WK 7 (Carbohydrate metabolism) Flashcards
Glycolysis
the mechanism by which saccharides or sugars are catabolised in the cytoplasm of cells to liberate both common intermediates (building blocks for biosynthesis) and energy in the form of the activated carrier molecules ATP and NADH.
What are the 3 phases of glycolysis?
- A phase of energy investment
- A phase involving the cleavage of a 6 carbon sugar to 2x 3 carbon sugars
- A phase in which energy is generated
What is the net result of glycolysis?
The formation of 2 molecules of pyruvate, 2 molecules of NADH and 2 molecules of ATP (4 produced, yet 2 used in early steps of pathway) per molecule of glucose that enters the cycle.
What is the overall free energy change of glycolysis?
Highly negative
Glycolysis:
Catalysed by the enzyme hexokinase whose activity can be inhibited by elevated concentrations of glucose 6-phophate
Glycolysis is regulated by feedback mechanisms at 3 points in the pathway. These points are characterised by reactions that are not easily reversible. What are the 3 feedback points and how are they regulated?
Step 1: catalysed by hexokinase whose activity can be inhibited by elevated concentrations of glucose 6-phosphate
Step 3: Catalysed by Phosphofructokinase whose activity is inhibited by ATP and citrate, yet stimulated by fructose 6-phosphate
Step 10: catalysed by pyruvate kinase whose activity is inhibited by ATP and alanine
Fermentation of pyruvate
A secondary pathway of carbohydrate metabolism (occurs in the absence of oxygen)
Forms lactate or lactic acid and can occur in most cells. It utilises NADH to regenerate NAD+ that is required for glycolysis to proceed. Pyruvate can subsequently be metabolised to reform pyruvate when oxygen becomes available. This mechanism is important in exercising animals (such as horses) and can also be a source of energy in ruminants whose rumen microflora can contribute pyruvate from otherwise indigestible carbohydrates. The regeneration of NAD+ can also be facilitated in some anaerobic organisms in fermentation of pyruvate to form ethanol and carbon dioxide. This pathway has industrial applications (beer and bread production).
Pentose Phosphate pathway
Secondary pathway of carbohydrate metabolism (occurs in the absence of oxygen)
Glucose is oxidised for the generation of NADPH (utilised in reductive biosynthesis as an electron donor) and ribose 5-phosphate (which is utilised in the synthesis of nucleotides). These is also a non-oxidative branch of the pathway in which un-utilised ribose 5-phosphate can be completely converted into intermediates of glycolysis.
Sorbitol Pathway
glucose is converted via the intermediate sorbitol to fructose which is required for either energy or osmoregulation in the testes, pancreas, brain and eye lens. Disorders of this pathway may be observed when elevated blood glucose concentrations occur (eg. diabetes mellitus). In this situation and overproduction of the sorbitol intermediate results in precipitation of lens proteins that form an opaque cataract.
Gluconeogenesis
the synthesis of glucose from non-carbohydrate precursors.
How long are the bodies’ reserves of glucose (glycogen) able to sufficiently meet the glucose requirements?
24hrs
How many reversible reactions does gluconeogenesis share with glycolysis?
7
Where does glycolysis occur?
Skeletal muscle
Where does gluconeogenesis occur?
Liver
The central pathway of glycolysis can be conceptualised as:
a) A pathway that enables molecules of glucose to be manipulated to facilitate the extraction of energy subsequently stored in the form of NADPH
b) A pathway that enables molecules of glucose to be manipulated to facilitate the formation of glycogen
c) A pathway that enables molecules of glucose to be manipulated to facilitate extraction of energy and for the cleavage of a 6 carbon molecule (glucose) into 2 x 3 carbon molecules (pyruvate)
d) none of the above
c)
