Metabolism S4 - Energy Storage Flashcards
What is glycogenesis?
Synthesis of glycogen
What enzymes are involved with glycogenesis?
- Hexokinase
- Phophoglucomutase (reversible reaction)
Irreversible:
- Glycogen synthase - adds non-branched α1-4 glycosidic bonds
- Branching enzymes - adds branched α1-6 glycosidic bonds about every 10 subunits
Where does glycogenesis occur?
Liver and skeletal muscle
What is glycogenolyis?
- Glycogen degradation in skeletal muscle in response to exercise and in the liver in response to fasting or stress
- The pathway is NOT a reversal of glycogen synthesis
What enzymes are involved in glucogenolyis?
- Glycogen phosphorylase - attacks the α1-4 bonds.
- De-branching enzymes - attacks the α1-6 branch points
- Phosphoplucomutase (reversible reaction)
Produces glucose 6-phosphate which can be used in glycolysis (skeletal muscle)
- Glucose 6-phosphotase (found only in the liver to release free glucose)
Compare the functions of liver and muscle glycogen
Liver - Glucose store for all body
Muscle - Glucose 6-phosphate store, only used by muscle cells - glycolysis intermediate
Explain the clinical consequences of glycogen storage diseases
- Abnormality in Glycogen phosphorylase, Phosphoglucomutase or glucose 6-phosphatase
Increased/decreased amounts of glycogen:
- Tissue damage if excessive storage
- Fasting hypoglycaemia (low blood glucose)
- Poor exercise tolerance
- Usually liver and/or muscle affected
What is gluconeogenesis?
The production of glucose in the absence of carbohydrates.
Particularly important for glucose-dependant tissues such as the CNS
What are the possible substrates for gluconeogenesis?
- Pyruvate
- Lactate
- Glycerol
- Glycogenic amino acids
What enzymes bypass the reversible steps of glycolysis in gluconeogenesis?
1) Glucose 6-phosphatase
3) fructose 1,6-bisphosphatase
10) PEPCK & pyruvate carboxylase
How is gluconeogenesis regulated?
Two key control sites:
1) PEPCK activity
- increased by glucagon and cortisol
- Decreased by insulin
2) Fructose 1,6-bisphosphatase
- Increased by glucagon and cortisol
- Decreased by insulin
Outline β-Oxidation of fatty acids
- The major catabolic, mitochondrial pathway for fatty acids
- 2 carbons (acetate) are removed successively from the fatty acyl CoA
- Continues until 2 carbons remain - acetyl CoA
- Produces NADH and FAD2H
- Lost acetate links with Coenzyme A, converting to Acetyl-CoA
Where does β-Oxidation of fatty acids occur?
Mitochondria
Where does lipogenesis (fatty acid synthesis) take place?
Cell cytoplasm
What is lipogenesis?
The generation of fatty acids from Acetyl CoA at the expense of ATP and NADPH
Outline lipogenesis
- Most steps of this pathway are carried out by the fatty acid synthase complex
- Fatty acids are built up sequentially by adding the 3C malonyl-CoA and losing a carbon via carbon dioxide
- Net gain of 2 carbons
How is malonyl-CoA produced?
Produced from Acetyl-CoA by the enzyme acetyl-CoA carboxylase in a reaction that requires vitamin biotin
What enzyme plays a key role regulating lipogenesis?
Acetyl CoA carboxylase
- Insulin activates by promoting dephosphorylation
- Glucagon and adrenaline inhibit the enzyme promoting phosphorylation
Outline amino acid metabolism
- Each amino acid has its own pathway
- Early step is removal of amino group by transamination or deamination
- The amino group is converted into urea and excreted in the urine
- The remaining C-skleton can be converted into - Pyruvate, oxaloacetate, α-ketogluterate, succinate (all glycogenic) , Acetyl-CoA (ketogenic)
What is transamination?
The funnelling of amino groups to α-ketogluterate to form glutamate and importantly an α-keto acid that provides the back bone for Pyruvate, oxaloacetate, α-ketogluterate, succinate etc to be used in krebs cycle etc
What type of enzymes are involved in transamination?
aminotransferases
What is Phenylketonuria?
An inherited defect in phenylalanine metabolism
Phenylalanine is then metabolised by other pathways producing various products including phenylpyruvate - inhibits pyruvate uptake which inhibits brain development
What enzyme is defective in phenylketonuria?
Phenylalanine hydroxylase
Oxidises phenylalanine to tyrosine
How is phenylketonuria diagnosed?
- High phenylketones in urine (think of name)
- High concentration of phenylalanine in the blood
How is phenylketonuria treated?
A diet low in phenylalanine
What is homocystinuria?
A rare autosomal recessive defect in methionine metabolism
What enzyme is defective in type I homocystinuria?
Cystathionine synthase
How is homocystinuria detected?
Elevated levels of homocysteine and methionine in blood plasma and homocystine in the urine
What does chronic elevated levels of homocysteine cause?
Disorders of the connective tissue, muscle, CNS and cardiovascular system. In children the symptoms are very similar to Marfan’s syndrome and can be easily misdiagnosed
What does an enzyme activity of 13U/g tissue mean?
13 units of product formed per minute per gram tissue
Why does the absence of glucose 6-phosphate dehydrogenase produce a sensitivity to primaquine?
- Primaquine requires NADPH to reduce drug to its metabolites
- NADPH levels are already low in RBCs
- NADPH is further depleted when administered primaquine
Compare and contrast triacylglycerols and glycogen as energy storage materials in humans
- Both energy storage molecules
- Triacylglycerol is the major energy store molecule (10-15kg) compared to glycogen (0.4kg)
- Triacylglycerol stored in highly specialised cells (adipose cells) while glycogen is stored in the liver and muscles
- TAGS more efficient store as they are hydrophobic and stored in anhydrous form while glycogen is polar and stored with water
- TAGs are more reduced and contain more stored energy per C-atom than glycogen
How is glutamine (amino acid) produced?
Produced from glutamate + ammonia
Catalysed by glutamine synthase
How is glutamine metabolised?
Enzyme glutaminase dehydrogenase breaks down glutamine to glutamate and ammonia
What are the consequences of hyperammonaemia?
- Damage to CNS resulting in blurred vision, tremors, slurred speech, comma and eventually death
- removal of α-ketogluterate from TCA cycle to form glutamate disrupts energy supply including to brain
- Alters pH of cells in CNS
- Interferes with neurotransmitter synthesis and release
What is the function of urea?
Nitrogen storage molecule
How is urea suited to its role?
- Water soluble meaning it can be excretes in urine
- Metabolically inert
- Non-toxic
- High N-content
Not all urea is excreted in the urine, where does a small proportion go?
Some diffuses across the intestinal wall where bacteria breaks it down to ammonia in the intestine for reabsorption
How does PKU inhibit brain development?
Phenylpyruvate is produced instead of tyrosine which prevents the uptake of pyruvate in to the mitochondria of cells in the brain
Describe in general terms how amino acids are degraded in the body
- The removal of the NH2 amino group by transamination or deamination
- The C-atoms are converted to intermediates of carbohydrate metabolism (glycogenic amino acids) or lipid metabolism (ketogenic amino acids)
- The N-atoms are usually converted to urea for excretion in the urine but some may be excreted directly as ammonia and some may be converted to glutamine and used for the synthesis of purines and pyrimidines
Explain why blood ammonia levels are normally kept low
- Ammonia is toxic, especially to the CNS
- Interferes with mitochondrial energy metabolism by removing α-ketogluterate (forms glutamate) from the krebs cycle
Explain how ammonia is removed from the body
- Conversion to urea in the liver. The urea is removed from the body in the urine via the kidney
- Conversion to glutamine using glutamate and used for prime and pyramiding synthesis
- Excretion as ammonium ion in urine
With respect to fatty acid catabolism, which enzyme facilitates the activation of fatty acids by linking coenzyme A
Fatty acyl CoA synthase
What does malonyl CoA inhibit
The carnitine shuttle
Little point of fatty acids being mad in cytoplasm to be transported back into mitochondria to undergo b-oxidation
Define osmolarity
The number of osmoles per litre solution
Define osmolality
The number of osmoses per kg of solution
