METABOLIC PATHWAYS
Section 6
SOME BASIC CONCEPTS OF METABOLISM
ATP
ADP
adenosine triphosphate
adenosine diphosphate
adenine - a carbon ring structure
ribose - five carbon sugar
phosphate - an energy rich structure
ATP + H2O ⇔ ADP + Pi + 7 kcal/mole
NAD nictinamide adenine dinucleotide
Lactic acid + NAD ⇔ Pyruvic acid + NADH2
NAD molecules acts as a temporary H+ carrier
NADH2 + 1/2 O2 ⇔ NAD + H2O + 52 kcal/mole
If 52 kcal were released at once, then 7 kcal (ADP ATP) + 45 kcal of heat would be wasted.
Instead, this reaction is slowed down so that 3 ATP molecules are formed: 52 - 21 = 3 ATP
molecules + 31 kcal heat, efficiency is 40%.
If O2 is not available to the cytochrome system, ATP will not be formed by the mitochondria.
FAD flavine adenine dinucleotide is similar to NAD
I. Carbohydrate metabolism
A. Primary function of carbohydrates in the body is to provide a readily available source of
energy that can be coupled to the synthesis of ATP
1. major carbohydrate in the body is glucose
a. glucose is metabolized with oxygen to CO2 and H2O
b. C6H12O6 + 6O2 ⇔ 6H2O + 6CO2 + 686 kcal
c. 38 molecules of ATP are formed from one molecule of glucose, the remainder is
lost as heat
2. the breakdown of glucose to form ATP molecules is in two parts
a. glycolysis - glucose is converted to pyruvic acid
b. Kreb's cycle - pyruvic acid to ATP, CO2, and water
B. Glycolysis is a series of enzyme-mediated reactions changing one molecule of glucose to
two molecules of either pyruvic or lactic acid.
1. absorption of glucose
a. glucose will not pass through the cell membrane without pancreatic insulin present
b. once inside the cell, glucose passes through intermediate stages and becomes an
ionized phosphate group
c. once glucose has been phosphorylated the intermediates leak back across the
membrane
2. reactions of glycolysis
a. the first reaction is the transfer of phosphate from ATP to glucose to form
glucose 6-phosphate
b. a second molecule of ATP transfers a phosphate to fructose 6-phosphate
c. fructose 1, 6-phosphate (six carbon molecule) is split into 3 phosphoglyceraldehyde
and dihydroxyacetone (three carbon molecules)
Concepts of Metabolism
d. dihydroxyacetone phosphate converts to 3 phospho-glyceraldehyde with an overall
result to convert to two identical three carbon molecules
e. 1, 3 diphosphoglycerate forms 2 phosphoglyceric acid and transfers a phosphate
radical to ADP forming ATP
f. phosphoenolpruvate form pyruvic acid and forms another ATP molecule
3. aerobic and anerobic metabolism
a. three energy rich ATP molecules can be formed from each NADH2 formed
(6/molecule of glucose) oxidative phosphorlation leads to the transfer of two
hydrogen atoms to oxygen and the release of NAD which can reenter the reactions
b. in the absence of O2, NADH2 cannot be converted to NAD
c. NADH2 can donate 2H+ to pyruvic acid and convert to NAD, pyruvic acid is
converted to lactic acid (anerobically)
d. anerobic metabolism produces only 2 ATP molecules and lactic acid
C. The second part of glucose degradation involves the conversion of pyruvic acid to CO2 and
water using O2 to form ATP
1. this enzyme-mediated reaction occurs in the mitochondria
2. pathways D. Summary
1. 2 molecules of ATP by glycolysis (4-2)
2. 36 molecules of ATP are formed by oxidative phosphorlation
3. 6 molecules of O2 are consumed/molecule of glucose
6 molecules of CO2 are produced/molecule of glucose
6 molecules of H2O are produced/molecule of glucose
4. 686 kcal of energy are produced from each Mole of glucose
a. 266 kcal are used to form 38 ATP molecules
b. 420 kcal are liberated as heat
5. glucose is either broken down to ATP or stored
a. small amounts are stored as glycogen in muscles and liver
b. the rest of what is not used may be converted to fat
II. Lipid metabolism
A. Neutral fats
1. consist of three fatty acid chains and a glycerol 2. metabolism requires the splitting of the three fatty acids from glycerol
3. glycerol forms dihydroxyacetone phosphate and enters the carbohydrate pathway in
glycolysis
4. fatty acids enter the system through a series of reactions and energy transfers as
Acetyl CoA
B. Energy production
1. each molecule of glycerol forms 22 ATP molecules
2. each of the 18 carbon fatty acids form 147 ATP molecules
a. 3 fatty acid chains form 441 ATP molecules
b. one molecule of fat forms 463 ATP molecules
3. enzymes for fatty acids are located in the cytoplasm rather than the mitochondria of the
cell
4. reverse the direction of the reaction - formation of fat
III. Protein metabolism
A. Proteins must be broken down to amino acids before they can be used to form ATP
Molecules
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Concepts of Metabolism
B. Amino acids contain nitrogen in addition to carbon, hydrogen, and oxygen
1. once nitrogen is removed, the remainder of the intermediates of the molecules can enter
the pathways of glucose metabolism
2. nitrogen removal
a. oxidative deamination b. transamiantion c. only two examples given, the other 18 amino acids are formed into intermediates
that can enter the cycle at some point
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