Metabolism Review Slides: Glycolysis Flashcards
Why are NAD+ and NADP+ so important?
They are the most important electron carriers. They are needed as oxidizing agents
What are the fuel preferences of the liver?
Fatty acids, glucose and amino acids
What are the fuel preferences of skeletal muscle?
At rest: fatty acids
Exertion: glucose
What are the fuel preferences of the brain?
Fed state: glucose
Starvation: ketone bodies/glucose
What are the fuel preferences of adipose tissue?
Fatty acids
What are the fuel preferences of heart muscle?
Fatty acids, but it will pretty much use anything
How does the liver use energy during fasting?
The liver expends energy through the metabolism of fatty acids in order to make glucose.
Fatty acid oxidation in the liver provides most of the ATP needed for gluconeogenesis
What is the driving force for the coordination of metabolism?
Provide the normal range of glucose necessary for the brain
When does the brain lose its requirement for glucose?
Never
The majority (76%) of our caloric stores are in which form?
Fat
What is the second most abundant form of energy stored in our bodies?
Protein- at 23%
Fasting glucose blood levels must be maintained above what concentration? What happens if it falls below?
Fasting blood glucose must be maintained above 60mg/100mL.
Acute hypoglycemia causes neurological problems, coma and death
Fasting blood glucose levels above what concentration is considered hyperglycemia? What are complications associated with hyperglycemia?
110mg/100mL
Multiple problems, including increased oxidative stress within cells, increased intracellular lipids, lipotoxicity. This ultimately leads to type 2 diabetes mellitus
Glucagon and epinephrine stimulate what two metabolic processes?
Glucagon and epinephrine stimulate glycogenolysis and gluconeogenesis
Insulin stimulates what two metabolic processes?
Insulin stimulates glycogenesis and glycolysis
What three enzymes catalyzes irreversible reactions in glycolysis?
Hexokinase/glucokinase: glucose –> glucose-6-phosphate
Phosphofructokinase-1: Fructose-6-phosphate –> Fructose-1,6-bisphosphate
Pyruvate Kinase: PEP –> pyruvate
Describe the three different “stages” of glycolylsis
Priming: ATP investment- 2 ATP invested
Splitting: Fructose-1,6-bisphosphate (6 carbon) split into two 3 carbon molecules
Oxidoreduction phosphorylation stage: ATP earnings- 4 total
Why is glucose “committed” to glycolysis after phosphorylation by hexokinase?
Glucose can exit the cell through a glucose transporter (GLUT2).
Glucose-6-phosphate is more polar, and therefore can no longer exit the cell. It is “trapped” and ready for further metabolism
Discuss the differences in location of hexokinase vs. glucokinase
Hexokinase is present in all cell types.
Glucokinase is present in the liver (for glycolysis) and pancreas (as a glucose- sensing enzyme in the pancreas)
Discuss the inhibition of hexokinase vs. glucokinase
Hexokinase is inhibited allosterically by its product (glucose-6-phosphate)
Glucokinase is inhibited via translocation into the nucleus, promoted by fructose-6-phosphate ( a downstream product)
Discuss the stimulation of hexokinase vs glucokinase
Hexokinase is constitutively active.
Glucokinase is inducible; enzyme synthesis is induced by insulin. Activity is also increased by glucose, which promotes translocation from the nucleus to the cytosol.
Describe the relative affinities for glucose between hexokinase and glucokinase
Hexokinase has a high affinity for glucose and is saturated at physiologic concentrations
Glucokinase has a low affinity for glucose and is not saturated at physiologic concentrations.
How does fructose-2,6-bisphosphate affect glycolysis in the liver?
High levels of fructose-2,6-bisphosphate activates hepatic PFK-1.
Fructose-2,6-bisphosphate is the major physiological activator of hepatic PFK-1.
What conditions increase vs. decrease levels of fructose-2,6-bisphosphate?
Increase: fructose-2,6-bisphosphate is formed when blood glucose (and insulin) levels are high. High blood glucose levels are indicative of sufficient substrates for glycolysis.
Decrease: low blood glucose levels (and low insulin) signals that there is not sufficient substrates for glycolysis to occur in the liver.
What is the enzyme responsible for the formation of fructose-2.6-bisphosphate?
PFK-2
How is the formation of fructose-2.6-bisphosphate affected by glucagon and epinephrine?
Glucagon and epinephrine inhibit the formation of fructose-2,6-bisphosphate.
Glucagon and epinephrine stimulate cAMP synthesis –> PKA activation, which phosphorylates the kinase domain of PFK-2
Is PFK-2 a glycolytic enzyme?
No, but it plays a MAJOR role in the regulation of glycolysis, through the formation of fructose-2,6-bisphosphate, a stimulator of PFK-1
How is pyruvate kinase regulated?
Covalent modification: phosphorylation inhibits kinase activity
Allosteric regulation: Fructose-1,6-bisphosphate stimulated activity, and ATP and alanine inhibits activity
How does glucagon and epinephrine affect pyruvate kinase activity?
In the liver: glucagon and epinephrine act via cAMP and PKA to phosphorylate and inactivate hepatic pyruvate kinase
In the skeletal muscle: glucagon and epinephrine stimulate pyruvate kinase activity
How does insulin affect hepatic pyruvate kinase activity
In the liver, the presence of insulin (indicating high blood glucose levels), stimulates dephosphorylation/activation of pyruvate kinase.
High blood glucose –> glycolysis for energy
How does glucagon and epinephrine affect the synthesis of the three irreversible enzymes of glycolysis in the liver?
In the liver, glucagon and epinephrine also decrease the synthesis of the three irreversible enzymes of glycolysis
What are the 4 fates of pyruvate?
Alanine via transamination
Oxaloacetate via pyruvate carboxylase
Lactate via lactate dehydrogenase
Acetyl CoA via pyruvate dehydrogenase
What is the result of pyruvate dehydrogenase deficiency?
Inability to convert pyruvate to acetyl CoA. Leads to a build up of pyruvate and lactate.
Causes microcephaly, poor muscle coordination and mental retardation
What is the result of pyruvate carboxylase deficiency?
Pyruvate cannot be converted to oxaloacetate. This leads to a build up of alanine, lactate and pyruvate levels.
Causes developmental delays, recurrent seizures and metabolic acidosis
Other than substrates, what else must be regenerated in order for glycolysis to continue?
NAD+. NAD+/NADH is in limited supply in the cytoplasm, and NAD+ must be regenerated for glycolysis to continue. NADH is oxidized when pyruvate is converted to lactate.
Under anaerobic conditions, when is NAD+ regenerated?
NAD+ is regenerated from production of lactate (utilizing LDH)
What is the cori cycle?
Cori cycle; recycling of lactate from the muscle to the liver where it can be converted back to glucose.
Aside from the production of lactate, what is another way that NAD+ can be regenerated?
Cytoplasmic NAD+ can be regenerated by using mitochondria-linked shuttles (the glycerophosphate and malate-aspartate shuttles). In these shuttles, reducing equivalents are transferred to the mitochondria. Mitochondrial FADH2 or NADH are formed as a result of this transfer. Since mitochondrial FADH2 or NADH can be reoxidized using the electron transport chain, far more ATP can be formed via this route than through the generation of lactate.
The malate-aspartate shuttles are necessary because the mitochondrial inner membrane is impermeable to NADH.
What reaction in glycolysis requires a steady stream of NAD+?
glyceraldehyde-3-P dehydrogenase.
What are the three mechanisms that regenerate NAD+ and where in the cell do they occur?
1) Lactate dehydrogenase: pyruvate –> lactate. Occurs in the cytosol
2) Malate-aspartate shuttle: occurs in the cytosol and mitochondria
3) Glycerol-phosphate shuttle: occurs in cytosol and mitochondria
Shuttles exists because the inner mitochondrial membrane is impermeable to NADH
How do low levels of NADH affect lactate formation?
Low levels of NADH will lead to decreased lactate formation. NADH is required for the formation of lactate.
What reaction is catalyzed by pyruvate dehydrogenase?
PDH catalyzes the reaction of pyruvate to acetyl CoA
pyruvate + CoASH + NAD+ –> Acetyl CoA + CO2+ NADH + H+
Describe the composition of pyruvate dehydrogenase.
PDH is a multienzyme complex consisting of E1, E2 and E3 that all have distinct function.
What vitamin cofactors are required for PDH activity?
Thiamine (B1)
Pantothenate (B5)
Riboflavin (B2)
Niacin (B3)
If you have deficiencies in B1, B2, B3 or B5, PDH is less efficient
What regulates the activity of pyruvate dehydrogenase?
The end products, Acetyl CoA and NADH allosterically inhibition of PDH.
Additionally, the end products cause phosphorylation of PDH.
What happens to glycolysis in someone with lactate dehydrogenase deficiency?
1) The reaction of pyruvate –> lactate by LDH is necessary for the regeneration of NAD+. When LDH is deficient, there is insufficient NAD+ for the glyceraldehyde-3-P dehydrogenase reaction to take place and glycolysis slows.
What is the net generation of ATP during anaerobic conditions?
2 ATP from glycolysis.
Glycolysis can take place in either aerobic or anaerobic conditions
What is galactosemia and what is it most commonly associated with?
Galactosemia is a genetic disorder caused by a deficiency in either galactokinase or galactose-1-phosphate uridyltransferase. Accumulation of galactitol and galactose-1-phosphate is associated with cataracts.
What is the most common form of galactosemia?
The classic galactosemia is the genetic deficiency of galactose-1-phosphate uridyltransferase. It is also the most severe form of galactosemia.
What is the root cause of hereditary fructose intolerance?
A deficiency in Aldolase B.
What is aldolase B?
Aldolase B, also known as fructose-bisphosphate aldolase B or liver-type aldolase catalyzes the cleavage of fructose 1 phosphate into glyceraldehydes and DHAP (a glycolysis intermediate)
What are the symptoms and treatment for fructose intolerance?
Symptoms: hypoglycemia, vomiting, jaundice, hepatic failure
Treatment: avoid fructose in the diet
What is the connection between fructose, aldolase B and glycolysis?
Fructose must first be phosphorylated to Fructose 1-phosphate and then cleaved to glyceraldehyde and DHAP. DHAP is a glycolytic intermediate, and can be funneled into the glycolytic cycle.
Conversion of fructose to a usable form, fructose-1-phosphate, requires ATP. If fructose is around but aldolase B is not functioning, ATP is used in the phosphorylation, but no ATP is generated downstream.
PHosphorylated sugars are toxic to the cell.
