Glycolysis Flashcards
Glycolysis is the oxidation of _____ to ______
glucose (6 carbon molecule); pyruvate (or lactate)
Glycolysis occurs where?
cytosol
Three Stages of glycolysis
- Conversion of glucose to fru-1,6-bisphosphate (uses 2 ATPs)
- Cleavage of fru-1,6-bisphosphate to DHAP and glyceraldehyde-3-phosphate (G3P)
- Conversion of G3P to pyruvate (produces 4 ATPs and 2 NADHs)
Pyruvate has at least two fates:
- complete oxidation to CO2
- fermentation to lactate
Anaerobic glycolysis is the only energy source in:
- Red blood cells
- Cornea and lens of eye, and certain regions of the retina
- Renal medulla
- Testis
- Leukocytes
- White (fast-twitch) muscle fibers
Main fuel for brain
Glucose
Glycolysis is an important pathway in tumor cells the ________effect and in embryonic stem cells in which a ________ occurs during implantation
Warburg; “metabolic shift”
Step #1:
Irreversible (i.e., equilibrium lies far to right)
In muscle allosterically inhibited by:
Cofactor
Conversion of Glucose to Glucose-6-Phosphate (via Hexokinase)
Inhibited by: Glu-6-P (in muscle)
Irreversible step
Cofactor: Mg2+
Step #2
Isomerization of Glucose-6-Phosphate to Fructose-6-Phosphate (via Phosphoglucose isomerase)
Reversible
Step #3
Cataylst Inhibitors:
Activators:
Cofactor:
Conversion of Fructose-6-Phosphate to Fructose-1,6-Bisphosphate (via PFK-1)
Irreversible step
Inhibitors: ATP (liver & muscle), H+ (muscle), citrate (liver)
Activators: AMP (muscle), Fru-2,6-bisP (liver)
Cofactor: Mg2+
Step #4
Cleavage of Fructose-1,6-Bisphosphate (via Aldolase)
yields Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde 3-phosphate (GAP)
GAP continues on down pathway
Step #5
The Triose Phosphate Isomerase (TIM)-Catalyzed Reaction
- Isomerization of ketose into aldose: intramolecular redox reaction
- Involves formation of “enediol” intermediate
- “reversible”
Dihydroxacetone phosphate to glyceraldehyde 3-phosphate
Step #6
Converstion of Glyceraldhyde-3-Phosphate to 1,3-Bisphosphoglycerate
- Reaction occurs in two stages:
- Oxidation of –CHO to –CO2- using NAD+
- Joining of –CO2- and Pi to form an acyl-P product, 1,3-BPG
- Two stages must be coupled: favorable oxidation drives unfavorable formation of acyl-P compound
- Coupling occurs via formation of high-energy thioester intermediate involving active site cysteine (Cys-149)
- Overall, two NADHs produced from two NAD’s
Step #7
Cofactor:
The Phosphoglycerate Kinase-Catalyzed Reaction
1,3-Bisphosphoglycerate to 3-Phosphoglycerate
Cofactor: Mg2+
Substrate-level phosphorylation occurs
Generates two ATPs
Step #8
Phosphoglycerate Mutase
reversible
- Catalyzes intramolecular group transfer
- Employs active site phospho-histidine residue
- Catalytic amounts of 2,3-BPG needed to maintain histidine in phosphorylated state
- 2,3-BPG synthesized from 1,3-BPG by special mutase
- Levels of 2,3-BPG low in most cells (but high in RBCs
Step #9
Enolase
reversible
Dehydration of substrate markedly elevates P-group transfer potential
Step #10
Pyruvate Kinase
irreversible
- Energy that’s trapped by P within unstable enol form is released when enol changes to more stable ketone form upon loss of P: enol-ketone conversion drives reaction
- Substrate-level phosphorylation occurs; 2 more ATPs produced
Step #11
Regeneration of NAD+ under anaerobic conditions
- Lactic acid fermentation; it occurs under O2-limiting conditions
- Is necessary to maintain REDOX balance
- (NADH, NAD+) in limited supply, so regeneration critical
- NAD+ necessary for G3P DHase step; without NAD+, glycolysis would come to a standstill
Modes of Regulation of Glycolytic (and other) Enzymes
- Allosteric control
- Covalent modification via protein phosphorylation and dephosphorylation
- Different isozymic forms
- Transcriptional control (enzyme induction)
- Sequestration by binding another protein
- Subcellular translocation
How is glycolytic flux regulated?
At the irreversible steps catalyzed by the enzymes:
–hexokinase,
–phosphofructokinase, and
–pyruvate kinase
Hexokinase
Muscle:
Liver:
- Muscle: Hexokinases I-III are allosterically inhibited by glucose 6-phosphate
- Liver: Glucokinase (HK IV)
- Not inhibited by glucose-6-phosphate
- Fructose 6-phosphate “substitutes” for Glu-6-P in inhibiting HK IV
- Fru-6P in equilibrium with Glu-6-P in cell
- Fru-6P’s binding to a nuclear protein, GK-RP, promotes the binding of HK IV to GK-RP, causing its translocation into the nucleus
- This removes it from cytosol
- High levels of glucose reverse the effect of Fru-6-P
- Insulin indirectly increases HK IV protein level through stimulation of transcription
Pyruvate Kinase
Two isozymes:
- Muscle type:
- Allosterically regulated
- inhibited by ATP, which indicates high energy charge, and by alanine (one step away from pyruvate), which indicates an abundance of building blocks
- Activated by fructose-1,6-bisphosphate
- Allosterically regulated
- Liver type:
- Allosterically regulated like M-type
- Subject to reversible covalent modification (phosphorylation) that is under hormonal control
- When blood glucose levels low, glucagon-triggered cAMP cascade leads to phosphorylation of PK; this inhibits it
- Protein levels also subject to hormonal control
- Insulin stimulates PK gene expression; glucagon inhibits it
Phosphofructokinase-1
- Allosterically controlled
- In muscle, PFK activity regulated by energy charge
- Inhibited by ATP
- Very sensitive to level of AMP, an activator
- In cell, AMP, as opposed to ADP, is a good regulator, due to:
- Presence of adenylate kinase (ADP + ADP = ATP + AMP)
- The fact that ATP › ADP › AMP, so that a small decrease in ATP level results in a large increase in AMP level; AMP is more exquisite “sensor” of energy charge
- In liver, allosterically controlled by ATP and citrate (both inhibitors)
- In liver, activated by fructose 2,6-bisphosphate (Fru-2,6-BP), whose level is under hormonal control
- Increased by insulin, which indirectly causes dephosphorylation and activation of a the kinase activity of a bifunctional enzyme (phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2/FBPase2)) that makes and degrades Fru-2,6-BP
- Decreased by glucagon, which indirectly causes phosphorylation (and inactivation) of the kinase domain of the bifunctional enzyme
Glucose Transporters
- Twelve GLUTs identified, but only five have known functions
- Each comprises single polypeptide chain, is ≈ 500 amino acids long and has 12 transmembrane segments; forms pore in membrane
- Each is an “enzyme” whose function is to transport glucose
- Km-values differ according to physiological role & tissue location
