Biochemistry lecture notes metabolism_glycolysis, pentose phosphate pathway

1. REGULATION OF METABOLIC
PATHWAYS
Dr.B.RENGESH | M.Tech., Ph.D.
Associate Professor, Department of Pharmaceutical Technology,
Mahendra Engineering College (Autonomous),
Namakkal District, Tamil Nadu, India

2. Metabolism
• Metabolism is the sum of all the chemical transformations taking place in a
cell or organism, occurs through a series of enzyme-catalyzed reactions that
constitute metabolic pathways.
• Each of the consecutive steps in a metabolic pathway brings about a specific,
small chemical change, usually the removal, transfer, or addition of a
particular atom or functional group. The precursor is converted into a product
through a series of metabolic intermediates called metabolites.
• The term intermediary metabolism is often applied to the combined activities
of all the metabolic pathways that interconvert precursors, metabolites, and
products of low molecular weight

3. Catabolism
• Catabolism is the degradative phase of metabolism in which organic nutrient
molecules (carbohydrates, fats, and proteins) are converted into smaller, simpler end
products (such as lactic acid, CO2, NH3).
• Catabolic pathways release energy, some of which is conserved in the formation of
ATP and reduced electron carriers (NADH, NADPH, and FADH2); the rest is lost as
heat.
Anabolism
• In anabolism, also called biosynthesis, small, simple precursors are built up into
larger and more complex molecules, including lipids, polysaccharides, proteins, and
nucleic acids.
• Anabolic reactions require an input of energy, generally in the form of the
phosphoryl group transfer potential of ATP and the reducing power of NADH,
NADPH, and FADH2.

4. Energy relationships between catabolic and
anabolic pathways:
Catabolic pathways deliver chemical energy in
the form of ATP, NADH, NADPH, and FADH2.
These energy carriers are used in anabolic
pathways to convert small precursor molecules
into cell macromolecules.

5. Glycolysis
• In glycolysis (from the Greek glykys, meaning “sweet,” and lysis, meaning “splitting”), a
molecule of glucose is degraded in a series of enzyme-catalyzed reactions to yield two
molecules of the three-carbon compound pyruvate.
• During the sequential reactions of glycolysis, some of the free energy released from
glucose is conserved in the form of ATP and NADH.
• The breakdown of the six-carbon glucose into two molecules of the three-carbon pyruvate
occurs in ten steps, the first five of which constitute the preparatory phase.

6. Glycolysis
a) two molecules of glyceraldehyde 3-phosphate are
formed; both pass through the payoff phase
b) Pyruvate is the end product of the second phase of
glycolysis.
For each glucose molecule, two ATP are consumed in the preparatory phase and four ATP are produced in the payoff
phase, giving a net yield of two ATP per molecule of glucose converted to pyruvate.

7. Glycolysis
• The first five steps constitute the preparatory phase (Fig. a). Glucose is first
phosphorylated at the hydroxyl group on C-6 (step 1).
• The D-glucose 6-phosphate thus formed is converted to D-fructose 6-phosphate (step 2),
which is again phosphorylated, this time at C-1, to yield D-fructose 1,6-bisphosphate (step
3).
• For both phosphorylations, ATP is the phosphoryl group donor.
• Fructose 1,6-bisphosphate is split to yield two three-carbon molecules, dihydroxyacetone
phosphate and glyceraldehyde 3-phosphate (step 4); this is the “lysis” step that gives the
pathway its name.
• The dihydroxyacetone phosphate is isomerized to a second molecule of glyceraldehyde 3-
phosphate (step 5), ending the first phase of glycolysis.
• To summarize: in the preparatory phase of glycolysis the energy of ATP is invested, raising
the free-energy content of the intermediates, and the carbon chains of all the metabolized
hexoses are converted into a common product, glyceraldehyde 3-phosphate.

8. Glycolysis
• The energy gain comes in the payoff phase of glycolysis (Fig. b). Each molecule of
glyceraldehyde 3-phosphate is oxidized and phosphorylated by inorganic phosphate (not by
ATP) to form 1,3-bisphosphoglycerate (step 6).
• Energy is then released as the two molecules of 1,3-bisphosphoglycerate are converted to
• two molecules of pyruvate (steps 7 through 10). Much of this energy is conserved by the
coupled phosphorylation of four molecules of ADP to ATP.
• The net yield is two molecules of ATP per molecule of glucose used, because two
molecules of ATP were invested in the preparatory phase. Energy is also conserved in the
payoff phase in the formation of two molecules of NADH per molecule of glucose.
• In the sequential reactions of glycolysis, three types of chemical transformations are
particularly noteworthy: (1) degradation of the carbon skeleton of glucose to yield
pyruvate, (2) phosphorylation of ADP to ATP by high-energy phosphate compounds
formed during glycolysis, and (3) transfer of a hydride ion to NAD+, forming NADH.

9. Glycolysis – Biological Importance
1. Energy production:
Ø anaerobic glycolysis gives 2 ATP.
Ø aerobic glycolysis gives 8 ATP.
2. Oxygenation of tissues: Through formation of 2,3 bisphosphoglycerate, which decreases
the affinity of Hemoglobin to O2.
3. Provides important intermediates:
Ø Dihydroxyacetone phosphate: can give glycerol-3phosphate, which is used for
synthesis of triacylglycerols and phospholipids (lipogenesis).
Ø 3 Phosphoglycerate: which can be used for synthesis of amino acid serine.
Ø Pyruvate: which can be used in synthesis of amino acid alanine.
4. Aerobic glycolysis provides the mitochondria with pyruvate, which gives acetyl CoA
Krebs' cycle.

10. Glycolysis – Hexoses catabolism
1. Other hexoses are also oxidized via the glycolysis pathway. They are also first primed by
phosphorylation (at C-1 or C-6).
2. Fructose is primed and cleaved to form dihydroxyacetone phosphate and glyceraldehyde,
which are further converted to glyceraldehyde 3-P.
3. Galactose is first converted to Glc-1-P via a UDP-galactose intermediate and UDP-glucose
intermediate, then to Glc-6-P.
4. Dietary poly- and disaccharides are hydrolyzed to monosaccharides in the digestive system.
Specific enzymes (e.g., lactase, sucrase, maltase,etc.) on the microvilli of the intestinal
epithelial cells finally hydrolyze all disaccharides into monosaccharides. The
monosacchrides are then absorbed at the intestinal microvilli and transported to various
tissues for oxidative degradation via the glycolytic pathway.

11. Glycolysis – Hexoses catabolism

12. Glycolysis – The Fate of Pyruvate
• The sequence of reactions that convert glucose to pyruvate is similar in all organisms.
However, the fate of the pyruvate as it is used to generate energy is variable.
• As the process occurs, NAD+ is reduced to NADH. The need for a continuous supply of
NAD+ for glycolysis is a key to understanding the fates of pyruvate.
– In each case, pyruvate is metabolized so as to regenerate NAD+, allowing glycolysis to
continue.

13. Glycolysis – The Fate of Pyruvate
• There are three things that can happen to pyruvate after glycolysis:
– oxidation to acetyl CoA under aerobic conditions
– reduction to lactate under anaerobic conditions
– reduction to ethanol under anaerobic conditions for some prokaryotic organisms

14. • Under aerobic conditions (a plentiful supply of oxygen), pyruvate is oxidized in the
mitochondria to form acetyl CoA:
– Most of the acetyl CoA formed can enter the citric acid cycle on its way to complete
oxidation to CO2.
– Some acetyl CoA serves as a starting material for fatty acid biosynthesis.
• NAD+ is regenerated when NADH transfers its electrons to O2 in the electron transport
chain
Glycolysis – The Fate of Pyruvate – Oxidation to Acetyl CoA

15. • Under anaerobic conditions (restricted O2 supply), such as those that accompany strenuous
or long-term muscle activity, the cellular supply of oxygen is not adequate for the
reoxidation of NADH to NAD+.
• Under these conditions, the cells begin reducing pyruvate to lactate as a means of
regenerating NAD+:
• Adding this equation to the net results of glycolysis produces the equation for lactate
fermentation:
Glycolysis – The Fate of Pyruvate – Reduction to Lactate

16. • This reaction does not produce as much energy as the complete oxidation of pyruvate under
aerobic conditions, but the two ATPs produced from lactate fermentation are sufficient to
sustain the life of anaerobic microorganisms.
– In human metabolism, those two ATPs play a critical role by furnishing energy when
cellular supplies of oxygen are insufficient for complete oxidation of pyruvate.
– During vigorous exercise, there is a shift to lactate production as a means for producing
ATP; the buildup of lactate in the muscles causes muscle pain and cramps, and causes a
slight decrease in blood pH, triggering an increase in the rate and depth of breathing,
providing more oxygen to the cells.
Glycolysis – The Fate of Pyruvate – Reduction to Lactate

17. • Several organisms, including yeast, regenerate NAD+ under anaerobic conditions by
alcoholic fermentation, by decarboxylation (removing CO2) of pyruvate to produce
acetaldehyde:
Glycolysis – The Fate of Pyruvate – Reduction to Ethanol
– The CO2 thus produced causes beer to foam and wine and champagnes to bubble.
• Acetaldehyde is then reduced by NADH to form ethanol (also regenerating NAD+ for
glycolysis):
• Combining the reaction for glycolysis with the reactions for reduction to ethanol gives the
following overall reaction:

18. The pentose phosphate pathway is an alternate route for the oxidation of glucose.
The pentose phosphate pathway has two main functions.
1. Generation of NADPH
a. mainly used for reductive syntheses of fatty acids, steroids, amino acids via
glutamate dehydrogenase; and production of reduced glutathione in
erythrocytes and other cells.
b. active in liver, adipose tissue, adrenal cortex, thyroid, erythrocytes, testes,
and lactating mammary gland
c. not active in non-lactating mammary gland and has low activity in skeletal
muscle.
2. Production of ribose residues for nucleotide and nucleic acid synthesis.
PENTOSE PHOSPHATE PATHWAY

19. Reactions of the pentose phosphate pathway occur in the cytosol in two phases:
Oxidative non-reversible phase →
← Non-oxidative reversible phase
PENTOSE PHOSPHATE PATHWAY

20. • The entry of glucose 6-phosphate into the pentose phosphate pathway is
controlled by the cellular concentration of NADPH
• NADPH is a strong inhibitor of glucose 6-phosphate dehydrogenase
• As NADPH is used in various pathways, inhibition is relieved, and the
enzyme is accelerated to produce more NADPH
• The synthesis of glucose 6-phosphate dehydrogenase is induced by the
increased insulin/glucagon ratio after a high carbohydrate meal.
Regulation of Pentose phosphate pathway