biochemistry methabolism ofcarbohydrates

1. METABOLISM AND
RADOX COENZYME
LECTURE 1

2. Metabolism
• Metabolism is a highly coordinated cellular activity in which many multi enzyme
systems (metabolic pathways) cooperate to:
1. Obtain chemical energy by capturing solar energy or degrading energy-rich
nutrients from the environment
2. Convert nutrient molecules into the cell’s own characteristic molecules,
including precursors of macromolecules
3. Polymerize monomeric precursors into macromolecules: proteins, nucleic
acids, and polysaccharides
4. Synthesize and degrade biomolecules required for specialized cellular
functions, such as membrane lipids, intracellular messengers, and pigments.
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3. Metabolism
• Metabolism is the sum of all the chemical transformations taking place in a cell
or organism which occurs through a series of enzyme catalyzed reactions that
constitute metabolic pathways.
• 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, and 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
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4. Metabolism
• In anabolism or 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.
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6. ATP- the Universal Currency of Free Energy
• Just as commerce is facilitated by the use of a common currency, the commerce
of the cell metabolism is facilitated by the use of a common energy currency,
adenosine triphosphate (ATP).
• Part of the free energy derived from the oxidation of foodstuffs and from light is
transformed into this highly accessible molecule, which acts as the free-energy
donor in most energy-requiring processes such as motion, active transport, or
biosynthesis.
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7. ATP- the Universal Currency of Free Energy
• ATP is a nucleotide consisting of an adenine, a ribose, and a triphosphate unit
• The active form of ATP is usually a complex of ATP with Mg2+ or Mn2+
• ATP is an energy rich molecule because its triphosphate unit contains two
phosphor anhydride bonds.
• A large amount of free energy(31 kj per mole) is liberated when ATP is hydrolyzed
to adenosine diphosphate (ADP) and orthophosphate (Pi) or when ATP is
hydrolyzed to adenosine monophosphate (AMP) and pyrophosphate (PPi).
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8. Hydrolysis of ATP to ADP and ADP to AMP
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9. Oxidation-reduction reactions
• In living organisms, both energy-capturing
and energy-releasing processes consist
largely of redox reactions
• Redox reactions occur when electrons are
transferred between an electron donor (a
reducing agent) and an electron acceptor
(an oxidizing agent)
• In some redox reactions, only electrons
are transferred
• For example, in the reaction
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10. Oxidation-reduction reactions
• In many reactions, both electrons and
protons are transferred
• For example, the reaction catalyzed by
lactate dehydrogenase begins with the
transfer of a hydride ion (H:−), that is, a
hydrogen nucleus and two electrons,
from NADH to pyruvate
• A proton (H+) is gained from the
environment to form the final products
lactate and NAD+.
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11. Redox Coenzymes
• The coenzyme forms of the vitamin molecules nicotinic
acid and riboflavin are universal electron carriers
• NICOTINIC ACID : two coenzyme forms: NAD and NADP
• May occur in oxidized forms (NAD+ and NADP+) and
reduced forms (NADH and NADPH)
• The structures of NAD+ and NADP+ both contain
adenosine and the N-ribosyl derivative of nicotinamide
(derived from the vitamin niacin), which are linked
together through a pyrophosphate group
• NADP+ has an additional phosphate attached to the 2′
OH group of adenosine
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12. Function
• Both NAD+ and NADP+ carry electrons for several enzymes in a group known as
the dehydrogenases- (Dehydrogenases catalyze hydride transfer reactions)
• Many dehydrogenases that catalyze reactions involved in energy generation use
the coenzyme NADH
• The enzymes that require NADPH usually catalyze biosynthetic reactions (e.g.,
fatty acid synthesis)
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13. Redox Coenzymes
• RIBOFLAVIN (vitamin B2) is a component of two coenzymes: flavin
mononucleotide (FMN) and flavin adenine dinucleotide (FAD)
• FMN and FAD function as tightly bound prosthetic groups in a class of enzymes
known as the flavoproteins
• Flavoproteins are a diverse group of redox enzymes; they function as
dehydrogenases, oxidases, and hydroxylases
• FMN plays a key role in the link between two-electron transfer reactions in the
mitochondrial matrix and the one-electron transfer reactions of the electron
transport chain because it can transfer one hydrogen atom at a time
• Succinate dehydrogenase is a prominent example of a flavoprotein
• It catalyzes the oxidation of succinate by FAD to form fumarate and FADH2, an
important reaction in the citric acid cycle.
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14. FAD and FMN
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15. Coenzyme A
• In coenzyme A, a 3′-phosphate derivative of ADP is linked to pantothenic acid via
a phosphate ester bond
• The β-mercaptoethylamine group of coenzyme A is attached to pantothenic acid
by an amide bond
• Coenzyme A is a carrier of acyl groups that range in size from the acetyl group
to long-chain fatty acids
• Because the reactive SH group forms a thioester bond with acyl groups,
coenzyme A is often abbreviated as CoASH
• Note that sulfur is a better leaving group than oxygen
• Consequently, the carbon–sulfur linkage of a thioester is a high-energy bond that
is more easily cleaved than the carbon–oxygen bond of an ester
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16. Coenzyme A
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