Flashcards in Energy Storage Deck (84):
Why is it good that glycogen is branched?
There's lots of laces for the enzymes to work so can be broken down and mobilise glucose quicker.
Which cells have an absolute requirement of glucose?
Lens and cornea of eye
Which are the three main energy stores in humans and which one contains the most energy?
Triacylglycerol (the most)
Which out of glycogenesis and glycogen breakdown I'd catabolic and anabolic?
Glycogenesis is anabolic
Glycogen breakdown is catabolic
What is the process of glycogenesis?
Glucose + ATP makes glucose 6-phosphate and ADP. Catalyses by Hexokinase in most cells and glucokinase in the liver.
Glucose 6-phosphate to glucose 1-phosphate catalysed by phosphoglucomutase
Glucose 1-phosphate + UTP + H2O to UDP-glucose and 2Pi.
Glycogen(n residues) +UDP-glucose to glycogen(n+1 residues) +UDP. irreversible and catalysed by glycogen synthase (alpha 1,4 bonds) and branching enzyme (alpha 1,6 bonds)
What is UTP and what is its role in glycogen synthesis?
It is a molecule structurally similar and energetically equivalent to ATP. UDP- glucose is a highly activated form of glucose that allows it to be joined to the glycogen chain.
What is glycogen broken down in response to in the muscle and liver?
Skeletal muscle - response to exercise
Liver - in response to fasting or from stress response.
What does glycogen breakdown not being a complete reversal of glycogen synthesis mean?
Different enzymes in each pathway allow for the simultaneous stimulation of one pathway and inhibition of the other.
Is glycogen ever degraded fully?
No, small amount of primer is always conserved so that the chain can reform when needed.
What is the process of glycogen breakdown?
Glycogen(n residues) + Pi to glucose 1-phosphate + glycogen(n-1 residues). Catalysed by glycogen phosphorylase that attacks the alpha 1,4 bonds and debranching enzyme that attacks the alpha 1,6 bonds. Glucose 1-phosphate to glucose 6-phosphate catalysed by phosphoglucomutase. In the muscles this enters glycolysis to generate ATP for the exercising muscle cell. In the liver the glucose 6-phosphate is converted to glucose catalysed by glucose 6-phosphotase. The glucose is released into the bloodstream via glucose transporter and transported to other tissues.
What is the difference between glycogen breakdown in muscle and liver cells?
In liver cells there is the enzyme glucose 6-phosphotase that catalyses the reaction that converts glucose 6-phosphate into glucose. This can travel in the blood stream and so the liver is a glucose store for all tissues of the body.
This enzyme is absent in muscle cells and so the glucose 6-phosphate enters glycolysis to produce energy for the muscle cell only. Therefore the muscle cells are a glucose 6-phosphate store and the liver is a glucose store.
What affects the muscle and liver in glycogen breakdown and what doesn't?
Muscle - glucagon has no effect and AMP is an allosteric activator
Liver - AMP is not an allosteric activator and is activated by glucagon.
What do glycogen metabolism storage diseases result from and which enzymes can lead to glycogen storage diseases (+egs of 2 of them)?
Glycogen phosphorylase - McArdel disease (muscle deficiency)
Glucose 6-phosphotase (in the liver) - Von Gierkes disease
Why would you never see hypoglycaemia in a normal healthy person?
The bodys system can maintain the levels in other ways.
What may glycogen diseases cause?
Increased/decreased amounts of glycogen
- tissue damage is excessive storage
- fasting hypoglycaemia (low blood glucose)
- poor exercise tolerance
Glycogen structure may be abnormal
-usually liver and/or muscle are affected
What does gluconeogenesis allow and what is it for?
Production of glucose when carbs are absent.
Glucose dependent tissues eg CNS
How long can glycogen stores last?
Where is the main site for gluconeogenesis?
What are the possible substrates for gluconeogenesis?
Pyruvate, lactate and glycerol, essential and non essential aas whose metabolism involves pyruvate (can be converted to alanine) or intermediates of the TCA cycle.
How is glucose produced from pyruvate?
Using some of the steps from glycolysis (the reversible steps) and the irreversible steps are bypassed.
Which steps of glycolysis are bypassed in the gluconeogenesis from pyruvate? What are the bypass reactions called? What are they catalysed by?
Step 10 - bypassed by 2 reactions that are driven by GTP and ATP hydrolysis.
1. Pyruvate + CO2 + ATP + H2O to oxaloacetate + ADP + 2Pi + 2H+. Catalysed by pyruvate carboxylate (added CO2)
2. Oxaloacetate + GTP + 2H+ to phosphoenolpyruvate + GDP + CO2. Catalysed by PEPCK and pyruvate and oxaloacetate can be provided from Glucogenic aas (intermediates in TCA cycle)
Steps 1+3 - bypassed by thermodynamically spontaneous reactions catalysed by phosphotases.
8. Fructose 1,6-bisphosphate + H2O to fructose 6-phosphate + Pi. Catalysed by fructose 1,6-bisphosphatase (removes a phosphate)
10. Glucose 6-phosphate +H2O to glucose + Pi. Catalysed by glucose 6-phosphatase (removes a phosphate).
What mainly controls gluconeogenesis?
Where are the main control sites in gluconeogenesis?
PEPCK and fructose 1,6-bisphosphatase
What are the effects of glucagon, insulin and cortisol on PEPCK activity?
Glucagon, cortisol - increase
Insulin - decrease
What are the effects of glucagon and insulin on fructose 1,6-bisphosphatase activity?
Glucagon - increase
Insulin - decrease
What plays a major role in determining the rate of gluconeogenesis?
Insulin/anti insulin ratio
What happens in the absence of biologically effective insulin?
(Diabetes) increased gluconeogenesis rate contributing to hyperglycaemia
What is a Triacylglycerol made of?
3 fatty acids esterified to glycerol.
In what form is TAG stored and why?
Is hydrophobic so stored in anhydrous form in adipose tissue.
What is TAGs function? How is the storage controlled?
Store of fuel molecules for prolonged aerobic exercise, stress situations (eg starvation, pregnancy). Controlled by hormones.
What promotes and what depletes TAG storage?
Glucagon, adrenaline, cortisol, growth hormone and thyroxine deplete storage.
How can adipose tissue store a lot of TAG?
They can undergo hypertrophy as more fat is added which pushes the organelles to the edge and then hyperplasia where they begin to divide and increase the number of fat cells.
Where are the main dietary lipids hydrolysed, by what and what does is produce?
In the small intestine by pancreatic lipase to release fatty acids and glycerol.
What happens to glycerol produced by the hydrolysis of TAG? (simply)
Transported in chylomicrons in the blood stream to adipose tissue to be stored as TAG.
What happens during the beta-oxidation of fatty acids?
Sequence of reactions that oxidises that fatty acid and removes the C2 unit (acetate). The shortened fatty acid is cycled through the process removing another C2 each time until only 2C remain. The C atoms of the fatty acid are converted to Acetyl Co-A.
What does beta-oxidation require?
Mitochondrial NAD+ and FAD. It cannot occur in the absence of O2 as this is needed in oxidative phosphorylation to produce the NAD AND FAD.
Is any ATP produced during beta-oxidation?
What happens in lipogenesis? (Fatty acid synthesis) where does it occur?
Fatty acids are synthesised from Acetyl Co-A form carbs/aas at the expense of ATP AND NADPH. occurs in the cytoplasm. The FAs are built up sequentially by cycle that adds C2 each time in the form of Malonyl-CoA (C3) and subsequent loss of CO2. Activated FAs are then shuttled across the mitochondrial membrane via cartinine. Not just reverse of beta-oxidation.
Where do the substrates for fatty acid synthesis come from?
NADPH produced in cytoplasm by the pentose phosphate pathway.
Acetyl Co-A from the mitochondria when cleaved from citrate
Malonyl-CoA produced from Acetyl-CoA by enzyme Acetyl-CoA carboxylase (adding CO2) which requires biotin. This enzyme is not a part of the fatty acid synthase complex.
What are most of the steps in FA synthesis carried out by?
Multi-enzyme complex known as fatty acid synthase complex.
What has an important role in controlling the rate of FA synthesis? How can it be regulated?
Allosteric, citrate activates, AMP inhibits (want to use Acetyl-CoA to make ATP not FA)
Covalent modification, reversible phosphorylation/dephosphorylation. Insulin activates by promoting dephosphorylation of bulky PO4, glucagon and adrenaline inhibit by promoting phosphorylation.
Comparison of FA oxidation and synthesis
O cycle of reactions that remove C2, S cycle that add C2.
O C2 atoms removed as Acetyl-CoA, S C2 added as malonyl-CoA
O produces Acetyl-CoA, S consumes it.
O occurs in mitochondria, S cytoplasm.
O enzymes separate in mitochondrial matrix, S multi enzyme complex in cytoplasm.
O oxidative, produces NADH and FADH2, S reductive, requires NADPH
Requires small amount ATP to activate FA, S requires large amount ATP to drive process.
O intermediates linked to Acetyl-CoA, S intermediates linked to fatty acid synthase by carrier protein.
O regulated indirectly by FA availability in mitochondria, S regulated directly by activity of Acetyl-CoA carboxylase.
O glucagon and adrenaline stimulate, S inhibit
O insulin inhibits, S stimulates.
When can excessive breakdown of aas occur?
In Cushing's syndrome. Excess cortisol stimulates aa catabolism and weakens structure of dermis of skin which leads to formation of striae.
What happens in catabolism of aas?
Early step where the NH2 group is removed (transamination/deamination). This is converted to urea and excreted in the urine. The C skeleton can be converted into one or more of the following:
Pyruvate, Oxaloacetate, fumarate, alpha-ketoglutarate, succinate, Acetyl-CoA.
What types of aas are they (plus example) and what can they be used to make?
Ketogenic - produce Acetyl-CoA that can be used to synthesise ketone bodies. Eg lysine
Glucogenic - can be used to synthesise glucose by gluconeogenesis. Eg alanine
Some can be both, eg tyrosine
What kind of aas can the body synthesise? Where do the C atoms for this come from? Where does the amino group come from?
Non essential aas
Intermediates of glycolysis, pentose phosphate pathway, Krebs cycle
Provided by other aas by transamination or from ammonia.
What is transamination?
Moving the amino group from one aa to another
What is the transamination for alanine and Aspartate? How do they link?
Alanine + alpha-ketoglutarate reversibly to pyruvate + glutamate. Catalysed by ALT (alanine aminotransferase)
Aspartate + alpha-ketoglutarate reversibly to oxaloacetate + glutamate. Catalysed by AST (aspartate aminotransferase)
Glutamate from the alanine reaction can be put into the aspartate reaction to produce aspartate that can enter the urea cycle to excrete the amino group in the urine.
What can we measure as part of a liver function test and why?
Routinely measure AST and ALT as they don't normally appear in the plasma unless there is a problem. Eg in viral hep, autoimmune liver disease.
What does cortisol do in relation to transamination?
Stimulates transaminase synthesis in the liver.
What does deamination do?
Removes the amino group from an aa.
What deamination would allow NH3 to be excreted?
Glutamine to glutamate + NH3. Catalysed by glutaminase. Can be excreted in urine directly if in the kidney or enter the urea cycle if not.
What deamination would allow NH3 to be stored?
Glutamate + NH3 to glutamine. Allows for the safe storage of NH3. Can then be safely moved around the body to be used.
Why do we have to get rid of NH3?
Ammonia is very toxic and reduces TCA activity (reacts with intermediates which causes a depletion of the substrate) and affects neurotransmitter synthesis.
Why else is deamination important in aa metabolism by the liver?
Glutamate dehydrogenase is high specificity enzyme that catalyses: glutamate + NAD+ + H2O reversibly to alpha-ketoglutarate + NH4+ + NADH + H+.
Involved in the disposal of aas (glutamate to alpha... which is intermediate in TCA cycle) and the synthesis of non essential aas (alpha... To glutamate)
What is phenylketonuria?
Inherited disorder in which the urine contains large amounts of phenylketones produced from the phenylalanine.
What happens in phenylketonuria?
First step in metabolism of phenylalanine is oxidation to tyrosine catalysed by phenylalanine hydroxylase. This enzyme is defective in most PKU cases. As a result, phenylalanine accumulates in the tissues and blood. It is metabolised by other pathways to produce phenylpyruvate (by transamination) that is excreted in the urine.
How is PKU diagnosed?
By the detection of phenylketones in the urine (phenylpyruvate breaks down to form these) or high phenylalanine blood conc (normal is less than 0.1mM).
How is PKU treated?
Diet low in phenylalanine
What can happen if PKU is left untreated?
Can inhibit brain development due to inhibition of pyruvate uptake by phenylpyruvate
What effect does PKU have on tyrosine and what does it mean?
It prevents tyrosine from being made which is an important precursor for a lot of neurotransmitter synthesis. Tyrosine isn't normally an essential aa but can become one here as it can't be synthesised by the body.
What is homocystinuria?
Rare inherited autosomal recessive defect in methionine metabolism.
What happens in type 1 homocystinuria?
Deficiency in cystathione beta-synthase (which requires vit b6 as a co factor) which normally converts homocysteine into cystathione which is further converted into cysteine. Homocysteine increases in the blood and some can be converted into methionine. This is toxic but less so than homocysteine. This requires vit b12. Also if the homocysteine levels are high enough, 2 can form a disulphide bond to form homocystine which is detected in the urine along with methionine and homocysteine being detected in the plasma.
How do you treat homocystinuria?
There's no cure so give low methionine diet and supplement with b12 for the conversion to methionine.
What can elevated plasma levels of homocysteine cause?
Disorders of the connective tissue (affects collagen and elastic fibres by binding to lysine), muscle, CNS and cardiovascular system. In children the symptoms can appear similar to Marfans syndrome.
What is creatinine, when is it produced and what does it provide us?
Breakdown of creatine, produced at a constant rate by a spontaneous reaction in muscles unless the muscle is wasting (atrophy) or in a high protein diet. Creatinine is excreted via the kidneys into urine and the amount of excretion in 24 hrs is proportional to the muscle mass of the individual so provides a measure of muscle mass.
What is very sensitive to ammonia and what can high levels of ammonia cause?
CNS, hyperammonaemia is associated with blurred vision, tremors, slurred speech, coma and eventually death.
What may the toxic effect of ammonia be due to?
May involve its reaction with alpha-ketoglutarate to form glutamate in mitochondria via Glutamate dehydrogenase. This removes alpha... From the TCA cycle which slows, disrupting the energy supply to brain cells. Also affects pH inside cells of the CNS which interferes with neurotransmitter synthesis/release.
In addition to protein synthesis that requires all 20 aas, what else are aas required for? 5 Examples.
Synthesis of other important compounds that need specific aas.
Tyrosine - melanin, thyroid hormones, neurotransmitters
Cysteine - hydrogen sulphide (signalling molecule), glutathione
Tryptophan - serotonin, melatonin.
Arginine - nitric oxide (important role in vasodilation and memory formation)
Glycine - purines (DNA), glutathione, Haem, creatine
Why is the removal of the amino group important?
Allows C skeleton to be utilised in oxidative metabolism. Glucogenic feed into gluconeogenesis pathway. Ketogenic converted to Acetyl-CoA to make ketone bodies.
What are the features of urea?
V soluble in water so excreted in urine. It is non-toxic, metabolically inert and has a high N content so a good way to dispose of unwanted N.
How and where is urea synthesised?
In the liver by the urea cycle. Happens in cytoplasm and mitochondria, 5 enzymes. The urea is then transported via the blood to the kidneys for excretion. It is induced and not regulated. High protein diet = v active, low protein = less active
Why are the enzymes of the urea cycle not subject to feedback inhibition?
The function of the cycle is to dispose of ammonia as urea.
How do you treat starvation/low protein diet? Why?
Gradual reintroduction of protein to prevent hyper ammonia. Referring syndrome - urea cycle can't deal and process the extra nitrogen as its working at a reduced level so the toxic ammonia builds up and affects the body.
When would you see symptoms of inherited diseases of the urea cycle?
Severe- 1 day after birth
Mild deficiencies- may not show symptoms until early childhood
How do you manage inherited diseases of the urea cycle?
Low protein diet, replace essential aas (in diet) with keto acids to reduce the number of amine groups they have to deal with in the urea cycle. They use up NH4+ when converted to aas therefore lowering NH4+ conc.
What are inherited diseases of the urea cycle?
Autosomal recessive disorders involving a deficiency in one of the 5 enzymes. Mutations cause partial loss of enzyme function.
What would happen if there was a total loss in the working of one of the enzymes in the urea cycle?
Urea cycle would be completely inactive and so would die v early on in embryonic period as can deal with the toxic ammonia.
What do the defects in the urea cycle cause?
Hyperammonaemia, accumulation and/or excretion of particular urea cycle intermediates.
What does the severity of inherited diseases of the urea cycle depend on? What do symptoms include?
Extent of the defect, amount of protein eaten.
Include vomiting, lethargy, irritability, severe cases may see mental retardation, seizures, coma and eventually death.
When may hyperammonaemia also arise?
As a secondary consequence of liver disease such as cirrhosis where the livers ability to remove NH3 from the portal blood is impaired.
As well as being carried to the kidney from the liver cells via the blood where it is filtered and excreted, what else can happen to urea?
It can diffuse across the intestinal wall into the intestine where here, bacteria breaks it down, releasing ammonia that can be reabsorbed.
What else can contribute to hyperammonaemia apart from liver disease and disease of the urea cycle?
Kidney failure, conc of urea in blood is high so production of ammonia by gut bacteria can contribute to hyperammonaemia.