1
Metabolism: The Generation of Energy
I.
Energy and work
A.
Energy:
Capacity to do work
Three types of work occur in living organisms:
Chemical work:
Biosynthesis
Transport work:
Movement of molecules against a concentration gradient
Mechanical work:
Movement
Sunlight:
Ultimate source of all biological energy:
Converted to complex organic molecules by photoautotrophs:
Complex organic molecules serve as carbon and energy sources for
heterotrophs
Cells obtain energy by carrying out chemical reactions:
2000 chemical reactions can occur in a single cell:
Each reaction (with a few exceptions) mediated by a different enzyme:
Cell must synthesize at least 2000 enzymes
B.
Enzymes:
Protein catalysts:
Increase the rate of reactions but do not alter their equilibrium constant
Specific for:
Reaction catalyzed  Molecules acted upon
Bring substrates together at the active site (catabolic site):
Enzyme-substrate complex
Speed up reactions hundreds of thousands of times
Active at temperatures ranging between 0o- 37oC
During a chemical reaction energy available for the performance of work
is either released or absorbed:
G = energy available for work
 = change in
2
G = Free Energy Change:
Amount of usable energy liberated or absorbed during a
chemical reaction:
Expressed in calories
G chemical energy:
Exergonic
-8000 calories):
Release energy
Occur spontaneously
Endergonic
Absorb energy
Do not occur spontaneously:
Energy must be supplied before the reaction can
occur
Cells use energy released by exergonic reactions to drive endergonic
reactions:
Couple an exergonic with an endergonic reaction by means of a
common reactant:
1.
Exergonic reaction:
A  B
G = -10,000 calories
2.
Endergonic reaction:
C D
G = +5,000 calories
3.
Coupled reaction:
A+X  B+Y
G = -2,000 calories
Know:
A  B has a G of -10,000 calories
Calculate that:
Y has absorbed 8000 calories in going from X to Y
Add:
Y to C  D:
3
4.
Coupled reaction
G = -3000 calories
Y (common reactant):
Releases 8000 calories in going from Y to X
Uses 5000 calories in going from C to D:
Leaving a G of -3000 calories
C.
Reactions involved in production of cellular energy are oxidation reactions:
Oxidation:
Loss of electrons & a gain in positive valance
Electron donor:
Oxidized substance gives up electrons
Oxidation reactions always accompanied by reduction reactions:
Reduction:
Gain of electrons & a loss of positive valance
Requires an:
Electron acceptor:
Reduced substance
Accepts electrons
Examples of a coupled oxidation/reduction reaction:
H2  2H+ + 2e(electron donating)
-  2O + 2e O
(electron accepting)
2H+ + O2-  H2O
(water formation)
Hydrogen is oxidized (loses electrons)
Oxygen is reduced (gains electrons)
Glucose is the primary energy source of most microorganisms:
Complete oxidation of glucose:
Glucose (C6H12O6) + 6O2  6CO2 + 6H2O
Oxidation of glucose releases electrons:
Glucose  6CO2 + 12 electrons + 12H+
Reduction of oxygen consumes electrons:
4
6O2 + 12 electrons + 12H+  6H2O
Electrons never exist as free electrons:
Must be part of a molecule
D.
Electron carriers:
Carry electrons between reactions:
Able to:
Accept & donate electrons readily:
Undergo reversible oxidation & reduction
Two most important electron carriers:
Nicotinamide Adenine Dinucleotide (NAD):
Electron carrier involved in catabolic processes
Nicotinamide Adenine Dinucleotide Phosphate (NADP):
Electron carrier involved in biosynthetic processes
Electrons carriers:
Transfer electrons from one redox pair to another:
Act as common reactants
E.
Energy transfer compounds:
Energy released by oxidation must be saved for use by the cell:
Much energy released during oxidation is transferred to a phosphate
compound via a high energy phosphate bond ~P (squiggle P)
Several types of energy transfer compounds in cells:
Able to transfer large amounts of free energy:
Energy which is not stored or transferred is usually released as heat:
Lost to the cell for useful work
Most common high energy phosphate compound in the cell
ATP (Adenosine Triphosphate):
Other high energy compounds in cells:
GTP (Guanidine Triphosphate)
UTP (Uridine Triphosphate)
II.
Metabolism:
A.
Total of all the organized chemical activity of the cell:
Made possible by:
5
1. Flow of energy through cell:
Release of energy from reduced organic compounds
Use of energy in:
The synthesis of macromolecules
Movement
2. The activities of enzymes
B.
Metabolism has two major parts:
1.
Catabolism:
Reactions by which energy producing molecules are degraded:
Reactions which liberate energy
2.
Anabolism:
Reactions leading to the synthesis of biopolymeres:
Making large molecules from smaller molecules:
Requires energy
Albert Lehninger:
Catabolism may be divided into three stages:
Stage one:
Proteins
Carbohydrates
Lipids
Large nutrient molecules are hydrolyzed to their component
parts:
Amino acids
Monosaccharides
Fatty acids
Glycerol
Hydrolysis does not release much energy
Stage two:
Products of Stage one are degraded to a few simpler
molecules:
Acetyl coenzyme A
Pyruvate
Tricarboxylic acid cycle intermediates
Stage two reactions may occur under aerobic or anaerobic
conditions:
Produces small amounts of:
6
ATP
NADH
Stage three:
Nutrient carbon fed into Tricarboxylic acid cycle:
Oxidized completely to CO2:
Generates much energy:
Produces:
ATP
NADH
FADH2
C.
Catabolism begins with wide variety of molecules:
Number and complexity reduced at each step:
Nutrient molecules  smaller and smaller number of metabolic
intermediates:
Finally fed into the TCA cycle
Nutrients serve two functions in heterotrophic metabolism:
Oxidized to provide energy
Supply carbon or building blocks for synthesis of new cell
constituents
D.
Amphibolic pathways function both:
Catabolically and anabolically
Glycolysis - Most important
TCA cycle
Most reactions in these two pathways are reversible:
Can be used to synthesize and degrade molecules
E.
The break down of Glucose to pyruvate:
Three Major pathways:
1.
Glycolytic (Embden-Myrerhof) Pathway
2.
Pentose Phosphate Pathway (hexose monophosphate pathway)
3.
Entner-Doudoroff:
Converts glucose to pyruvate and glyceraldhyde 3phosphate by producing 6-phosphoglyconate and then
dehydrating it
Found primarily in some Gram-negative bacteria:
7
Pseudomonas
Rhizobium
Agrobacterium
III.
Glycolysis:
A.
Most common biochemical pathway to pyruvate
Found in all major groups of microorganisms
Occurs in the cytoplasm in the presence or absence of oxygen
Two major parts:
Six-carbon stage
Three-carbon stage
B.
Six-carbon stage:
Glucose phosphorylated twice  fructose 1,6 bisphosphate:
Uses 2 ATP molecules:
Does not release energy
C.
Three-carbon stage:
Fructose 1,6 bisphosphate cleaved:
Two 3-carbon molecules result:
Glyceralaldehyde-3-P
Dihydroxyacetone-P
(converted to Glyceralaldehyde-3-P)
These three carbon molecules:
Converted to Pyruvate in five steps:
Glyceralaldehyde-3-P is oxidized:
NAD+ is electron acceptor
bisphosphoglycerate (high energy molecule)
~P on carbon 1 transferred to ADP  ATP:
D.
Substrate level phosphorylation:
ATP synthesized by direct transfer of a ~P to ADP from an intermediate
of a catabolic pathway
8
E.
GLYCOLYTIC PATHWAY
F.
SUMMARY OF GLYCOLYSIS:
4 ATP molecules are synthesized:
2 from each 3 carbon fragment
2 ATP used to phosphorylate glucose subtracted from the total of 4:
 net gain of 2 ATP molecules
Net energy yield from glycolysis small:
Not efficient in retaining energy for cell use:
26% of the energy released is retained:
Rest lost as heat
The two pyruvate molecules still have most of the energy stored in glucose
IV.
The Tricarboxylic Acid (TCA) Cycle:
9
A.
B.
Generalities
In eukaryotic cells:
Occurs in the mitochondria
In prokaryotic cells:
Occurs in the cell membrane
Degrades pyruvate to CO2
Pyruvate Dehydrogenase Complex
Oxidizes pyruvate to CO2 and Acetyl Coenzyme A (acetyl-CoA)
Acetyl-CoA produced by the catabolism of:
Carbohydrates
Lipids
Amino acids
B.
C.
Acetyl-CoA is the substrate of the TCA cycle
TCA Cycle
Summary of the TCA Cycle
Two complete cycles are needed to oxidize the two pyruvate
molecules produced by glycolysis
During each cycle 8 e- & 8 H atoms are removed from the
substrate:
For each glucose molecule the TCA cycle removes 16 e- & 16 H
10
atoms
12 of the e- are transferred to NADH (each NAD accepts 2 e-)
4 e- are transferred to FADH2
(each FAD accepts 2 e-)
6 NADH & 2 FADH2 are produced for each molecule of glucose
that is oxidized during the TCA cycle
Other NADH Production:
2 NADH are produced during the conversion of pyruvate to
acetate
2 NADH are produced during glycolysis
D.
Total production of reduced electron carriers:
NADH:
6 from TCA cycle
2 from oxidation of pyruvate to acetyl-CoA
2 from glycolysis
FADH2:
2 from TCA cycle
V.
Electron Transport and Oxidative Phosphorylation:
A.
Electron Transport Chain:
Composed of a series of e- carriers
Transfer e- from NADH and FADH2 to terminal e- acceptor (O2)
Allows electrons to flow down a chain of electron carrier enzymes of
successively lower energy levels
B.
Electron transport chain carriers located in:
Membranes of mitochondrial cristae in eukaryotic cells
Plasma membrane of prokaryotic cells
Arranged in 4 complexes of carriers:
Each complex capable of transporting e- part of way to O2
Complexes connected by:
Coenzyme Q
Cytochrome C
11
As electrons pass from one carrier to next:
Lose energy:
Some saved in ATP (OXIDATIVE PHOSPHORYLATION)
VI.
ATP Yield from the Aerobic Oxidation of Glucose:
A.
Glycolysis:
Substrate-level phosphorylation:
2 ATP
Oxidative Phosphorylation with 2 NADH:
6 ATP
B.
Two pyruvate molecules converted to 2 Acetyl-Co A molecules:
Oxidative Phosphorylation with 2 NADH
6 ATP
C.
Tricarboxylic Acid Cycle
Substrate-level phosphorylation
2GTP  2 ATP
Oxidative Phosphorylation with 6 NADH
18 ATP
Oxidative Phosphorylation with 2 FADH2
4 ATP
Total Aerobic Yield
38 ATP
12
VII.
FERMENTATION:
A.
Energy yielding process in which organic molecules serve as both electron donors
and acceptors.
During Fermentation:
NADH oxidized to NAD
Pyruvate or a pyruvate derivative acts as the terminal e- acceptor:
End product of the reaction acts as electron acceptor
B.
Alcoholic fermentation:
Many fungi (yeast), some bacteria, algae and protozoa ferment sugars to
CO2 & ethyl alcohol
Ethyl alcohol is terminal electron acceptor
No ATP molecules generated from NADH
Fermentation occurs in the presence or absence of oxygen
Oxygen is not the terminal electron acceptor
13
C.
D.
VII.
Only 2 ATP molecules for each molecule of glucose fermented.
Produced during glycolysis by substrate level phosphorylation
Many different types of Fermentations:
Often characteristic of particular microbial groups
Organisms with different enzymes convert pyruvate to other organic
compounds:
Acetic acid
Lactic acid
Succinic acid
Isopropanol
Formic acid
Butanol
Propionate
Butyrate
Lactic fermentation:
Lactobacillus, Bacillus, Chorella (alga) convert pyruvate to lactic acid:
Lactobacillus:
Responsible for the souring of milk & the production of fermented
milk products
Anaerobic Respiration:
A. Some bacteria use terminal electron acceptors other than oxygen:
Nitrate
Nitrite
Sulfate
Carbonate
B. Oxidation with a terminal electron acceptor other than O2 is anaerobic respiration
Less efficient than aerobic respiration:
Only 2 NADH molecules produced (during glycolysis)
TCA cycle does not occur
C. Biochemistry not well understood:
Many variations:
Actual steps depend on species
D. Electron transport system functions in anaerobic respiration:
But electrons are handed to the terminal electron acceptor earlier than in
aerobic respiration:
The third ATP molecule is not made:
Two ATP molecules produced for each NADH
VIII.
Comparison of fermentation, anaerobic respiration, & aerobic respiration:
14
IX
A.
Fermentation produces 2 ATP molecules
B.
Anaerobic respiration produces about 6 ATP molecules
C.
Aerobic respiration produces 38 ATP molecules
Biosynthetic Pathways:
A.
Catabolic pathways produce:
Reduced electron carriers
ATP
B.
Biosynthesis uses:
Energy saved in ATP to synthesize cell components
Biosynthesis = Anabolism
C.
Extra Cellular Digestion:
Nutrients are not supplied to cells as small easily used molecules:
Usually supplied as macromolecules:
Bacteria cannot ingest solid materials:
Nutrients must be in a soluble form:
Large molecules must be broken down outside the cell
Extra cellular digestion:
Extra cellular enzymes:
Produced in the cell:
Secreted from the cell
Act outside of the cell
Hydrolyze macromolecules outside the cell:
H2O added to complex molecule: Breaks it
into its simpler components:
Small enough to enter the cell:
Polysaccharides  6 Carbon Sugars
Proteins  Amino Acids
Lipids  Fatty Acids
Inside the cell smaller molecules enter various metabolic pathways
D.
Carbohydrate anabolism:
For many microorganisms:
Hexose sugars serve as the:
Primary energy source
15
Primary carbon source
Glucose most important 6 carbon sugar that enter the cell:
Other 6 C sugars:
Often converted to glucose before being metabolized
Hexose sugars often complexed as polysaccharides:
Starch
Glycogen
Pectin
Cellulose
Agar
Must be broken down outside the cell
E.
As glucose enters the cell:
Phosphorylated:
Glucose-6-phosphate  glycolysis Pyruvate
Pyruvate feeds into:
Aerobic respiration
or
Anaerobic respiration
or
Fermentation
F.
G.
X.
Excess glucose may be converted to polysaccharide:
Stored by the cell
Used to construct cell structures
Uridine Diphosphoglucose (UDP-glucose): Key metabolic intermediate in the
synthesis of:
Polysaccharides within the cell
Heteropolysaccharides:
Make up:
Cell wall
Capsule
Slime layers
Amino Acids
A. Amino Acid Synthesis:
20 found in proteins
B. Essential amino acids:
Cannot be made by the organism:
16
Must be supplied by the environment:
Escherichia coli:
Makes all 20 from ammonia
Leuconostoc mesenteroids:
Makes 4
C. Ability to synthesize amino acids depends on cell's genetic information:
Enzymes present in cell determined by its genes
D.
E.
Amino acid synthesis requires reduced nitrogen source:
Ammonia
Nitrite
Nitrate
Amino group (NH2) is added to intermediates from:
Glycolysis
Krebs cycle
Amino acid catabolism:
Amino acids may be:
Used as energy sources:
Amino group removed:
Amino acid converted to organic acid
Organic acid oxidized via:
Glycolysis
Krebs cycle
Carboxyl group removed:
Amino acid synthesis and amino acid catabolism:
Closely linked to other metabolic pathways