13 - Absorptive state Flashcards
What is glycolysis?
Glycolysis is the sequence of reactions that metabolises one molecule of glucose to two molecules of pyruvate.
Net production of 2 ATP molecules.
What occurs in glycolysis?
- Two phosphorylation reactions forming fructose 1,6-bisphosphate. (2 ATP -> 2 ADP)
- Cleavage to two molecules of triose phosphate
A series of molecular rearrangements of triose phosphate:
(2x) 2ADP -> (2x) 2ATP
(2x) NAD^+ -> (2x) NADH
Following the molecular rearrangements 2 molecule of pyruvate are formed.
What is the overall reaction of glycolysis?
D-glucose + 2 NAD^+ + 2ADP + 2Pi -> 2 pyruvate + 2 ATP + 2 NADH + 2 H^+ + 2 H2O
Why is it necessary for NAD^+ to be regenerated through the metabolism of pyruvate?
Without the regeneration of NAD^+ glycolysis will stop.
What are the 2 roles of the glycolytic pathway?
- Degradation of glucose to generate ATP.
- Provision of building blocks for synthetic reactions.
What are the 3 sites of control in glycolysis?
Hexokinase:
Inhibited by its product glucose 6-phosphate.
Phosphofructokinase (committed step): Inhibited by - ATP, low pH, citrate.
activated by: AMP, fructose 2,6-bisphosphate
Pyruvate kinase:
Inhibited by - ATP, alanine.
Activated by - 1,6-bisphosphate.
What is the function of phosphofructokinase 2
Bifunctional enzyme responsible for the synthesis and hydrolysis of 2,6-bisphosphate.
essentially 2 enzymes stuck together:
phosphofructokinase-phosphofructobisphosphatase.
How is phosphofructokinase 2 regulated?
Reciprocal control by phosphorylation of serine 460 by protein kinase A.
Phosphorylated form - phosphatase is active
Dephosphorylated form - kinase is active
What causes the conversion of pyruvate into lactic acid?
Lactate is produced by muscles when the body cannot supply enough oxygen.
The body makes more ATP at a cost of creating an oxygen debt.
lactate is oxidised back into pyruvate in the muscle and liver cells using oxygen.
What is mitochondrial oxidative decarboxylation of pyruvate and what happens in the process?
Conversion of pyruvate into acetyl CoA - catalysed by pyruvate dehydrogenase complex:
- Decarboxylation of pyruvate (3C) to a 2C alcohol.
- Oxidation of alcohol to acetic acid and reduction of NAD^+ to NADH.
- Esterification of acetic acid to coenzyme A producing acetyl CoA.
What is the function of glycogen and where is it mainly stored?
Glycogen functions as a reserve of glucose when metabolic demand for glucose outpaces the cells ability to obtain it from extracellular sources.
Mainly stored in the liver and skeletal muscles.
What are the 2 stages of glycogen anabolism (synthesis), and what enzymes are involved in each stage?
Initiation - catalysed by glycogenin.
elongation - catalysed by glycogen synthase and the branching enzyme.
How does glycogenin catalyse the first step of glycogen synthesis?
It acts as the primer, to which further glucose monomers can be added.
Glycogenin binds glucose from UDP-glucose to a hydroxyl group of tyrosine 194.
How does glycogen synthase catalyse the second step of glycogen synthesis?
Glycogen synthase catalyses glycogen polymerisation.
It can only elongate an existing chain of at least four glucose residues.
How is glycogen synthase regulated?
Glycogen synthase is phosphorylated by protein kinase A and glycogen synthase kinase 3.
Phosphorylation converts the active form into the inactive form.
However the inactive form is still active when high levels of the allosteric activator glucose 6-phosphate is present.
