What is the glycolytic pathway?
Employed by all tissues for oxidation of glucose to provide energy (ATP) and intermediates for other metabolic pathways
- the hub of carbohydrate metabolism (all sugars can be converted to glucose)
- RBCs only do glycolysis (never enter TCA) so would die without it
What is the end product of glycolysis?
Pyruvate
- in aerobic environments -> pyruvate enters TCA cycle for oxphos/ETC
- in anaerobic environments -> pyruvate reduced to lactate
What is Na-independent facilitated diffusion?
Transport of glucose by a family of membrane transporters (GLUT-1 to GLUT-14)
- tissue-specific expression
- GLUT-4 abundant in muscle and adipose, increased by insulin
What is Na+ co-transport?
Energy requiring (against the monosaccharide gradient), Na-dependent transport of glucose -found in intestine and renal tubules
What are the 2 stages of glycolysis?
1) Energy investment prep phase: 1st 5 rxns
2) Energy generation phase: net 2 ATPs (total 4)
What is the E-investment prep phase (step 1) of glycolysis?
Glucose -> Glucose 6-phosphate
- Enzyme: Hexokinase I-III or Glucokinase
- Irreversible reaction #1 (phosphorylation prevents sugar from being able to leave the cytosol)
What is the Km/Vmax of Hexokinase I-III?
- Low Km (high affinity): important to RBCs because they need high affinity for glucose since they only perform glycolysis
- Low Vmax
- Inhibited by product
What is the Km/Vmax of Glucokinase (Hexokinase IV)?
- Much higher Km (lower affinity): present in the liver parenchymal cells and beta-cells of pancreas (only needs to be effective at high levels of glucose for glycogen production)
- High Vmax
- Not inhibited by product
What is step 2 of glycolysis?
Isomerization of Glucose 6-P -> Fructose 6-P
- Enzyme: phosphoglucose isomerase
- Reversible
- Not a rate-limiting or regulated step
- Most likely not on test
What is step 3 of glycolysis?
Fructose 6-P -> Fructose 1,6-biphosphate
- Enzyme: phosphofructokinase 1
- Irreversible, most rate-limiting and committed step (aka point of no return)
- Inhibited by: increase in ATP and increase in Citrate
- Activated by: Fructose 2,6-biphosphate and AMP
- Phosphorylation adds an additional phosphate (uses energy)
What is step 4 of glycolysis?
Fructose 1,6-biphosphate -> glyceraldehyde 3 phophate + dihydroxyacetone phosphate
- Enzyme: Aldolase
- Reversible, not rate-limiting or regulated
- Aldolase B also cleaves fructose 1-P (dietary fructose)
- DHAP -> G3P via Triose phosphate isomerase (so you end up with 2 G3P)
What is step 5 of glycolysis?
Glyceraldehyde 3-P -> 1,3-biphosphoglycerate (1,3-BPG)
- Enzyme: Glyceraldehyde 3-phosphate dehydrogenase
- 1st redox (oxidation-reduction) rxn
- Requires 1 ATP and produces NADH (2 G3P -> 2 NADH)
- High-energy phosphate at C1 drives following ATP synthesis
What is step 1 of the E-generation phase of glycolysis?
Each step occurs twice because we started with 2 G3P
1,3-biphosphoglycerate -> 3-phosphoglycerate
-Enzyme: Phosphoglycerate kinase
-Makes 1st ATP via substrate level phosphorylation (does not require oxygen, energy for production of high-energy P comes from substrate)
-3-phosphoglycerate -> 2-phosphoglycerate
What side step can RBCs take during step 1 of the E-generation phase?
1,3-BPG -> 2,3-BPG
- Enzyme: Mutase
- Does not make ATP
- Bypass done to regulate O2 affinity
What is step 2 of the E-generation phase of glycolysis?
2-phosphoglycerate -> phosphoenolpyruvate
- Enzyme: Enolase
- Dehydration reaction causes phosphoenolpyruvate to become high-energy
What is step 3 of the E-generation phase of glycolysis?
Phosphoenolpyruvate -> pyruvate
- Enzyme: Pyruvate kinase
- Irreversible
- ATP is created via substrate level phosphorylation
- Activated by fructose 1,6-biphosphate
What does fluoride inhibit?
Enolase
-Water fluoridation reduces lactate production by mouth bacteria, decreasing dental cavities
How is pyruvate reduced to lactate?
By lactate dehydrogenase
- Reversible enzyme
- Major fate in lens/cornea of eye, kidney, medulla, testes, leukocytes and RBCs because they are all poorly vascularized (low O2) or lack mitochondria (no TCA)
- Important in low O2 situations because it allows the oxidation of NADH -> NAD+ (a build-up of NADH is rate limiting in many reactions so this would inhibit glycolysis)
What occurs in exercising skeletal muscle?
- When NADH production exceeds the oxidative capacity of the respiratory chain -> elevated NADH/NAD+ ratio -> favors reduction of pyruvate to lactate
- During intense exercise: lactate accumulates in muscle -> drop in intracellular pH -> cramps
What does LDH activity depend on?
Intracellular concentration of pyruvate, lactate and on NADH/NAD+ ratio
-In liver and heart, NADH/NAD+ ratio is lower than in exercising muscle -> oxidize lactate (obtained from the blood) back to pyruvate
What is lactic acidosis?
Elevated [lactate] in plasma (a type of metabolic acidosis) when there is a collapse of circulatory system (such as in MI, PE, and uncontrolled hemorrhage)
- O2 is not brought to tissues effectively -> limits oxphos and decreases ATP synthesis
- Cells rely on anaerobic glycolysis -> increase in lactate
What is the effective yield of anaerobic glycolysis?
- Net 2 ATP (total 4, but use 2 in the 1st steps)
- 2 lactate molecules
- Net 0 NADH (create 2 but oxidize 2 in conversion of pyruvate -> lactate)
What is the effective yield of aerobic glycolysis?
- Net 2 ATP (total 4, but use 2 in the 1st steps)
- 2 pyruvate molecules
- Net 2 NADH molecules (each can create about 3 ATP in ETC)
What hormonal regulation occurs in glycolysis?
- Allosteric or covalent phosphorylation (short-term)
- Slower hormonal changes in gene expression (hours to days)
- Insulin turns ON
- Glucagon turns OFF
- 1st major regulatory step: F6P -> F1,6P
- 2nd major regulatory step: phosphoenolpyruvate -> pyruvate
