Glycolysis Flashcards
How many steps in glycolysis?
10, employed by all tissues to break down glucose and provide energy in the form of ATP and intermediates for other pathways
what types of sugars can be converted to glucose?
almost all of them
anaerobic pathway
glycolysis does not require O2 to proceed
Where does glycolysis take place?
cytosol
What does the fate of pyruvate in human cells depend on?
- whether or not an adequate supply of oxygen is available
2. mitochondria are present
aerobic glycolysis
in cells with sufficient O2 and mitochondria
What is the end product of in aerobic glycolysis?
pyruvate
NADH during glycolysis
oxidized to NAD+ via oxidative phosphorylation where O2 is the final electron acceptor
What is require for glycolysis to proceed?
NAD+
anaerobic glycolysis
in cells that lack mitochondria or have insufficent O2 supply
what is the final electron acceptor for anaerobic glycolysis?
pyruvate
What allows for the regeneration of NAD+ in anaerobic glycolysis?
pyruvate being converted into lactate via lactic acid fermentation
How does glucose enter cells?
- CANNOT diffuse directly into cells
- Na+ independent facilitated diffusion transport
- Na+ monosaccharide cotransporter system (SGLT)
GLUT mechanism
- spans membrane
- ATP-independent (passive transport)
- changes conformation upon glucose binding which allows to cross membrane
Which GLUT are always in plasma membrane?
1,2,3
GLUT isoforms
- tissue specific
- specific regulation
- specificity for substrate (GLUT-5, fructose)
GLUT-1
- most tissues, abundant in brain and RBC
- Basal uptake of glucose
GLUT-2
- Liver, pancreatic beta-cells
- glucose uptake and release by liver; beta-cell glucose sensor
GLUT-3
- most tissues, abundant in neurons
- basal uptake of glucose
GLUT-4
- skeletal muscle, adipose tissue
- insulin stimulated glucose uptake; stimulated by exercise in muscle
Which GLUT is not always on plasma membrane?
4
SGLT mechanism
- transport glucose against its can gradient by co-transporting Na+ down its gradient (active)
- requires energy supplied by an Na+, K+-ATPase that pumps Na+ outside the cell to create the Na+ gradient
SGLT location
- epithelial cells of the intestines
- renal tubules
- choroid plexus (BBB)
significance of phosphorylation of glucose to glucose 6-phosphate
- once phosphorylated, glucose cannot cross membrane (+deltaG, NEEDS ATP)
- there are no transported for G-6-P
- therefor phosphorylation traps the molecule inside the cell because molecule is now more polar.
- overall delta G is negative
hexokinase
- phosphorylation of glucose to glucose 6-phosphate
- found in most tissues
- broad substrate specificity
- inhibited by product
- high affinity for glucose (low Km)
- low Vmax-cannot trap glucose inside the cells
glucokinase
- phosphorylation of glucose to glucose 6-phosphate
- similar to hexokinase (not specific)
- has much higher Km
- high Vmax-allows liver to effectively clear up glucose from the blood
glucokinase location
- phosphorylation of glucose to glucose 6-phosphate
- found only in hepatocytes and pancreatic beta cells
What inhibits glucokinase?
-phosphorylation of glucose to glucose 6-phosphate
indirectly inhibited by fructose-6-phosphate
What stimulates glucokinase?
- phosphorylation of glucose to glucose 6-phosphate
- indirectly stimulated by glucose
mutations of glucokinase
cause a rare form of diabetes called maturity-onset diabetes of the young (MODY)
First step of glycolysis
phosphorylation of glucose to glucose 6-phosphate
second step of glycolysis
-isomerization of glucose 6-phosphate
significance of isomerization of glucose 6-phosphate
- aldo-keto isomerization
- readily reversible
- NOT rate limiting
enzyme of isomerization of glucose 6-phosphate
phosphoglucose isomerase
What is the third step of glycolysis?
phosphorylation of fructose 6-phosphate
What is the rate determining step of glycolysis?
phosphorylation of fructose 6-phosphate
What is the most important step of glycolysis?
phosphorylation of fructose 6-phosphate
significance of phosphorylation of fructose 6-phosphate
- irreversible
- rate-limiting step
- the most important control point
enzyme involved with phosphorylation of fructose 6-phosphate
phosphofruktokinase-1 (PFK-1)
PFK-1 activity
-requires ATP
-regulated by
-energy status of the cell
-fructose 2,6-bisphosphate produced by
PFK 2
AMP being a positive regulator
cannot have high amounts of AMP and ATP. if ATP is low, AMP is high
2,6-bis-phosphate produced by PFK-2 as a positive regulator
2 different binding sites for different affinity for ATP
- allosteric site
- atp will bind to allosteric site when there’s a lot
PFK1 regulation by energy status of the cell
allosterically inhibited by ATP and citrate; activated by high AMP levels
negative feedback
4th step of glycolysis
cleavage of fructose 1,6-bisphosphate
significance of cleavage of fructose 1,6-bisphosphate
cleaves one 6-carbon sugar to form two 3-carbon sugars
enzyme of the cleavage of fructose 1,6-bisphosphate
aldolase
what does aldolase do?
cleaves one 6-carbon sugar to form 2 3-carbon sugars
whats the 5th step of glycolysis?
isomerization of dihydroacetone-P (DHAP)
significance of isomerization of dihydroacetone-P (DHAP)
- interconverts two 3-carbon sugars
- necessary for he glycolysis for proceed
- reversible
- dependent on cxn of products and substrates
enzyme associated with isomerization of dihydroacetone-P (DHAP)
triose phosphate isomerase
What is the 6th step of glycolysis?
oxidation of glyceraldehyde 3-phosphate
significance of the oxidation of glyceraldehyde 3-phosphate
- coupled to the reduction of NAD+ to NADH
- attachment of a phosphate group to the carboxyl group to form 3-BPG
- this Pi will drive the synthesis of ATP in the next reaction
enzyme associated with the oxidation of glyceraldehyde 3-phosphate
glyceraldehyde 3-P dehydrogenase
glyceraldehyde 3-P dehydrogenase
- removes electrons
- inhibited by pentavalent arsenic
step 6.1 of glycolysis
alternating reaction in RBCs: synthesis of 2,3bisphosphoglycerate
Where does synthesis of 2,3bisphosphoglycerate happen?
RBC only
synthesis of 2,3bisphosphoglycerate
- 2,3-BPG is in trace levels in most cells
- in RBCs is VERY HIGH and serves to increase O2 delivery
- this step is a modification of glycolysis found in RBCs only
step 7 of glycolysis
synthesis of 3-phosphoglycerate producing ATP
significance of synthesis of 3-phosphoglycerate producing ATP
- phospho- group is directly transferred to ADP to produce ATP (substrate level phosphorylation)
- reversible under physiological conditions (unusual for kinases)
enzyme involved with synthesis of 3-phosphoglycerate producing ATP
phosphoglycerate kinase
phosphoglycerate kinase
- synthesis of 3-phosphoglycerate producing ATP
- unusual kinase
- transfers phosphate group
- actually dephosphorylates too
- irreversible
what is the 8th step of glycolysis?
shift the phosphate group from carbon 3 to carbon 2
what is the enzyme to shift the phosphate group from carbon 3 to carbon 2
phosphoglycerate mutase
What is the ninth 9th step of glycolysis
dehydration of 2-phosphoglycerate
what is the significance of dehydration of 2-phosphoglycerate
- produce PEP which contain high energy phosphate
- substrate level phosphorylation
what is the enzyme associated with the dehydration of 2-phosphoglycerate
enolase
what is the 10th step of glycolysis?
formation of pyruvate and production of ATP
what is the significance of the formation of pyruvate and production of ATP
- the 3rd irreversible action
- high energy enos-phosphate is used to produce ATP from ADP
- substrate level phosphorylation
what is the enzyme used in the formation of pyruvate and production of ATP
pyruvate kinase (PK)-complex regulation
Pyruvate kinase (PK)-complex regulation
- formation of pyruvate and production of ATP
- feedforward regulation by fructose 1,6-bisphosphate (allosteric activator)
- phosphorylation via cAMP-dependent PKA
Deficiency in pyruvate and ATP
- affects RBC function mostly
- they don’t have mitochondria and can result in hemolytic anemia
glucagon
- fasting
- phosphorylation=inactive pyruvate kinase
insulin
- well-fed
- dephosphorylation=active pyruvate kinase
PK regulation via cAMP-dependent PKA and hormonal status
- pyruvate kinase only active in liver in well-fed state (high glucose availability)
- this is necessary in the liver because the liver will synthesize glucose (gluconeogenesis) during fasting
Fate of Pyruvate
-in order for glycolysis to proceed, NAD+ has to be regenerated
fate of pyruvate if there is sufficient O2 and mitochondria available
- will be done through the ECT
- pyruvate will be converted into acetyl CoA in the mitochondrial matrix
fate of pyruvate if there is a lack of O2 and/or mitochondria
- reduced to lactate by lactate dehydrogenase (REVERSIBLE)
- this will regenerate NAD+
Fate of lactate and human health
-lactate is normal in RBCs and exercising muscle
excessive exercise
build up of lactate in muscle cells that results in lowered pH leading cramping
lack of O2 in tissues
- causes excessive lactate buildup inside cells
- denatures enzymes
- death of cells
- MI, pulmonary embolism, uncontrolled hemorrhage
lactic acidosis
- excessive lactate production
- lactate reaching the plasma builds to excessive levels
- lowers the pH of the blood
alternative fates of pyruvate
- oxidative decarboxylation of pyruvate to acetyl coA
- carboxylation of pyruvate to oxaloacetate
- reduction to ethanol (not in humans)
oxidative decarboxylation of pyruvate to acetyl coA
- alternative fate of pyruvate
- enzyme: pyruvate dehydrogenase complex
- important in tissues with high oxidative capacity (cardiac muscle)
- irreversible step
carboxylation of pyruvate to oxaloacetate
- alternative fate f pyruvate
- enzyme: pyruvate carboxylase
- can be used for gluconeogensis
- to replenish TCA cycle intermediates
Net energy yields in anaerobic glycolysis
2 ATP + 2 NADH per 1 glucose
NADH oxidation does not yield additional ATP
Net energy yields for aerobic glycolysis
- 2 ATP + 2 NADH per 1 glucose
- oxidation of NADH through the ETC, we will get an additional 3 ATP per molecule NADH
- net yield about 8 ATP per 1 glucose
What is under the transcriptional control in he liver?
the three key enzymes catalyzing the irreversible steps of glycolysis (glucokinase, PFK, pyruvate kinase)
what stimulates the transcription of the gene of the irreversible enzymes?
insulin
what inhibits the transcription of the gene of the irreversible enzymes?
glucagon
prolonged higher insulin levels
- high carb diet, insulin therapy for diabetic, hyperinsulinemic pt
- results in net increase in the amount of these key enzymes
prolonged higher glucagon levels
- prolonged fasting, low carb, untreated type 1 diabetes
- result in a net decrease in these key enzymes
