II. NADH must be oxidized to NAD+ ==> Final steps depend upon whether O2 is available;
1. Structure of Biological Molecules
a) Small molecules
- carbohydrates -- fuel for metabolism; structural roles; nucleotides
- lipids -- energy storage (ATP); membranes
- amino acids (building blocks for proteins)
- H2O -- solvent for cytoplasm
b) Macromolecules -- polymers of small "building block molecules"
- proteins -- enzymes and transport proteins ==> catalyze reactions
- membranes -- isolate compartments where different processes occur
2. Cell Structure
a) Organization of Cellular Functions into compartments (organelles)b) Replication of Cells
3. Energetics -- how energy can be stored and transformed
a) Chemical Energy -- ATPb) Gradients -- talk about this later
1. Goal of Catabolic Metabolism -- extract chemical energy from food ==> ATP
a) ATP -- energy storage in high energy chemical bondsb) ATP is intermediate in energy among phosphorylated compounds
2. Cytoplasm -- Glycolysis - Figure 7.7
a) Anaerobic (no O2)-- no net oxidation==> 2 Lactate + 2 ATP per glucoseb) Aerobic ==> 2 Pyruvate + 2 ATP + 2 NADH per glucose
3. Mitochondria - Figure 7.9
a) Pyruvate Dehydrogenase ==> 2 pyruvate --->2 Acetyl-CoA + 2 CO2 + 2NADH per glucoseb) Citric Acid Cycle ==> 2Acetyl-CoA ---> 2 CO2 + 3 NADH + 1 FADH2 + 1 GTP per glucose
c) Fatty Acids -- for each 2 carbons ==> Acetyl-CoA + FADH2 + NADH
d) Electron Transport -- Inner Mitochondrial Membrane Figure 7.10 & 7.11
- electrons from NADH and FADH2 (high energy) transferred to O2 to produce H2O (low energy)
- free energy capture as ATP -- 3 ATP/NADH and 2 ATP/FADH2
- produces 36 ATP/glucose ---> extracts much more energy than anaerobic glycolysis
1. Stage 1 Consumes ATP when glucose and then fructose are phosphorylated
In the energy consuming stage of Glycolysis, 2 molecules of ATP are used to phosphorylate first glucose and then fructose producing Fructose-1,6-diphosphate. Fructose-1,6-diphosphate is then split into two triose phosphates, Dihydroxyacetone phosphate and Glyceraldehyde-3-phosphate. Dihydroxyacetone phosphate is converted (isomerized) to Glyceraldehyde-3-Phos, so the overall yield is two molecules of Glyceraldehyde-3-phosphate. The products of all subsequent steps are doubled.
2. Stage 2 Produces ATP - a Net production of 2 ATP per glucose
In the energy producing stage of glycolysis, Glyceraldehyde-3-Phos. is oxidized and phosphorylated producing 1,3 diphosphoglycerate and NADH (from NAD+); the new phosphate is then transferred to ADP to yield ATP (actually 2 ATP since all molecules are doubled in the last half of glycolysis). In a subsequent step the second phosphate group is transferred to ADP making another pair of ATP's and the molecule Pyruvate.
The two ATP producing steps produce a total of 4 ATP molecules for each molecule of glucose metabolized. Since 2 ATP molecules were used in the energy consuming stage, the net production of ATP is 2/glucose.
1. Energy Yield: Glucose ----->2 Lactate + 2H+ ---> DG°' =-47.0 kcal/mole
2. Energy Storage: 2ADP + 2Pi -----> 2ATP + 2H2O ---> D G°' =+14.6 kcal/mole
3. Sum: Glucose + 2ADP + 2Pi -----> 2
Lactate + 2H+ + 2ATP + 2 H2O
---> DG°'
= -32.4 kcal/mole; XS Free Energy drives the reaction to
completion
4. This is the Standard Free Energy change (everything at 1M concentrations). At actual intracellular concentrations of reactants and products >60% of available free energy is recovered as ATP.
5. Even this represents only a small fraction of the free energy available from glucose oxidation.
===> Anaerobic Glycolysis uses only about
7% of available energy. To make the remaining 93% available,
Pyruvate must be completely oxidized to
CO2
and H2O
via
Krebs
Cycle and Electron
Transport==>
Oxidative
Phosphorylation
1. Conversion of Glucose
a) hexokinase -- enzyme which converts glucose in our diet to glucose-6-phosphate using up one ATP.b) glycogen phosphorylase -- an allosteric enzyme which hydrolyzes glucose units from the end of glycogen; the glycosidic bond is broken by adding phosphate to the glucose product producing glucose-1-phosphate which is then converted to glucose-6-phosphate.
2. Stage 1 "Energy Investment Stage" (Figure 7.7 Steps 1-5)-- Consumes ATP to convert carbohydrates to Triose Phosphates
3. Stage 2 "Energy-Yielding Phase" (Figure 7.7 Steps 6-10)-- The two molecules ofGlyceraldehyde-3-phosphate produced from each glucose unit undergo a series of enzyme catalyzed reactions which produce two molecules of ATP per Glyceraldehyde-3-Phos. for a total of 4 ATP. NAD+ is used to oxidize the carbonyl group of Glyceraldehyde-3-Phos. producing the reduced form of NAD+, NADH; NAD+ is regenerated in anaerobic glycolysis by reducing pyruvate to lactate (lactic acid). Thus, in Lactic Fermentation there is no net oxidation.
In Lactic Fermentation Glucose is converted to two molecules of Lactate with no net oxidation. However, the rearrangement of the oxidation state (one end fully oxidized and the other fully reduced) is lower in free energy allowing the production of two molecules of ATP. Figure 7.15
Complete oxidation of glucose (or other carbohydrates) in Glycolysis + Krebs Cycle leads to the production of far more ATP than can be gained from Glycolysis alone.
1. At entry of glucose units.
a) Hexokinase-b) Glycogen Phosphorylase-
2. At steps within the glycolytic sequence.
a) Phosphofructokinase-b) Pyruvate Kinase-
E. OVERALL ENERGY BALANCE SHEET.
1.Priming stage------->2 ATP/Glucose used
2.ATP-producing stage------>4 ATP/Glucose produced
3. Sum: Glucose + 2 ADP + 2 Pi------>2 Lactate + 2 ATP Net production--2 ATP
4. No Net Oxidation
1. Oxidation of Acetyl-CoA -- Sum of Reactions (from one Glucose, get 2X everything here)
CH3-C-S-CoA + 3 NAD+ + FAD + GDP + Pi ----> 2 CO2 + 3 NADH + 3 H+ + FADH2 + GTP + CoA-SH
2. What are the important products?
a) NADH and FADH2 are reduced nucleotides with high potential energy; they can be oxidized by enzymes in the inner mitochondrial membrane and O2 to produce 3 ATP/NADH and 2 ATP/FADH2b) GTP (equivalent to ATP) produced by Substrate Level Phosphorylation, NADH and FADH2 used to make ATP via Oxidative Phosphorylation
1. FATS -- converted to glycerol and fatty acids
a) fatty acids are oxidized to Acetyl-CoA producing both NADH and FADH2b) glycerol oxidized to glyceraldehyde-3-P
2. Proteins -- hydrolyzed to enzymes to produce amino acids which can be degraded to Acetyl-CoA and intermediates in the Citric Acid Cycle.
1. The Citric Acid Cycle and Glycolysis are a sources of metabolic intermediates for Biosynthesis--many amino acids can be synthesized from intermediates of the Citric Acid Cycle.
2. Glucose can be made from pyruvate--but two of the steps are not identical to those by which glucose is degraded.
In yeast Pyruvate is decarboxylated to form acetaldehyde which is then reduced to Ethanol regenerating NAD+. Figure 7.16
Pyruvate can enter the mitochondrial matrix where it is oxidized to Acetate producing NADH; the acetate group is bound to a Coenzyme-A molecule forming Acetyl-CoA. Acetyl-CoA combines with the four carbon dicarboxylic acid, oxaloacetate, to form the six carbon tricaboxylic acid, citric acid, to begin the Krebs Cycle. In a sequence of reactions, citric acid is oxidized releasing two molecules of CO2. There are four oxidation steps in the Krebs Cycle which produce 3 NADH and 1 FADH2 regenerating oxaloacetate. Another product is 1 GTP (equivalent to ATP) produced by substrate level phosphorylation.
The NADH and FADH2 produced by the Krebs Cycle as well as the NADH produced by oxidation of pyruvate and by glycolysis have electrons with high potential energy which pass through the electron transport chain in the inner mitochondrial membrane producing many ATP molecules by oxidative phosphorylation (see next lecture).
