1.5 - Key Control of Glycolysis and Gluconeogenesis Flashcards
isoenzymes
group of enzymes that catalyse same reaction but have different enzyme forms and catalytic efficiencies (Km)
erythrocytes
red blood cells
hepatocytes
functional cells of the liver
what inhibits glycolysis?
ATP
which enzymes are regulated when glycolysis is inhibited? (2)
- phosphofructokinas kinase
(accumulates inhibiting hexose kinase) - pyruvate kinase
key control points in glycolysis (3)
- phosphofructokinase (PFK)
- pyruvate kinase (PK)
- hexokinase (HK)
phosphofructokinase (PFK) isozymes in human genome (3)
- PFKM - muscle type
- PFKL - liver type
- PFKP - platelet type
pyruvate kinase (PK) isozymes in human genome (2)
- PKM
- PKLR
isoforms of PKM gene (2)
- PKM1
- PKM2
PKM1
predominantly expressed through alternative splicing in muscle and brain tissues
PKM2
found in various tissues, including tumour cells
PKLR gene isozymes (2)
- PKL
- PKR
where are PKL and PKR expressed (separately) (2)
- PKL - expressed in liver
- PKR - found in erythrocytes (red blood cells)
primary pyruvate kinase (PK) isozymes (2)
- M-type - muscle and brain
- L-type - liver
what happens to L-type pyruvate kinase isozyme when blood glucose is low?
covalently phosphorylated
why is L-type pyruvate kinase isozyme phosphorylates?
less active compared to non phosphorylated, prevents liver cells consuming glucose when there is demand in other parts of body
Km of hexokinase I (HK1) in erythrocytes and skeletal muscle cells?
Km = 0.05 - 0.1mM
Km of hexokinase II (HK2) in skeletal muscle cells and adipose cells
Km = 0.1mM
Km in glucokinase/hexokinase IIII (HK4) in hepatocytes and pancreatic beta cells
kM = 5 - 6mM
how is hexokinase (HK1) allosterically inhibited?
high levels of G-6-P
how is hexokinase (HK1) an important control step in glycolysis?
prevents over consumption of cellular ATP to form G-6-P when glucose is not limiting
What happens to HK1 in hepatic cells/pancreatic B-cells when glucose is not limiting glycolysis?
replaced by glucokinase/HK4
Why is HK1 replaced by glucokinase/HK4 in hepatic cells/pancreatic B-cells when glucose is not limiting glycolysis? (4)
- has high Km
- activated by high blood glucose and insulin, not inhibited by G-6-P
- Allows liver to remove excess glucose in CVS for glycogen synthesis
- helps reduce blood glucose after eating (reducing hyperglycaemia)
GLUT2 Km/affinity for glucose (2)
- highest Km
- lowest affinity for glucose
why do hepatocytes and pancreas express GLUT2
exposed to relatively high glucose concentration via hepatic portal vein
why do hepatocytes and pancreatic B-cells express GK instead of HK for phosphorylation of glucose?
GKs have higher Km than HK (relatively low affinity to glucose)
lactic acid fermentation for NAD+ (3)
- accumulation of pyruvate
- accumulation of NADH without enough oxygen
- pyruvate converted to lactate
fate of lactic acid (4)
- lactate is biproduct of NAD+ recovery by lactate dehydrogenase
- in excess, lactate inhibits many cellular activities in cytoplasm
- monocarboxylate transporter (MCT) transports lactate across plasma membranes of muscle cells (allow lactate to move to CVS)
- lactates recycled by hepatic cells
Cori cycle (4)
- in oxygen deficit anaerobic respiration muscle cells produce lactate
- lactate released into blood
- in liver, conversion lactate to pyruvate and to glucose uses 6 ATP by gluconeogenesis
- glucose released from liver cells into blood (taken up by peripheral tissues)
where does gluconeogenesis occur?
primarily in liver and kidney cells
major precursors of gluconeogenesis (3)
- glycerol
- amino acids
- lactic acid
ATP economy of gluconeogenesis (2)
- pyruvate -> glyceraldehyde 3-phosphate = 3 ATP
- need 2x GAP = 6 ATP - glycerol -> DAHP = 1 ATP
- need 2x DAHP = 2 ATP
carboxylases to regenerate phosphoenolpyruvate (2)
- phosphoenolpyruvate carboxylase
- pyruvate carboxylase
How can glycerol be used to regenerate DHAP? (dihydroxyacetone phosphate) (2)
- glycerol –> glycerol phosphate
- via glycerol kinase - glycerol phosphate –> dihydroxyacetone phosphate (DHAP)
- via glycerol phosphate dehydrogenase
4 key enzymes of gluconeogenesis (4)
- glucose 6-phosphatase (hexokinase)
- fructose 1,6-bisphosphatase
(phosphofructokinase) - phosphoenol-pyruvate carboxylase
(pyruvate kinase) - pyruvate carboxylase
(pyruvate kinase)
why do human erythrocytes (RBCs) rely entirely on glycolysis for ATP?
no mitochondria, nucleus or other cellular organelles
why do human erythrocytes (RBCs) require ATP?
for Na+/K+ ATPase pump keeping membrane potential, which contributes to maintenance of biconcave shape
sole source of ATP for human erythrocytes (RBCs)
glycolysis in the cytosol
what can result in haemolytic anaemia?
genetic conditions such as deficiency in glycolytic processes
why is pyruvate kinase (PK) so important for RBC availability? (2)
- pyruvate kinase (PK) is last enzymatic reaction in glycolysis
- aids conversion of phosphoenolpyruvate (PEP) to pyruvate, thereby producing ATP
effect of deficit pyruvate kinase (PK)/defficiency in glycolytic processes on erythrocytes (RBC)
amount of ATP insufficient for erythrocyte survival
how are deficient glycolytic processes countered in erythrocytes? (2)
- corrupted erythrocytes removed by reticuloendothelial cells (particularly by spleen)
- build up of 2,3-DPG occurs which aids oxygen offloading into tissues
Warburg effect
in tumours/other proliferating or developing cells, rate of glucose uptake dramatically increases and lactate is produced, even in presence of oxygen and fully functioning mitochondria
cancer cell expression of hexokinases
1 and 2, speed up glycolytic pathway
hypoxia-inducible factor (HIF) regulated genes role
oxygen deprived hypoxia known to allow HIF transcription factor to turn on many enzymes of glycolysis
how do cancer cells fall short for oxygen?
due to spread of cell division and proliferation (tumour cells in core become hypoxic)
result of tumour cells becoming hypoxic (oxygen deficient)
makes tumour cells more likely to rely on glycolysis
2 enzymes of glycolysis cancer cells have alterations of (2)
- hexokinase
- pyruvate kinase
2 forms of pyruvate kinase M2 (PKM2)
- low activity dimeric form
- high activity tetrameric form
role of less active dimeric form of PKM2
phosphorylated by tyrosine kinases and promotes conversion of pyruvate to lactate
role of high activity tetrameric form of PKM2
promotes conversion of pyruvate to acetyl-CoA
role of tumour form of pyruvate kinase M2 (PKM2)
undergoes tyrosine phosphorylation and gives rise to Warburg effect
