Glycolysis is the breakdown of glucose into pyruvate with simultaneous ATP production. It occurs in the cytoplasm and involves 10 enzyme-catalyzed reactions split into two phases. The process yields ATP both with and without oxygen present. In the presence of oxygen, pyruvate undergoes further processing in the citric acid cycle and electron transport chain to fully oxidize glucose and generate more ATP. In anaerobic conditions, pyruvate is converted to lactic acid or ethanol. Cellular respiration consists of three stages: pyruvate processing in the citric acid cycle, electron transport across protein complexes in the mitochondria, and ATP synthesis through oxidative phosphorylation.
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Carbohydrate metabolism
1. Glycolysis
Glycolysis (glycos = sugar (sweet); lysis =
dissolution) is the breakdown of glucose into
pyruvate with the simultaneous production of
ATP.
• Takes place in the extramitochondrial part of the
cell (or the soluble cytoplasm).
•Also referred as Embden-Meyerh-Parnas or EMP
pathway
•Occurs in absence or presence of Oxygen
2. • During glycolysis, the 6-carbon glucose is
broken down into two moles of 3-carbon
pyruvate via 10 enzyme-catalyzed sequential
reactions. These reactions are grouped under 2
phases: phase I (preparatory) and II (pay off)
10. THREE STAGES OF CELL
RESPIRATION
• First stage: Oxidative decarboxylation of
pyruvate to acetyl CoA and CO2
• Second stage: Citric acid cycle or Acetyl CoA
catabolism
• Third stage: Electron transport and oxidative
phosphorylation
16. ELECTRON TRANSPORT CHAIN
• Complexes I and II catalyze electron
transfer to ubiquinone from two different
electron donors: NADH (Complex I) and
succinate (Complex II). Complex III carries
electrons from reduced ubiquinone to
cytochrome c, and Complex IV completes the
sequence by transferring electrons from
cytochrome c to O2.
17. Complex I, also called
NADH:ubiquinone oxidoreductase
NADH dehydrogenase, is
a large enzyme
composed of 42
different polypeptide
chains, including an
FMN-containing
flavoprotein and at least
six ironsulfur centers.
18. Complex II, Succinate dehydrogenase
• The only membrane-bound enzyme in the citric acid cycle
• It contains five prosthetic groups of two types and four
different protein subunits
• Subunits C and D are integral membrane proteins, each with
three transmembrane helices. They contain a heme group,
heme b, and a binding site for ubiquinone, the final electron
acceptor in the reaction catalyzed by Complex II. Subunits
A and B extend into the matrix (or the cytosol of a
bacterium); they contain three 2Fe-2S centers, bound FAD,
and a binding site for the substrate, succinate. The path of
electron transfer from the succinate-binding site to FAD,
then through the Fe-S centers to the Q-binding site
19. Complex III, also called cytochrome
bc1 complex or ubiquinone:cytochrome c
oxidoreductase
Couples the transfer of electrons from
ubiquinol (QH2) to cytochrome c with the
vectorial transport of protons from the
matrix to the intermembrane space.
20.
21.
22. Complex IV, also called cytochrome
oxidase
• Carries electrons from cytochrome c to
molecular oxygen, reducing it to H2O.
• Complex IV is a large enzyme (13 subunits; Mr
204,000) of the inner mitochondrial
membrane.
23. Mitochondrial subunit II contains two Cu
ions complexed with the -SH groups of two
Cys residues in a binuclear center that
resembles the 2Fe-2S centers of iron-sulfur
proteins.
Subunit I contains two heme groups,
designated a and a3, and another copper ion
(CuB). Heme a3 and CuB form a second
binuclear center that accepts electrons from
heme a and transfers them to O2 bound to
heme a3.
29. Significance of HMP shunt
• HMP shunt is unique in generating two important
products-pentoses and NADPH needed for the
biosynthetic reactions and other functions.
• lmportance of pentoses
• In the HMP shunt,hexoses are converted into
pentoses, the most important being ribose 5-
phosphate. This pentose or its derivatives are
useful for the synthesis of nucleic acids (RNA and
DNA) and many nucleotides such as ATP, NAD+,
FAD and CoA.
30. Importance of NADPH
• Biosynthesis of fatty acids and steroid
• NADPH is used in the synthesis of certain
amino acids involving the enzyme glutamate
dehydrogenase.
• Special functions of NADPH in RBC :
• NADPH produced in erythrocytes has special
functions to Derform. lt maintains the
concentration of reduced glutathione which is
essential to preserve the integrity of RBC
membrane.
Editor's Notes
When animal tissues cannot be supplied with sufficient oxygen to support aerobic oxidation of the pyruvate and NADH produced in glycolysis, NAD1 is regenerated from NADH by the reduction of pyruvate to lactate. As mentioned earlier, some tissues and cell types (such as erythrocytes, which have no mitochondria and thus cannot oxidize pyruvate to CO2) produce lactate from glucose even under aerobic conditions.
Yeast and other microorganisms ferment glucose to ethanol and CO2, rather than to lactate. Glucose is converted to pyruvate by glycolysis, and the pyruvate is converted to ethanol and CO2 in a two-step process: In the first step, pyruvate is decarboxylated in an irreversible reaction catalyzed by pyruvate decarboxylase. This reaction is a simple decarboxylation and does not involve the net oxidation of pyruvate. Pyruvate decarboxylase requires Mg21 and has a tightly bound coenzyme, thiamine pyrophosphate, which is discussed below. In the second step, acetaldehyde is reduced to ethanol through the action of alcohol dehydrogenase, with the reducing power furnished by NADH derived from the dehydrogenation of glyceraldehyde 3-phosphate.
