Flashcards in Metabolism S4 - Energy Storage Deck (46)
What is glycogenesis?
Synthesis of glycogen
What enzymes are involved with glycogenesis?
- Phophoglucomutase (reversible reaction)
- 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?
- 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
- 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?
Where does lipogenesis (fatty acid synthesis) take place?
What is lipogenesis?
The generation of fatty acids from Acetyl CoA at the expense of ATP and NADPH
- 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?
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?
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?
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