Glycolysis Flashcards
What glucose transporters do humans have?
GLUT 1-4
GLUT 1
Present in most cell types, including fetal tissues, responsible for basal glucose transport
Km is lower (1mm) than normal glucose levels (4-8mm)
GLUT 2
Present in liver and pancreatic beta cells and transports only when glucose levels are high
Km is very high (15-20mm)
Bidirectional transporter; liver: glucose uptake for glycolysis and when liver levels are high, glucose is released into the circulation (e.g. during gluconeogenesis)
GLUT 3
Present mostly in neurons and the placenta
GLUT 4
Present in muscle and fat cells.
The number of transporters increases rapidly in the presence of insulin; endurance training increases the number in the muscles
Km is 5mM
Glucose metabolism in anaerobic conditions
e.g. a sprint
pyruvate is metabolized to lactate
Glucose metabolism in aerobic conditions
e.g. a long, slow run
pyruvate is more completely oxidized to CO2 with additional ATP generated
Stage 1 of Glycolysis
Preparative Phase:
Glucose is turned into fructose 1,6-bisphosphate, which can be cleaved into two, 3-C units of glyceraldehyde 3-phosphate
(requires 2 ATP)
Stage 2 of Glycolysis
ATP generating phase:
GAP is oxidized to generate pyruvate and ATP
4 ATP generated (net gain of 2 ATP)
How is glucose trapped in the cell after entry?
Glucose is phosphorylated by Hexokinase to create Glucose-6-phosphate
G6P cannot pass through the membrane because of its negative charge. It can also NOT be transported by a glucose transporter
Glycolysis Preparative phase
traps glucose into frucose 1,6-bisphosphate that can be cleaved into 3C units of glyceraldehyde-3-P
(Requires 2 ATP)
Glycolysis Stage 2 (ATP generating phase)
involves oxidation of GAP to generate pyruvate and ATP
4 ATP generated
How is glucose trapped in cells?
Hexokinase converts glucose to glucose-6-phosphate, which is unable to pass through the cell (is negatively charged)
Importance of phosphofructokinase-1 in glycolysis
PFK-1 is a key regulatory enzyme that controls the pace of glycolysis
-is a committed, rate-limiting step
Aldolase function in glycolysis
Aldolase converts fructose 1,6-bisphosphate into glyceraldehyde-3-phosphate and DHAP (which is then converted to GAP)
-GAP can generate ATP, DHAP cannot
NAD+
Niacin, or vitamin B3, derivative
Can accept 2 electrons and a H+ ion
FAD
Riboflavin, or vitamin B2 derivative
Can accept 2 electrons and 2 H+ ions
fructose metabolism
Fructose is metabolized eventually to glyceraldehyde-3-P, which enters the glycolysis pathway.
Fructose –> fructose-1-P (by fructokinase)
Fructose-1-P –> DHAP / glyceraldehyde (by aldolase B)
DHAP & Fructose-1-P –> glyceraldehyde-3-P
Aldolase B
Rate-limiting enzyme in fructose metabolism
Fructose Intolerance: Defect in aldolase B results in accumulation of fructose-1-P and phosphate & ATP depletion
Overall result is decreased glucose, lactic acidosis, hypoglycemia, nausea, convulsions, weakness, and jaundice
Essential Fructosuria
Defect in fructokinase
Asymptomatic, because fructose is able to exit cells, resulting in no buildup
Galactose metabolism
galactose is metabolized directly to glucose
Classical galactosemia
Defect in galactose 1-phosphate uridylytransferase (step 2), results in accumulation of galactose-1-P and galactose
Presentation: intellectual disability, cataracts, liver disease
Treatment: reduce lactose consumption
Non-classical galactosemia
Defect in galactokinase (step 1), results in accumulation of galactose
Presentation: cataracts, mild symptoms
Lactate production and NAD+ Regeneration
NAD+ is reduced to NADH in many steps of metabolism but NAD+ supply is limited (can only get from diet - Vitamin B3).
Production of lactate during anaerobic glycolysis by lactate dehydrogenase generates NAD+
Cori Cycle
RBCs can only undergo glycolysis, can generate lactate from glycolysis. The lactate can then enter the liver cells, which convert it to glucose, which is then sent back to RBCs.
Glycerol-3-Phosphate Shuttle
Shuttles electrons across mitochondrial membranes to regenerate NAD+ (There are no NADH mitochondrial membrane transporters).
Cytoplasmic glycerol-3-P dehydrogenase transfers electrons from NADH to DHAP to glycerol-3-P, which diffuses into the mitochondrial membrane and donates the electrons to FAD
Malate-Aspartate Shuttle
In the cytosol, oxaloacetate is converted to malate, which can be shuttled into mitochondria and once there, will contribute reducing agents to ETC.
Key sites of glycolysis regulation
Hexokinase: glucose –> glucose-6-phosphate
Phosphofructokinase-1: fructose-6-phosphate –> fructose 1,6-bisphosphate
Pyruvate kinase: PEP –> pyruvate
glucokinase
(Liver)
a high Km enzyme (unlike hexokinase) not inhibited by glucose-6-phosphate (also unlike hexokinase)
-when glucose levels are low, glycolysis is low so that there is sufficient glucose for other functions/cells
Major site of regulation in glycolysis
Phosphofructokinase-1
Senses energy status (ATP vs. AMP)
High ATP, High Citrate, and low pH inhibit PFK1
High AMP and high fructose 2, 6 bisphosphate stimulate
Fructose 2,6-bisphosphate
Responds to glucose levels and when glucose levels are high, it stimulates PFK-1 and inhibits fructose 1,6-bisphosphatase
Promotes glycolysis
**is a regulatory molecule, not a glycolytic pathway intermediate
Regulated by 2 enzyme activities, a kinase and a phosphatase, that reside on the same polypeptide, PFK-2
Phosphofructokinase-2
Has kinase and phosphatase activities
- The Kinase domain phosphorylates fructose-6-phosphate for form F-2,6-BP (increases levels)
- The phosphatase domain dephosphorylates F-2,6,-BP to form fructose-6-P (lower levels of F-2,6-BP)
Insulin and PFK2 Relationship
High Glucose Levels/Insulin INHIBIT PFK2 and thus PROMOTE GLYCOLYSIS
- Insulin signaling activates PFK2 phosphatase activity, removing the phosphate from the PFK2 kinase domain - this ACTIVATES the kinase and INHIBITS phosphatase
- Increases fructose 2,6-BP, stimulates PFK-1, and glycolysis predominates
Pyruvate Kinase Regulation
In normal cells:
ATP allosterically inhibits pyruvate kinase - this makes sense, because when ATP is high you want to slow glycolysis
In liver:
When glucose levels are low, glucagon signaling leads to the phosphorylation of pyruvate kinase and reduced activity, so that the other cells can have the glucose
Hemolytic anemia
Caused by pyruvate kinase deficiency
Results in lysis of RBCs because these cells need glycolysis for ATP. Other cells are usually okay.
