Lipid Metabolism Flashcards
How does Polysaccharide storage form ?
Forms of glucose in the body
Where is this predominately stored in ?
Liver and muscle as an energy reserve
What is the difference between liver glycogen and muscle glycogen ?
Liver glycogen is utilised to maintain plasma glucose levels between meals, whereas muscle glycogen is required to sustain muscle contraction
Glucose-1-Phosphate and Glucose-6-Phosphate are interconvertible by ?
Phosphoglucomutase
What is the pathway of Glycogen synthesis ?
Glucose-1-Phosphate + UTP = Uridine Diphosphate Glucose (UDP-Glucose)
What enzyme allows this formation of UDP-Glucose?
The enzyme is UDP-glucose pyrophosphorylase
Explain the formation of Glycogen from UDP-Glucose ?
Glycogen synthase adds glucose units in α-1:4- linkage onto the non-reducing end of the glycogen chain
What does synthesis involve ?
Addition of new glucose units (sometimes called residues) onto the end of an existing chain -sometimes called a primer
Glycogen synthase can add glucose to only ?
A pre-existing chain of more than four glucosyl residues
What is the priming function carried out by ?
A protein, glycogenin
Where is Glycogenin found ?
Free in the cell but also remains at the core of the glycogen granule it has initiated
What is Glycogenin ?
It is a protein dimer composed of two identical subunits of Mr 37,000. An oligosaccharide of four glucosyl units is built up on each molecule. Glycogen synthase can then elongate the chains. A separate branching enzyme introduces the branches
What does UDP donate ?
The first glucosyl residue to glycogenin. Carbon 1 off the glucose forms a covalent link with the hydroxyl group of a specific tyrosine residue in the glycogenin protein sequence
How is Glycogen synthesis controlled by ?
Insulin
What can Glycogen synthase only catalyse ?
The formation of α-1,4 glycosidic linkages. Another enzyme is required to generate the α-1,6 linkages needed to form a branch
The branching enzyme transfers ?
A block of 7 residues (including the non-reducing end) to a more interior site, creating a new α-1,6 glycosidic linkage
What must the branch not be ?
Within 4 residues of a pre-existing branch
Why is Glycogen a good energy store ?
Because it can be mobilised very rapidly:
- The enzymes phosphorylase and glycogen synthase are very sensitive to regulation by hormones, stress and muscle contraction;
- The branched structure helps it to be mobilised rapidly.
But why is Glycogen also a bad energy store because ?
Because glucose is hydrophilic. Therefore water associates with glycogen granules, increasing the overall weight and bulk
Glycogen Phosphorylase in muscle is subject to ?
Allosteric regulation by AMP, ATP, and glucose-6-phosphate
What does AMP activate?
AMP (present significantly when ATP is depleted due to contraction) activates phosphorylase
What does ATP & glucose-6-phosphate inhibit ? and how ?
ATP & glucose-6-phosphate, which both have binding sites that overlap that of AMP, inhibit phosphorylase. They are signals that energy status if OK
So glycogen breakdown is inhibited when ?
ATP and glucose-6-phosphate are plentiful
Glycogen synthase is allosterically activated by?
Glucose-6-phosphate (opposite to the effect on phosphorylase)
So when is glycogen synthesis activated ?
When glucose-6-phosphate is plentiful
Glycogen mobilisation is greatly accelerated:
- In liver during starvation, when Glucose is required for Glycolysis by the brain and blood cells
- In muscle to fuel glycolysis during vigorous exercise
- Glycogen synthesis is activated to replenish liver glycogen stores after feeding or muscle stores after feeding or muscle stores when exercise ceases. It is promoted by insulin
- The pathway for glycogen synthesis is NOT a simple reversal of breakdown and requires energy input
Explain the structure of Fatty acids and its role ?
- Only the carboxyl group is polar. The hydrocarbon chains are hydrophobic
- This is important for their role as an energy store, as it means that their mass is not increased by the association of water
- They are also very reduced, giving high potential for energy yield when they are oxidised
- So, for this reason fats are a more efficient energy store for the body than glycogen
