L14 Carbohydrate Metabolism Flashcards
What is hereditary fructose intolerance?
Hereditary fructose intolerance (HFI) is a metabolic disease caused by the absence of an enzyme called aldolase B. In people with HFI, ingestion of fructose (fruit sugar) and sucrose (cane or beet sugar, table sugar) causes severe hypoglycemia (low blood sugar) and the build up of dangerous substances in the liver.
What is galactosemia?
Galactosemia, which means “galactose in the blood,” refers to a group of inherited disorders that impair the body’s ability to process and produce energy from a sugar called galactose. When people with galactosemia injest foods or liquids containing galactose, undigested sugars build up in the blood.
What are the possible fats of glucose?
- Metabolised for ATP
- Conversion to glycogen for storage
- Synthesis of other cellular components e.g. used to make ribose phosphate for RNA and DNA
- Conversion to fat for storage
Added to lipids and proteins
What occurs if glucose concentration diverts from around 5 mM?
Maintaining blood glucose at ~ 5 mM is crucial for survival; the brain relies on glucose for ATP synthesis
Below 3 mM blood glucose – confusion, coma
Above 8 mM long term vascular damage occurs through protein glycation in arteries. This then leads to cardiovascular disease.
For glycolysis, give the following details:
What the pathway does?
What their products are?
Which tissues they occur in?
Where in the cell they take place?
Where the control points are?
How they are controlled?
(a) Glycolysis is an anaerobic pathway. It breaks down glucose to a 3 carbon sugar. It uses up 2 ATP molecules.
(b) 2 NADH molecules, a net gain of 2 ATP molecules and 2 pyruvate molecules per glucose molecules
(c) All tissues
(d) Cytosol
(e) In the glycolytic pathway many of the reactions are reversible e.g. glucose-6-phosphate to fructose-6-phosphate. Reactions that use ATP are irreversible. This allows control of the pathway. The cell tries to minimise its use of energy where it is not need and so the first reaction requires energy. The step involving phosphofructokinase is the main control step in the pathway.
(f) Product inhibition of hexokinase by glucose-6-phosopahte. Allosteric activation of phosphofructokinase by AMPK, AMP, Fructose-2,6,-bisphosphate but inhibited by ATP and glucagon. Allosteric inhibition of pyruvate kinase by ATP.
What are the two phases of glycolysis?
preparative phase – glucose to fructose 1,6 bisphosphate - requires ATP
generating phase – fructose 1,6 bisphosphate to (2) pyruvate – generates ATP and NADH (later converted into ATP under aerobic conditions via the electron transport chain)
What are the key control points of glycolysis?
Phospofructokinase
What are the steps of glycolysis?
- Glucose -> Glucose-6-phosphate by Hexokinase (ATP)
- Glucose-6-phosphate -> Fructose-6-Phosphate by phosphohexose isomerase
- Fructose-6-phosphate -> fructose 1,6-bisphosphate by phosphofructokinase (ATP)
- Fructose-1,6-bisphosphate is cleaved to glyceraladehyde-3-phosphate (this changes reversibly to dihydroxyacetatone phosphate)
- Glyceraladehyde-3-phosphate is converted to phosphenoenolpyruvate generating 2 ATP
- Phosphenoenolpyruvate is converted to pyruvate by pyruvate kinase generating ATP.
What is the storage molecule for glucose?
Glycogen - Glycogen has a branches tree like structure and is made up of 1-4 glyosidic bonds and 1-6 glyosidic bonds. This means glucose can be rapidly added to glycogen and rapidly released as there is many points that enzymes can act.
What is required for glycogen synthesis?
Glycogenin
How does glycogen synthesis occur?
Synthesised from glucose-6-phosphate. The first step is conversion to glucose-1-phosphate. This is then transferred onto UDP (a carrier). This is then transferred onto the primer glycogenin, a protein, on a growing glycogen chain. The amount of glucose that can be stored is limited by the amount of glycogenin you have and the overall size glycogen you can store. Unlike fats which you can store an unlimited amount of fats.
In UTP is converted to UDP as an inorganic phosphate group is released. This transfers the glucose onto glyogenin. There is a free tyrosine molecule on glycogenin which is what the glucose molecule can then attach to.
Unimaginatively branching enzyme puts on the branches.
How does glycogenolysis occur? Where does it occur?
Glycogen phosphorylase removes glucose units. This converts it back into glucose-1-phosphate and this is interconverted to glucose-6-phsopahte which can enter glycolysis.
In muscle cells the glucose-6-phsophate enters glycolysis. The glycogen in muscle is there to provide glucose for that muscle. The enzyme for converting glucose-6-phosphate is found in the liver and kidneys only. It is the liver that is involved in releasing glucose to maintain blood glucose levels.
How are fatty acids synthesised from glucose>
The link reaction occurs in the mitochondria. High levels of Acetyl coA in the mitochondria signals that fat is going to be made. Acetyl CoA is converted to citrate which can then be moved out.
Citrate is broken down in the initially stage to produce cytosolic acetyl CoA. This is carboxylated via Acetyl CoA carboxylase. This is the key control point in fat synthesis. This produces malonyl CoA - a 2 carbon sugar. Acetyl CoA is added to this and another carbon is lost producing a 4 carbon sugar. This continues until a long carbon chain is made - usually 16 carbons.
Acetyl CoA + malonyl CoA - a series of reactions requiring several molecules of NADPH – so fatty acid synthesis is an expensive process requiring both ATP and reducing power. All these reactions are catalysed by a single multi-functional enzyme – fatty acid synthase.
How are fatty acids synthesised from glucose?
The link reaction occurs in the mitochondria. High levels of Acetyl coA in the mitochondria signals that fat is going to be made. Acetyl CoA is converted to citrate which can then be moved out.
Citrate is broken down in the initially stage to produce cytosolic acetyl CoA. This is carboxylated via Acetyl CoA carboxylase. This is the key control point in fat synthesis. This produces malonyl CoA - a 2 carbon sugar. Acetyl CoA is added to this and another carbon is lost producing a 4 carbon sugar. This continues until a long carbon chain is made - usually 16 carbons.
Acetyl CoA + malonyl CoA - a series of reactions requiring several molecules of NADPH – so fatty acid synthesis is an expensive process requiring both ATP and reducing power. All these reactions are catalysed by a single multi-functional enzyme – fatty acid synthase.
Where are triacylglycerols synthesised?
Endoplasmic reticulum
