Metabolism and Nutrition

1
Chapter 25

• Metabolism and Nutrition
• The food we eat is our only source of energy for
  performing biological work.
• There are three major metabolic destinations for
  the principle nutrients. They will be used for
  energy for active processes, synthesized into
  structural or functional molecules, or
  synthesized as fat or glycogen for later use as
  energy.

2
Glucose Catabolism
3
Coenzymes

• Two coenzymes are commonly used by living cells
  to carry hydrogen atoms nicotinamide adenine
  dinucleotide (NAD) and flavin adenine
  dinucleotide (FAD).
• An important point to remember about
  oxidation-reduction reactions is that oxidation
  is usually an energy-releasing reaction.

4
Mechanisms of ATP Generation
ADP P ATP

• Phosphorylation is
• bond attaching 3rd phosphate group contains
  stored energy
• Mechanisms of phosphorylation
• within animals
• substrate-level phosphorylation in cytosol
• oxidative phosphorylation in mitochondria
• in chlorophyll-containing plants or bacteria
• photophosphorylation.

5
Phosphorylation in Animal Cells

• In cytoplasm (1)
• In mitochondria (2, 3 4)

6
Carbohydrate Review

• In GI tract
• polysaccharides broken down into simple sugars
• absorption of simple sugars (glucose, fructose
  galactose)
• In liver
• fructose galactose transformed into glucose
• storage of glycogen (also in muscle)
• In body cells --functions of glucose
• oxidized to produce energy
• conversion into something else
• storage energy as triglyceride in fat

7
Fate of Glucose

• Glucose can be used to form amino acids, which
  then can be incorporated into proteins.
• Excess glucose can be stored by the liver and
  skeletal muscles as glycogen, a process called
  glycogenesis.
• If glycogen storage areas are filled up, liver
  cells and fat cells can convert glucose to
  glycerol and fatty acids that can be used for
  synthesis of triglycerides (neutral fats) in the
  process of lipogenesis.

8
Glucose Movement into Cells

• In GI tract and kidney tubules
• Na/glucose symporters
• Most other cells
• GluT facilitated diffusion transporters
• insulin increases the insertion of GluT
  transporters in the membrane of most cells
• in liver brain, always lots of GluT
  transporters
• Glucose 6-phosphate forms immediately inside cell
  (requires ATP) thus, glucose is hidden when it
  is in the cell.
• Concentration gradient remains favorable for more
  glucose to enter.

9
Glucose Catabolism
10
Glucose Oxidation

• Cellular respiration
• 4 steps are involved
• glucose O2 producesH2O energy CO2
• Anaerobic respiration
• called glycolysis (1)
• formation of acetyl CoA (2)is transitional step
  to Krebs cycle
• Aerobic respiration
• Krebs cycle (3) and electron transport chain (4)

11
Glycolysis

• Glycolysis refers to the breakdown of the
  six-carbon molecule, glucose, into two
  three-carbon molecules of pyruvic acid.
• 10 step process occurring in cell cytosol
• use two ATP molecules, but produce four, a net
  gain of two (Figure 25.3).

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Glycolysis of Glucose Fate of Pyruvic Acid

• Breakdown of six-carbon glucose molecule into 2
  three-carbon molecules of pyruvic acid
• Pyruvic acid is converted to acetylCoA, which
  enters the Krebs Cycle.
• The Krebs Cycle will require NAD
• NAD will be reduced to the high-energy
  intermediate NADH.

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Glycolysis of Glucose Fate of Pyruvic Acid

• When O2 falls short in a cell
• pyruvic acid is reduced to lactic acid
• coupled to oxidation of NADH to NAD
• NAD is then available for further glycolysis
• lactic acid rapidly diffuses out of cell to blood
• liver cells remove lactic acid from blood
  convert it back to pyruvic acid

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Pyruvic Acid

• The fate of pyruvic acid depends on the
  availability of O2.

15
Formation of Acetyl Coenzyme A

• Pyruvic acid enters the mitochondria with help
  of transporter protein
• Decarboxylation
• pyruvate dehydrogenase converts 3 carbon pyruvic
  acid to 2 carbon fragment acetyl group plus CO2.

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Formation of Acetyl Coenzyme A

• 2 carbon fragment (acetyl group) is attached to
  Coenzyme A to form Acetyl coenzyme A, which enter
  Krebs cycle
• coenzyme A is derived from pantothenic acid (B
  vitamin).

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Krebs Cycle

• The Krebs cycle is also called the citric acid
  cycle, or the tricarboxylic acid (TCA) cycle. It
  is a series of biochemical reactions that occur
  in the matrix of mitochondria (Figure 25.6).

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Krebs Cycle
19
Krebs Cycle

• The large amount of chemical potential energy
  stored in intermediate substances derived from
  pyruvic acid is released step by step.
• The Krebs cycle involves decarboxylations and
  oxidations and reductions of various organic
  acids.
• For every two molecules of acetyl CoA that enter
  the Krebs cycle, 6 NADH, 6 H, and 2 FADH2 are
  produced by oxidation-reduction reactions, and
  two molecules of ATP are generated by
  substrate-level phosphorylation (Figure 25.6).
• The energy originally in glucose and then pyruvic
  acid is primarily in the reduced coenzymes NADH
  H and FADH2.

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Krebs Cycle (Citric Acid Cycle)

• The oxidation-reduction decarboxylation
  reactions occur in matrix of mitochondria.
• acetyl CoA (2C) enters at top combines with a
  4C compound
• 2 decarboxylation reactions peel 2 carbons off
  again when CO2 is formed

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Krebs Cycle

• Potential energy (of chemical bonds) is released
  step by step to reduce the coenzymes (NAD?NADH
  FAD?FADH2) that store the energy
• Review
• Glucose? 2 acetyl CoA molecules
• each Acetyl CoAmolecule that enters the
  Krebscycle produces
• 2 molecules of C02
• 3 molecules of NADH H
• one molecule of ATP
• one molecule of FADH2

22
Electron Transport Chain

• The electron transport chain involves a sequence
  of electron carrier molecules on the inner
  mitochondrial membrane, capable of a series of
  oxidation-reduction reactions.
• As electrons are passed through the chain, there
  is a stepwise release of energy from the
  electrons for the generation of ATP.
• In aerobic cellular respiration, the last
  electron receptor of the chain is molecular
  oxygen (O2). This final oxidation is
  irreversible.
• The process involves a series of
  oxidation-reduction reactions in which the energy
  in NADH H and FADH2 is liberated and
  transferred to ATP for storage.

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Electron Transport Chain

• Pumping of hydrogen is linked to the movement of
  electrons passage along the electron transport
  chain.
• It is called chemiosmosis (Figure 25.8.)
• Note location.

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Chemiosmosis

• H ions are pumped from matrix into space between
  inner outer membrane
• High concentration of H is maintained outside of
  inner membrane
• ATP synthesis occurs as H diffuses through a
  special H channels in the inner membrane

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Electron Transport Chain
26
Steps in Electron Transport

• Carriers of electron transport chain are
  clustered into 3 complexes that each act as a
  proton pump (expelling H)
• Mobile shuttles (CoQ and Cyt c) pass electrons
  between complexes.
• The last complex passes its electrons (2H) to
  oxygen to form a water molecule (H2O)

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Proton Motive Force Chemiosmosis

• Buildup of H outside the inner membrane creates
  charge
• The potential energy of the electrochemical
  gradient is called the proton motive force.
• ATP synthase enzymes within H channels use the
  proton motive force to synthesize ATP from ADP
  and P

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Summary of Aerobic Cellular Respiration

• The complete oxidation of glucose can be
  represented as follows
• C6H12O6 6O2 gt 36 or 38ATP 6CO2 6H2O
• During aerobic respiration, 36 or 38 ATPs can be
  generated from one molecule of glucose.
• Two of those ATPs come from substrate-level
  phosphorylation in glycolysis.
• Two come from substrate-level phosphorylation in
  the Krebs cycle.

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Review

• Table 25.1 summarizes the ATP yield during
  aerobic respiration.
• Figure 25.8 summarizes the sites of the principal
  events of the various stages of cellular
  respiration.

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Glycogenesis Glycogenolysis

• Glycogenesis
• glucose storage as glycogen
• 4 steps to glycogenformation in liver
  orskeletal muscle
• stimulated by insulin
• Glycogenolysis
• glucose release

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Glycogenesis Glycogenolysis

• Glycogenesis
• glucose storage as glycogen
• Glycogenolysis
• glucose release
• not a simple reversal of steps
• Phosphorylase enzyme is activated by glucagon
  (pancreas) epinephrine (adrenal gland)
• Glucose-6-phosphatase enzyme is only in
  hepatocytes so muscle can not release glucose
  into the serum.

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Gluconeogenesis

• Gluconeogenesis is the conversion of protein or
  fat molecules into glucose (Figure 25.12).

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Gluconeogenesis

• Glycerol (from fats) may be converted to
  glyceraldehyde-3-phosphate and some amino acids
  may be converted to pyruvic acid. Both of these
  compounds may enter the Krebs cycle to provide
  energy.
• Gluconeogenesis is stimulated by cortisol,
  thyroid hormone, epinephrine, glucagon, and human
  growth hormone.