Glycolysis and Glucose Oxidation Flashcards
How is glucose taken up by cells?
via GLUT receptors
What is a ‘Km value’ and what does it mean?
indicative of the affinity of a glucose transporter to glucose, a high Km indicates a low affinity and a low Km indicates a high affinity
Describe the GLUT1 receptor
low Km value (high affinity) and is found on erythrocytes and most cell membranes and provides basal glucose transport to cells at constant rate
Describe the GLUT2 receptor
found in liver and beta cells of pancreas and have a higher Km than GLUT1 (lower affinity) and these receptors are only active when there is a high blood glucose level (fed-state)
Describe the GLUT4 receptor
found in muscle and fat cells which ‘store’ these transporters in vesicles and release these in response to insulin to promote glucose uptake and have a low affinity for glucose ordinarily
Where is the hexokinase enzyme found?
in most tissues
Where is the glucokinase enzyme found?
liver (and the pancreas)
What is the role of hexokinase?
phosphorylates glucose to glucose-6-phosphate to prevent it from leaving the cell
What is the role of glucokinase?
converts glucose to glucose-6-phosphate
How is the action of hexokinase controlled?
inhibited by it’s product; glucose-6-phosphate
Describe the function of phosphofructokinase-1 (PFK1)
phosphorylates fructose-6-phosphate to fructose-1,6-phosphate using ATP which is irreversible and is the rate determining step of glycolysis
Describe how ATP is produced by substrate level phosphorylation in glycolysis
conversion of 1,3-BPG to 3-PG produces ATP and the conversion of PEP to pyruvate produces ATP
What is the net production of ATP per glycolysis cycle?
4 ATP produced, 2 ATP used –> 2 ATP
How does glycerol enter glycolysis?
IN THE LIVER: glycerol is converted to glyceraldehyde-3-phosphate by glycerol kinase (using ATP) and this GAP is then converted into DHAP (via reduction of NAD+ –> NADH) and enters into the glycolysis pathway directly
How does fructose enter glycolysis?
Can be metabolised by hexokinase to form fructose-6-phosphate to enter glycolysis direcly.
More often fructose –> fructose-1-phosphate (by fructokinase in the liver) –> converted to DHAP and glyceraldehyde –> both form GAP which enters the glycolysis pathway (converted by aldolase B)
Why can fructose be metabolised by hexokinase?
Due to low specificity of enzyme, mistakes it for glucose
Outline the potential fates of the pyruvate produced by glycolysis
aerobic respiration - converted to acetyl CoA by PDH –> TCA cycle –> ETC
anaerobic respiration - pyruvate converted back to lactate through lactate dehydrogenase to oxidise NADH back to NAD+
How may the deficiency of vitamin B1 affect metabolism?
vitamin B1 is the vitamin from which the coenzyme TPP is derived which is involved in stimulating the pyruvate dehydrogenase complex, so it could prevent aerobic respiration from taking place (as pyruvate wouldn’t be converted to acetyl CoA, so couldn’t enter Krebs cycle)
How is PDC regulated by allosteric control?
NADH (E3 enzyme competition) and acetyl CoA (E2 enzyme competition) compete for binding sites on the PDC enzymes to inhibit its action
How is PDC regulated by reversible phosphorylation?
Active when non-phosphorylated and inactive when phosphorylated. PDH kinase is involved in phosphorylating and deactivating the complex and is stimulated by high levels of ATP as this signifies there is enough energy so TCA cycle doesn’t need to be entered. Conversely, PDH phosphatase dephosphorylates to activate the enzyme and allow aerobic respiration and this is stimulated by insulin and Ca2+
Outline the role of NAD in glycolysis
NAD+ required to produce first ATP and this needs to be regenerated from the NADH product to feed the respiratory chain
How does glucokinase differ from hexokinase?
It has a higher Km than hexokinase –> lower glucose affinity, but has a greater specificity for glucose. Hexokinase is sensitive to feedback inhibition (causes reduced glucose uptake) whereas glucokinase is not
What is the location of the glucokinase enzyme controlled by?
Glucokinase regulator protein (GKRP)
What stimulates the glucokinase regulator protein?
Promoted by the presence of glucose and fructose-1-phosphate –> stimulates glucokinase to convert glucose into glucose-6-phosphate but only at high blood glucose levels
What is the rate-determining step of glycolysis?
The conversion of fructose-6-phosphate –> fructose-1,6-bisphosphate by PFK-1 as this is an irreversible step
Which substances inhibit the activity of PFK-1?
High ATP levels inhibit PFK1 (no more energy needed)
Citrate inhibits PFK1 (signifies enough metabolic intermediates are present)
Which substances promote the activity of PFK-1?
Increased levels of AMP and fructose-2,6-bisphosphate is strongest allosteric activator of PFK1
What is the most potent allosteric activator of PFK-1 (and subsequently, glycolysis)?
Fructose-2,6-bisphosphate; this is formed by phosphorylation of fructose-6-phosphate but using PFK-2 instead of PFK-1
Which enzymes require Mg2+ as cofactors in order to function?
All kinases
Where does most glycerol come from?
Breakdown of fat stores in the body, however, some also comes from dietary sources
What is the function of aldolase B?
Conversion of fructose-1-phosphate to DHAP and GAP which can enter glycolysis directly
What is the function of the pyruvate dehydrogenase complex?
Oxidative carboxylation of pyruvate to acetyl CoA in the mitochondrial matrix
What is the role of PDH phosphatase and what activates this enzyme?
Dephosphorylates, and thus activates the pyruvate dehydrogenase complex, and this enzyme is activated by Ca2+ and insulin (as these stimulate too much blood glucose –> need to use more up OR muscle respiration –> need to produce more energy)
What is the role of PDH kinase, and what activates this enzyme?
Catalyses the phosphorylation, and thus, inactivation of the pyruvate dehydrogenase complex, and this enzyme is activated by an increase in the ATP:AMP (signifies enough energy), NADH and acetyl CoA (in addition to their allosteric effect)
