Case 6 Flashcards
what are the different stages of carbohydrate metabolism?
- Glycolysis
- Link Reaction
- Krebs’ Cycle/ Citric Cycle
- Electron Transport Chain/ Oxidative Phosphorylation
glycolysis
- what is it
- what does it generate
- whats different if in aerobic or anaerobic conditions
• Glycolysis is the catabolism (breaking down) of glucose (and most other carbohydrates via glucose) in the cytoplasm of all tissues.
• This generates intermediates for other pathways of metabolism.
• In aerobic conditions, glycolysis generates energy.
• The end product of glycolysis depends on O2:
Aerobic conditions = Pyruvate (2 molecules)
Anaerobic conditions = Lactate
go through the process of glycolysis
- An ATP molecule is hydrolysed and the phosphate attached to the glucose molecule at C-6
- Glucose 6 Phosphate is turned into fructose 6 phosphate
- Another ATP is hydrolysed, and the phosphate attached to C-1
- The hexose sugar is activated by the energy release from the hydrolysed ATP molecules. It now cannot leave the cell and is known as Hexose-1,6-biphosphate
- It is split into two molecules of Triose phosphate
- Two hydrogen atoms are removed from each Triose Phosphate, which involved dehydrogenase enzymes.
- NAD combines with the Hydrogen atoms to form reduce NAD
- Two molecules of ATP are formed- substrate level phosphorylation
- Four enzyme-catalysed reactions convert each triose phosphate molecule into a molecule of pyruvate.
what is the net gain in ATP molecules by the entire glycolytic process?
2 molecules for each molecule of glucose.
link reaction
- where does this reaction take place
- what happens
- enzyme
- ATP
- what released
- This reaction takes place in the matrix of the mitochondrion.
- Pyruvate is actively transported into the mitochondria.
- In this step, 2 molecules of pyruvate, formed in glycolysis, are converted into 2 molecules of Acetyl Coenzyme A (Acetyl CoA).
- This reaction occurs under the influence of the enzyme pyruvate dehydrogenase.
- There is no ATP formation in this reaction.
- 4 hydrogen atoms are released which will be used later (oxidative phosphorylation) to form 6 molecules of ATP.
Krebs cycle
- where does it occur
- what happens
- This reaction occurs in the matrix of the mitochondrion.
- In this step, the acetyl CoA is degraded into carbon dioxide and hydrogen atoms.
- The release of hydrogen atoms will be used later (oxidative phosphorylation).
- Acetate is offloaded from CoA and joins with Oxaloacetate to form citrate.
- Citrate is decarboxlyated and dehydrogenated to form a 5C compound.
a. The hydrogen atoms are accepted by NAD, which take them to the Electron Transport Chain
b. The Carboxyl group becomes CO2. - The 5C compound is decarboxylated and dehydrogenated to form a 4C compound.
- The 4C compound is changed into another 4C compound, and a molecule of ATP is phosphorylated.
- The second 4C compound is changed into a third 4C compound and a pair of hydrogen atoms are removed, reducing FAD.
- The third 4C compound is further dehydrogenated to regenerate oxaloacetate.
what is the net reaction per molecule of glucose?
Enter into the cycle:
2 acetyl-CoA molecules
6 molecules of water
Release from the cycle: 4 carbon dioxide molecules 16 hydrogen atoms 2 molecules of coenzyme 2 molecules of ATP are formed (one acetyl CoA molecule = one ATP molecule) 6 NADH
for every molecule of glucose, how much ATP and hydrogen atoms do the first three stages of carbohydrate metabolism make?
4 molecules of ATP
24 molecules of hydrogen atoms
how many of the H atoms formed in first three stages combine with NAD+ and under influence of what? what happens to the NADH formed?
- 20/24 hydrogen atoms that were formed before combine with nicotinamide adenine dinucleotide (NAD+) under the influence of a dehydrogenase enzyme.
- This forms NADH and H+, which enter oxidative phosphorylation.
how much of ATP formation occurs at electron transport chain? what is undergoing oxidative phosphorylation?
• 90% of ATP formation occurs in this stage – oxidative phosphorylation of the hydrogen atoms that were released during the earlier stages of glucose degradation.
oxidative phosphorylation
- which stage
- what does it involve
- describe what happens
- Oxidative phosphorylation is the final stage of respiration.
- It involves electron carriers embedded in the mitochondrial membrane.
- These membranes are folded into cristae, which increases the surface area for electron carriers and ATP synthase enzymes.
- Oxidative phosphorylation is the formation of ATP by the addition of an inorganic phosphate to ADP in the presence of oxygen.
- As protons flow through ATPsynthase, they drive the rotation part of the enzyme and join ADP to Pi to make ATP.
- The electrons are passed from the final electron carrier to molecular oxygen, which is the final electron acceptor.
- Hydrogen ions also join, so oxygen is reduced to water
describe the process of chemiosmosis in oxidative phosphorylation
- Reduced NAD and FAD donate hydrogens, which are split into protons and electrons, to the electron carriers.
- The protons are pumped across the inner mitochondrial membrane using energy released from the passing of electrons down the electron transport chain.
- This builds up a proton gradient, which is also a pH gradient, and an electrochemical gradient
- Thus, potential energy builds up
- The hydrogen ions cannot diffuse through the lipid part of the inner membrane, but can diffuse through ATP synthase- an ion channel in the membrane. The flow of hydrogen ions is chemiosmosis.
- As H+ ions flow through ATPsynthase, they drive the rotation part of the enzyme and join ADP to Pi to make ATP.
give a summary of carbohydrate metabolism in terms of ATP and H gained
- Glycolysis = 2 ATP molecules and 4H gained. (actually four molecules of ATP are formed, and two are expended to cause the initial phosphorylation of glucose to get the process going. This gives a net gain of two molecules of ATP).
- Link Reaction = 0 ATP molecules and 4H gained.
- Citric Acid Cycle = 2 ATP molecules and 16H gained.
- Oxidative Phosphorylation = 20H go in and 30 ATP molecules gained.
(During the entire schema of glucose breakdown, a total of 24 hydrogen atoms are released during glycolysis and during the citric acid cycle. Twenty of these atoms are oxidized in conjunction with the chemiosmotic mechanism, with the release of 3 ATP molecules per two atoms of hydrogen metabolized. This gives an additional 30 ATP molecules.) - The remaining four hydrogen atoms are released by their dehydrogenase. Two ATP molecules are usually released for every two hydrogen atoms oxidized, thus giving a total of 4 more ATP molecules.
what is the maximum number of ATP molecules formed for each glucose molecule?
A maximum of 38 ATP molecules are formed for each glucose molecule degraded to carbon dioxide and water.
why is the maximum yield for ATP rarely reached?
Some hydrogens leak across the mitochondrial membrane
o Less protons to generate the proton motive force
Some ATP is used to actively transport pyruvate into the mitochondria
Some ATP is used to bring Hydrogen from reduced NAD made during glycolysis, into the cytoplasm, into the mitochondria.
how does the ATP yield from anaerobic respiration compare to that of aerobic respiration? why?
• Anaerobic respiration produces a much lower yield of ATP than aerobic respiration because only glycolysis takes place in anaerobic respiration.
The electron transport chain cannot occur, as there is no oxygen to act as the final electron acceptor.
This means that the Krebs cycle stops, as there are no NAD- they are all reduced.
This prevents the link reaction from occurring.
Anaerobic respiration takes the pyruvate, and by reducing it, frees up the NAD, so glycolysis can continue, producing two molecules of ATP per glucose molecule respired.
how do we get the fatty acids need by the body?
• Most of the fatty acids needed by the body are provided with a normal diet.
what happens to any carbohydrates or proteins in excess of the body’s needs?
• Any carbohydrates or proteins in excess of the body’s needs can be converted to fatty acids by the liver and ultimately stored as fats (triacylglycerols) in adipocytes.
describe fatty acid synthesis
- starting in the mitochondria
• Since most acetyl-Co-A is generated in mitochondria and cannot cross the membrane, it needs to be moved into the cytoplasm.
• In the mitochondria, high energy levels (high ATP/ADP) inhibit isocitrate dehydrogenase (*) and lead to an increase in citrate in mitochondria.
• Citrate can be moved to the cytoplasm and converted “back” to acetyl-CoA.
• The next step, catalysed by Acetyl CoA carboxylase (ACC) is the conversion of acetyl CoA into Malonyl-CoA.
• This is the rate limiting and regulated:
ACC is activated by citrate and insulin.
o The enzyme is active as a multi-subunit polymer stabilised by citrate.
ACC is inactivated directly by fatty acyl-CoA and by phoshorylation by AMPK.
- Next, the Malonyl CoA is converted into Fatty acyl-CoA, in the presence of the enzyme Fatty acyl synthase (FAS).
- Fatty acyl synthase (FAS) is a multi-tasking enzyme that catalyses multiple rounds of chain elongation, reduction, dehydration and reduction (actually a 7-step reaction).
describe triacylglycerol (TAG) synthesis
• Fatty acyl-CoA is now converted into Triacylglycerol (TAG).
• To produce TAG as storage form of fatty acids, fatty acyl-CoA need to be linked up (esterified) with glycerol-3-phosphate.
• Two reactions that produce glycerol-3-P are available:
Glycerol-3-P dehydrogenase
Uniquely in the liver, glycerol kinase
o This reaction allows the glycerol part of TAGs to be used in gluconeogenesis.
- Adipocytes do not express glycerol kinase and so cannot metabolise glycerol produced during TAG mobilisation.
- The liver packages TAGs into VLDL for delivery and storage to peripheral tissues.
describe fatty acid catabolism
- how much energy produced
- per 2-carbon unit
- per 16-carbon unit
• The β-oxidation of fatty acids produces large amounts of energy:
Per 2-carbon unit, one FADH2, one NADH and one acetyl-CoA are produced.
Ultimately, these produce 2, 3 and 12 ATP, respectively.
Per 16-carbon (palmitoyl-) CoA, that’s 129 ATP!
triacylgylcerol -> fatty acids -> fatty acyl-CoA -> acetyl-CoA
ketone bodies
- what are they
- what can use ketone bodies
- how soluble
- how transported
- what is seen in type I diabetes
- what is ketoacidosis
- Ketone bodies are an “emergency fuel” that the liver can produce to preserve glucose.
- The liver itself cannot use ketone bodies, though!
- During starvation, the ability of the liver to oxidise fatty acids released from adipocytes may be limited.
- The liver produces ketone bodies and releases them into the blood for peripheral tissues.
- Ketone bodies are highly soluble and unlike lipids can be transported without carriers.
- Increased levels of ketone bodies in blood (ketonemia) and urine (ketonuria) are observed in uncontrolled type 1 diabetes mellitus.
- The acidity of ketone bodies lowers blood pH (ketoacidosis).
what hormones does the pancreas secreted?
- The pancreas, in addition to its digestive functions, secretes two important hormones, insulin and glucagon, that are crucial for normal regulation of glucose, lipid, and protein metabolism.
- Although the pancreas secretes other hormones, such as amylin, somatostatin, and pancreatic polypeptide, their functions are not as well established.
the pancreas is composed of two major type of tissue, what are these?
- The acini, which secrete digestive juices into the duodenum.
- The islets of Langerhans, which secrete insulin and glucagon directly into the blood.
what are the islets of Langerhans organised around?
small capillaries into which its cells secrete their hormones.
what cells are the islets made up of?
- what percentage do they constitute
- what do they secrete
• The islets contain four types of cells which are distinguished from one another by their morphological and staining characteristics:
1. Alpha cells
Constitute about 25% of the cells of the islet.
Secrete glucagon.
2. Beta cells
Constitute about 60% of the cells of the islet.
Secrete insulin and amylin.
3. Delta cells
Constitute about 10% of the cells of the islet.
Secrete somatostatin.
4. PP cells
Constitute about 5% of the cells of the islet.
Secrete pancreatic polypeptide.
what does ghrelin do and where is it produced?
Ghrelin is a hormone that is produced and released mainly by the stomach with small amounts also released by the small intestine, pancreas and brain. Ghrelin has numerous functions. It is termed the ‘hunger hormone’ because it stimulates appetite, increases food intake and promotes fat storage.
what does pancreatic polypeptide do?
- promotes fluid secretion in the GI tract
- decreases food intake
describe interrelations between the cells in the islets
• The close interrelations among these cell types in the islets of Langerhans allow cell-to-cell communication and direct control of secretion of some of the hormones by the other hormones.
Insulin inhibits glucagon secretion.
Amylin inhibits insulin secretion.
Somatostatin inhibits the secretion of both insulin and glucagon.
insulin
- what does it affect
- what usually causes death in diabetic patients
- what does this mean in terms of function of insulin
- Insulin affects carbohydrate metabolism.
- Yet it is abnormalities of fat metabolism, causing such conditions as acidosis and arteriosclerosis that are the usual causes of death in diabetic patients.
- Also, in patients with prolonged diabetes, diminished ability to synthesize proteins leads to wasting of the tissues as well as many cellular functional disorders.
- Therefore, it is clear that insulin affects carbohydrate, fat and protein metabolism.
what is insulin secretion associated with energy-wise?
- Insulin secretion is associated with energy abundance.
- That is, when there is great abundance of energy-giving foods in the diet, especially excess amounts of carbohydrates, insulin is secreted in great quantity.
describe insulin’s important role in storing excess energy
In the case of excess carbohydrates, it causes them to be stored as glycogen mainly in the liver and muscles.
Also, all the excess carbohydrates that cannot be stored as glycogen are converted under the stimulus of insulin into fats and stored in the adipose tissue.
In the case of proteins, insulin has a direct effect in promoting amino acid uptake by cells and conversion of these amino acids into protein.
In addition, insulin inhibits the breakdown of the proteins that are already in the cells.
what is insulin? what composed of? how is the functional activity of insulin lost?
- Insulin is a small protein.
- It is composed of two amino acid chains, connected to each other by disulfide linkages.
- When the two amino acid chains are split apart, the functional activity of the insulin molecule is lost.
describe the synthesis of insulin
• Insulin is synthesized in the beta cells by the usual cell machinery for protein synthesis:
Beginning with translation of the insulin RNA by ribosomes attached to the endoplasmic reticulum to form an insulin preprohormone.
The preprohormone is then cleaved in the endoplasmic reticulum to form proinsulin.
The proinsulin is further cleaved in the Golgi apparatus to form insulin and peptide fragments.
Insulin is packaged into secretory granules.
when insulin is secreted into the blood, in which form does it circulate? what is its plasma half-life?
- When insulin is secreted into the blood, it circulates almost entirely in an unbound form.
- It has a plasma half-life that averages only about 6 minutes, so that it is mainly cleared from the circulation within 10 to 15 minutes.
what does insulin do in the blood?
- Binds to insulin receptors in target cells.
- The remainder is degraded by the enzyme insulinase mainly in the liver, to a lesser extent in the kidneys and muscles, and slightly in most other tissues.
why is this rapid removal from the plasma important?
because, at times, it is as important to turn off rapidly as to turn on the control functions of insulin.
activation of target cell receptors
- what does insulin do
- what causes the effect
- describe the receptor
- where does the insulin bind
- what happens
- thus, what is the insulin receptor an example of
- To initiate its effects on target cells, insulin first binds with and activates a membrane receptor protein.
- It is the activated receptor, not the insulin that causes the subsequent effects.
- The insulin receptor is a combination of four subunits held together by disulfide linkages: two alpha subunits that lie entirely outside the cell membrane and two beta subunits that penetrate through the membrane, protruding into the cell cytoplasm.
- The insulin binds with the alpha subunits on the outside of the cell, but because of the linkages with the beta subunits, the portions of the beta subunits protruding into the cell become autophosphorylated.
- Thus, the insulin receptor is an example of an enzyme-linked receptor.
activation of target cell receptors
- what is required for effect to manifest on receptor
- what does autophosphorylation of the beta subunits of the receptor activate
- what effect does this have
- therefore what is the effect of insulin
• One insulin molecule can only bind to one alpha subunit, therefore two insulin molecules are required for its effects to manifest.
• Autophosphorylation of the beta subunits of the receptor activates a local tyrosine kinase, which in turn causes phosphorylation of multiple other intracellular enzymes including a group called insulin-receptor substrates (IRS).
Different types of IRS (e.g. IRS-1, IRS-2, IRS-3) are expressed in different tissues.
The net effect is to activate some of these enzymes while inactivating others.
• In this way, insulin directs the intracellular metabolic machinery to produce the desired effects on carbohydrate, fat, and protein metabolism.
what are the ends effect of insulin stimulation?
- Within seconds after insulin binds with its membrane receptors, the membranes of about 80% of the body’s cells markedly increase their uptake of glucose.
- The cell membrane becomes more permeable to many of the amino acids, potassium ions, and phosphate ions, causing increased transport of these substances into the cell.
- Slower effects occur during the next 10 to 15 minutes to change the activity levels of many more intracellular metabolic enzymes.
- Much slower effects continue to occur for hours and even several days.
- Within seconds after insulin binds with its membrane receptors, the membranes of about 80% of the body’s cells markedly increase their uptake of glucose.
- which cells is this especially true for
- what does glucose uptake occur as a result of
- what happens then
- what happens when insulin is no longer available
This is especially true of muscle cells and adipose cells but is not true of most neurons in the brain.
Glucose uptake occurs as a result of translocation of multiple intracellular vesicles to the cell membranes; these vesicles carry in their own membranes multiple molecules of glucose transport proteins (GLUT4), which bind with the cell membrane and facilitate glucose uptake into the cells.
The increased glucose transported into the cells is immediately phosphorylated and becomes a substrate for all the usual carbohydrate metabolic functions e.g. glucose synthesis.
When insulin is no longer available, these vesicles separate from the cell membrane within about 3 to 5 minutes and move back to the cell interior to be used again and again as needed.
- The cell membrane becomes more permeable to many of the amino acids, potassium ions, and phosphate ions, causing increased transport of these substances into the cell.
- what does this cause to happen?
causes intracellular processes occurring such as protein synthesis and fat synthesis.
- Slower effects occur during the next 10 to 15 minutes to change the activity levels of many more intracellular metabolic enzymes.
- what do these effects mainly result from
the changed states of phosphorylation of the enzymes.
- Much slower effects continue to occur for hours and even several days.
- what do they result from
- what does this lead to
- what does insulin therefore do
They result from changed rates of translation of mRNAs at the ribosomes to form new proteins and still slower effects from changed rates of transcription of DNA in the cell nucleus.
This leads to controlled growth of gene expression of the cell.
In this way, insulin remoulds much of the cellular enzymatic machinery to achieve its metabolic goals.
what happens when carbohydrates are eaten?
- When carbohydrates are eaten, glucose is absorbed into the blood.
- This causes rapid secretion of insulin.
what does insulin in turn cause? where?
rapid uptake, storage, and use of glucose by almost all tissues of the body, but especially by the muscles, adipose tissue, and liver.
what are the effects of insulin on carbohydrate metabolism?
- insulin promotes muscle glucose uptake and metabolism
- insulin promotes liver uptake, storage and use of glucose
- insulin promotes conversion of excess glucose into fatty acids
- insulin inhibits gluconeogenesis in the liver
what do muscle tissues normally depend on for its energy? why? what is insulin secretion like between meals? what effect does it have?
- Normally, muscle tissue depends not on glucose for its energy but on fatty acids.
- This is because the normal resting muscle membrane is only slightly permeable to glucose, except when the muscle fibre is stimulated by insulin.
- Between meals, the amount of insulin that is secreted is too small to promote significant amounts of glucose entry into the muscle cells.
however, what are the two conditions in which muscles do use large amounts of glucose?
- Moderate to heavy exercise.
o This usage of glucose does not require large amounts of insulin, because exercising muscle fibres become more permeable to glucose even in the absence of insulin because of the contraction process itself. - During the few hours after a meal.
o At this time the blood glucose concentration is high and the pancreas is secreting large quantities of insulin.
o The extra insulin causes rapid transport of glucose into the muscle cells.
o This causes the muscle cell during this period to use glucose preferentially over fatty acids.
storage of glycogen in muscle
- what happens after a meal if the muscles are not exercising
- when is this particularly useful for the muscle
- If the muscles are not exercising after a meal and yet glucose is transported into the muscle cells in abundance, then most of the glucose is stored in the form of muscle glycogen instead of being used for energy.
- The glycogen can later be used for energy by the muscle.
- It is especially useful for short periods of extreme energy use by the muscles and even to provide spurts of anaerobic energy for a few minutes at a time by glycolytic breakdown of the glycogen to lactic acid, which can occur even in the absence of oxygen.
what happens to glucose levels between meals? what does this cause? what happens in the liver?
- Insulin helps store glucose as glycogen in the liver.
- Between meals, the blood glucose concentration begins to fall.
- This decreases the secretion of insulin into the bloodstream.
- The liver begins to break down glycogen into glucose, which is released back into the blood to keep the glucose concentration from falling too low.
what is the mechanism by which insulin causes glucose uptake into hepatocytes?
includes simultaneous steps:
1. Insulin binds to its receptor on the hepatocytes.
2. Autophosphorylation of the B subunit occurs.
3. Tyrosine kinase is activated and intracellular changes happen.
4. Insulin inactivates ‘glycogen phosphorylase’, the principal enzyme that causes liver glycogen to split into glucose.
This prevents breakdown of the glycogen that has been stored in the liver cells.
5. Insulin causes enhanced uptake of glucose from the blood by the liver cells.
It does this by increasing the activity of the enzyme ‘glucokinase’, which is one of the enzymes that causes the initial phosphorylation of glucose after it diffuses into the liver cells.
Once phosphorylated, the glucose is temporarily trapped inside the liver cells because phosphorylated glucose cannot diffuse back through the cell membrane.
6. Insulin also increases the activities of the enzymes that promote glycogen synthesis, including especially ‘glycogen synthase’, which is responsible for polymerization of the monosaccharide units to form the glycogen molecules.
• The net effect of this mechanism is to increase the amount of glycogen in the liver, without it being broken down.
When the blood glucose level begins to fall to a low level between meals, several events transpire that cause the liver to release glucose back into the circulating blood, what are these?
- The decreasing blood glucose causes the pancreas to decrease its insulin secretion.
- The lack of insulin then reverses all the effects for glycogen storage, essentially stopping further synthesis of glycogen in the liver and preventing further uptake of glucose by the liver from the blood.
- The lack of insulin (along with increase of glucagon) activates the enzyme ‘glycogen phosphorylase’, which causes the splitting of glycogen into the enzyme ‘glucose phosphate’.
- The enzyme glucose phosphatase, which had been inhibited by insulin, now becomes activated by the insulin lack and causes the phosphate radical to split away from the glucose; this allows the free glucose to diffuse back into the blood.
what percentage of the glucose in a meal is stored in the liver in this way and then returned later?
- Thus, the liver removes glucose from the blood when it is present in excess after a meal and returns it to the blood when the blood glucose concentration falls between meals.
- Ordinarily, about 60% of the glucose in the meal is stored in this way in the liver and then returned later.
instead of glucose being converted to glycogen, what else can it be used for? what does insulin do?
- As well as the glucose entering hepatocytes being used for glycogen, this glucose can be used for hepatic metabolism.
- Insulin promotes the conversion of all this excess glucose into fatty acids.
after glucose has been converted to fatty acids, what happens to it?
these fatty acids are subsequently packaged as triglycerides in very-low-density lipoproteins (VLDLs) and transported in this form by way of the blood to the adipose tissue and deposited as fat.
how does insulin inhibit gluconeogenesis?
Decreasing the quantities and activities of the liver enzymes required for gluconeogenesis.
Decreasing the release of amino acids from muscle and other extrahepatic tissues and in turn the availability of these necessary precursors required for gluconeogenesis.
what brain cells permeable to glucose? and do they need insulin? therefore what is crucial? what happens when blood glucose levels drop too low?
- Brain cells are permeable to glucose and can use glucose without the intermediation of insulin.
- Brain cells only use glucose for energy.
- Therefore, it is crucial for the blood glucose levels to always be maintained.
- When the blood glucose levels drop too low, symptoms of hypoglycemic shock develop, characterised by nervous irritability that leads to fainting, seizures, and even coma.
what are the effects of insulin on fat metabolism?
- insulin promotes fat synthesis
- insulin promotes fat storage
what are the two ways in which insulin influences fat storage in adipose tissue?
- Insulin increases the utilisation of glucose by the cells in the body, which decreases the utilisation of fat, thus
functioning as a fat sparer. - Insulin promotes fatty acid synthesis.
This occurs when more carbohydrates are ingested than can be used for immediate energy, thus providing the substrate for fat synthesis.
Fatty acid synthesis occurs in hepatocytes.
Fatty acids are then transported from the liver by way of the blood lipoproteins to the adipose cells to be stored.