Transcript Lecture 21
Glycolysis 5/9/03 Glycolysis The conversion of glucose to pyruvate to yield 2ATP molecules •10 enzymatic steps •Chemical interconversion steps •Mechanisms of enzyme conversion and intermediates •Energetics of conversions •Mechanisms controlling the Flux of metabolites through the pathway Historical perspective Winemaking and baking industries 1854-1865 Louis Pasture established that microorganisms were responsible for fermentation. 1897 Eduard Buchner- cell free extracts carried out fermentation no “vital force” and put fermentation in the province of chemistry 1905 - 1910 Arthur Harden and William Young • inorganic phosphate was required ie. fructose-1,6bisphosphate • zymase and cozymase fractions can be separated by diaylsis Inhibitors were used. Reagents are found that inhibit the production of pathway products, thereby causing the buildup of metabolites that can be identified as pathway intermediates. Fluoride- leads to the buildup of 3-phosphoglycerate and 2-phosphoglycerate 1940 Gustav Embden, Otto Meyerhof, and Jacob Parnas put the pathway together. Pathway overview 1. Add phosphoryl groups to activate glucose. 2. Convert the phosphorylated intermediates into high energy phosphate compounds. 3. Couple the transfer of the phosphate to ADP to form ATP. Stage I A preparatory stage in which glucose is phosphorylated and cleaved to yield two molecules of glyceraldehyde-3phosphate - uses two ATPs Stage II glyceraldehyde-3-phosphate is converted to pyruvate with the concomitant generation of four ATPs-net profit is 2ATPs per glucose. Glucose + 2NAD+ + 2ADP +2Pi 2NADH + 2pyruvate + 2ATP + 2H2O + 4H+ Oxidizing power of NAD+ must be recycled NADH produced must be converted back to NAD+ 1. Under anaerobic conditions in muscle NADH reduces pyruvate to lactate (homolactic fermentation). 2. Under anaerobic conditions in yeast, pyruvate is decarboxylated to yield CO2 and acetaldehyde and the latter is reduced by NADH to ethanol and NAD+ is regenerated (alcoholic fermentation). 3. Under aerobic conditions, the mitochondrial oxidation of each NADH to NAD+ yields three ATPs Hexokinase CH2OH O H H OH Mg++ H OH H OH Glucose O H H OH + ATP H OH CH2OPO32H + ADP + H+ H OH OH H OH Glucose-6-phosphate Isozymes: Enzymes that catalyze the same reaction but are different in their kinetic behavior Tissue specific Glucokinase- Liver controls blood glucose levels. Hexokinase in muscle - allosteric inhibition by ATP Hexokinase in brain - NO allosteric inhibition by ATP Hexokinase reaction mechanism is RANDOM Bi-Bi Glucose ATP ADP Glu-6-PO4 When ATP binds to hexokinase without glucose it does not hydrolyze ATP. WHY? The binding of glucose elicits a structural change that puts the enzyme in the correct position for hydrolysis of ATP. The enzyme movement places the ATP in close proximity to C6H2OH group of glucose and excludes water from the active site. H O H H OH OH H OH a-D-Xylose There is a 40,000 fold increase in ATP hydrolysis upon binding xylose which cannot be phosphorylated! Yeast hexokinase, two lobes are gray and green. Binding of glucose (purple) causes a large conformational change. A substrate induced conformational change that prevents the unwanted hydrolysis of ATP. Phosphoglucose Isomerase CH2OPO32O H H OH H O3POCH2 H OH OH CH2OH O H OH H -2 HO H OH OH H Uses an “ ene dione intermediate 1) Substrate binding 2) Acid attack by H2N-Lys opens the ring 3) Base unprotonated Glu abstracts proton from C2 4) Proton exchange 5) Ring closure Uncatalyzed isomerization of Glucose Phosphofructokinase -2 -2 O3POCH2 CH2OH O H HO H + ATP OH OH H Fructose-6-PO4 O3POCH2 CH2OPO3-2 O Mg++ H HO H + ADP OH OH H Fructose-1,6-bisphosphate 1.) Rate limiting step in glycolysis 2.) Irreversible step, can not go the other way 3.) The control point for glycolysis Aldolase CH2OPO3-2 CH2OPO3-2 C O HO C H H C OH H C OH HO C O C H H + O H CH2OPO3-2 H Fructose -1,6-bisphosphate (FBP) Dihydroxyacetone phosphate (DHAP) C OH Glyceraldehyde-3phosphate (GAP) CH2OPO3-2 Aldol cleavage (retro aldol condensation) There are two classes of Aldolases Class I animals and plants - Schiff base intermediate Step 1 Substrate binding Step 2 FBP carbonyl groups reacts with amino LYS to form iminium cation (Schiff base) Step 3. C3-C4 bond cleavage resulting enamine and release of GAP Step 4 protonation of the enamine to a iminium cation Step 5 Hydrolysis of iminium cation to release DHAP CH2OPO3-2 C14 CH2OH + NH CH2OPO3-2 (CH2)4 Lys + NaBH4 H C14 CH2OH NH 3 (CH2)4 Lys Class II enzymes are found in fungi and algae and do not form a Schiff base. A divalent cation usually a Zn+2 polarizes the carbonyl intermediate. CH2OPO32- CH2OPO3-2 C HO C O - - O C Zn 2+ HO Zn 2+ H H Probably the occurrence of two classes is a metabolic redundancy that many higher organisms replaced with the better mechanism. Aldolase is very stereospecific When condensing DHAP with GAP four possible products can form depending on the whether the proS or pro R hydrogen is removed on the C3 of DHAP and whether the re or si face of GAP is attacked. CH2OPO3 H H H OH OH CH2OPO3 D-Fructose 1,6 bisphosphate 2- CH2OPO32- O O O HO CH2OPO32- CH2OPO32- 2- O H OH HO H H H OH HO H HO H OH H CH2OPO32D-Psicose 1,6 bisphosphate OH CH2OPO32- D-Tagatose 1,6 bisphosphate H OH H OH CH2OPO32- D-Sorbose 1,6 bisphosphate Triosephosphate isomerase DHAP K eq GAP DHAP GAP 2 4.7 x10 1 96 TIM is a perfect enzyme which its rate is diffusion controlled. A rapid equilibrium allows GAP to be used and DHAP to replace the used GAP. TIM has an enediol intermediate H H O H OH H H C OH CH2OPO32- H C OH CH2OPO3 2- C OH C O CH2OPO32- GAP enediol DHAP Transition state analogues Phosphoglycohydroxamate (A) and 2-phosphoglycolate (B) bind to TIM 155 and 100 times stronger than GAP of DHAP HO B. A. OH O H N O C - CH2OPO3 2- O32-POH2C O O32-POH2C O- TIM has an extended “low barrier” hydrogen bond transition state Hydrogen bonds have unusually strong interactions and have lead to pK of Glu 165 to shift from 4.1 to 6.5 and the pK of Geometry of the eneolate intermediate prevents formation of methyl glyoxal Orbital symmetry prevents double bond formation needed for methyl glyoxal Glyceraldehyde-3-phosphate dehydrogenase The first high-energy intermediate H O H C OPO32- O OH + NAD+ + Pi CH2OPO32- H C OH + NADH CH2OPO32- Uses inorganic phosphate to create 1,3 bisphosphoglycerate Reactions used to elucidate GAPDH’s mechanism Mechanistic steps for GAPDH 1. GAP binds to enzyme. 2. The nucleophile SH attacks aldehyde to make a thiohemiacetal. 3. Thiohemiacetal undergoes oxidation to an acyl thioester by a direct transfer of electrons to NAD+ to form NADH. 4. NADH comes off and NAD+ comes on. 5. Thioester undergoes nucleophilic attack by Pi to form 1,3 BPG. The acid anhydride of phosphate in a high energy phosphate intermediate Arsenate uncouples phosphate formation O 3PG O O As O O As GAP DH + O O O H H O O- O O C H C OH OH CH2OPO32- CH2OPO32H C + OH CH2OPO32- O O As O O