73

What is the rate determining step of the urea cycle?

Answer 73

Ammonium is converted to carbamoyl phosphate by carbamoyl phosphate synthase.

• Activated by an intermediate called N-acetyl glutamate, generated when glutamate levels in the liver are high.
• Enzyme is inhibited by increasing levels of protons.

Question 74

What are the periportal and perivenous cells?

Answer 74

The hepatic portal provides blood rich in oxygen and nutrients to the periportal cells. Periportal cells are close to the hepatic portal vein.

The perivenous cells are closer to the central vein and the centre of the hexagon. These receive blood relatively poor in nutrients.

Periportal and perivenous cells have different metabolic priorities, giving rise to metabolic zonation within the liver lobules.

Question 75

Describe metabolic zonation in terms of ammonia detoxification.

Answer 75

1. Periportal cells take up amino acids from the blood and ammonium form the gut.
2. Transaminases incorporate the amino groups into glutamate.
3. Glutamate can be made into ammonia or aspartate, which can enter the urea cycle.
4. Any ammonia that escapes exits the periportal cells but is mopped up by the perivenous cells, which utilise the ammonia to generate an amino acid called glutamine, catalysed by glutamine synthetase. This dual system means all the ammonium is detoxified.
5. Glutamine is exported from the venous cells and goes to the kidney for metabolism, where it generates bicarbonate and ammonium.
6. This ammonium can be directly excreted in the urine.
7. Some of the glutamine carbon skeletal can be used for gluconeogenesis and hence the production of glucose in the kidney.

Question 76

How is glutamine synthesis important in acidosis?

Answer 76

In acidotic conditions, urea synthesis is suppressed due to carbamoyl phosphate synthetase being inhibited by protons.

• Inhibition of urea cycle in acidosis is important, as the cycle consumes bicarbonate, an important buffer.
• The inhibition of urea synthesis means more ammonium escapes the periportal cells and its detoxification via glutamine synthesis in the perivenous cells becomes more important.

Glutamine synthesis is stimulated by acidosis.

Question 77

What is glutaminase and how is it affected by acidosis?

Answer 77

The enzyme that converts glutamine to glutamate.

• Liver glutaminase is inhibited by acidosis. This decreases the production of N-acetyl glutamate, the allosteric activator of carbamoyl synthase.
• Kidney glutaminase is activated by acidosis, stimulating ammonium release into the urine and generating bicarbonate to counter act acidosis.

Question 78

Describe the metabolism of haem groups in the liver.

Answer 78

Old red blood cells are broken down and the haemoglobin polypeptide chain is hydrolysed to free amino acids.

1. Haem group is metabolised by a number of steps to form a compound called bilirubin, which binds to albumin in the circulation.
2. For easier secretion of bilirubin, it is modified by conjugation to polar groups to make it more water-soluble.
3. Conjugation reaction occur in the smooth endoplasmic reticulum of the liver.
4. Conjugation is catalysed to UDP glucuronic transferase, producing conjugated bilirubin/bilirubin glucuronide.
5. This conjugated bilirubin can be excreted in the bile.
6. Secreted into the gut and acted on by gut bacteria to from a variety of compounds, such as urobilinogen and stercobilin.
7. Stercobilin is secreted in the faeces but some urobilinogen can be reabsorbed in the blood.

Question 79

What is jaundice? What are the causes of jaundice?

Answer 79

Presents as yellowing of the sclera and mucous membranes due to the deposition of bilirubin.

• Pre-hepatic jaundice is a term given to conditions that lead to increased haem breakdown.
• Hepatic jaundice is caused by liver disease, which interferes with the ability of the liver to take up, conjugate or secrete bilirubin.
• Post-hepatic jaundice is caused by obstruction tit e biliary system.