Protein-Metabolism.pptx

Protein Digestion and Absorption
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Section 26.1
Copyright ©2016 Cengage Learning. All Rights Reserved. 1

Chapter 26
Chapter Outline
26.1 Protein digestion and absorption
26.2 Amino acid utilization
26.3 Transamination and oxidative deamination
26.4 The urea cycle
26.5 Amino acid carbon skeletons
26.6 Amino acid biosynthesis
26.7 Hemoglobin catabolism
26.8 Proteins and the element sulfur
26.9 Interrelationships among metabolic pathways
26.10 B vitamins and protein metabolism

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Section 26.1
• Protein digestion starts in the stomach
– Dietary protein present in the stomach stimulates the
release of gastrin
• Gastrin promotes secretion of pepsinogen and HCl
– HCl in the stomach has 3 functions
• Antiseptic properties kill most bacteria
• Denaturing action “unwinds” globular properties
• Acidic property leads to activation of pepsinogen
– Pepsin affects the hydrolysis of 10% peptide bonds

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Section 26.1
• Production of secretin is stimulated by the passage of
small amounts of acidic protein content into the small
intestine
• Secretin stimulates bicarbonate (HCO3
-) production,
which in turn helps neutralize acidified gastric content
– Promotes secretion of pancreatic digestive enzymes
trypsin, chymotrypsin, and carboxypeptidase in their
inactive forms

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Section 26.1
Protein Digestive Enzymes in the Intestine
• Proteolytic enzymes
– Enzymes that attack peptide bonds
– Pepsin
– Trypsin
– Chymotrypsin
• Zymogens
– Proteolytic enzymes produced in inactive form

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Section 26.1
• Liberated amino acids are transported into the
bloodstream via active transport process
• The passage of polypeptides and small proteins across
the intestinal wall is uncommon in adults
– In infants, the transport of polypeptides allows the passage
of proteins such as antibodies in colostrum milk from a
mother to a nursing infant to build up immunologic
protection in the infant

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Section 26.1
Figure 26.1 - Digestion of Protein in Humans

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Section 26.1
Protein digestion begins in the _____ and is
completed in the _____, resulting in the release of
amino acids.
a. mouth; stomach
b. mouth; small intestine
c. stomach; small intestine
d. small intestine; liver

Section 26.2
Amino Acid Utilization
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Amino Acid Pool
• Amino acids formed through digestion process enter the
amino acid pool in the body
• Amino acid pool: The total supply of free amino acids
available for use in the human body
• Sources
– Dietary protein
– Protein turnover: The repetitive process in which proteins
are degraded and resynthesized
– Biosynthesis of amino acids in the liver
– Only nonessential amino acids are synthesized

Section 26.2
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Nitrogen Balance
• The state that results when the amount of nitrogen taken
into the human body as protein equals the amount of
nitrogen excreted from the body in waste materials
• Types of nitrogen imbalance
– Negative nitrogen imbalance - Protein degradation
exceeds protein synthesis
• Amount of nitrogen in urine exceeds consumed amount
• Results in tissue wasting
– Positive nitrogen imbalance - Rate of protein synthesis
(anabolism) is more than protein degradation (catabolism)
• Indicated by the synthesis of large amounts of tissue

Section 26.2
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Uses of Amino Acids in the Human Body
• Protein synthesis
‒ Uses approximately 75% of free amino acids
• Synthesis of non-protein nitrogen-containing compounds
‒ Synthesis of purines and pyrimidines
‒ Synthesis of heme for hemoglobin
• Synthesis of nonessential amino acids
‒ Essential amino acids cannot be synthesized due to the
lack of an appropriate carbon chain
• Production of energy
‒ Amino acids are not stored in the body
• Excesses are degraded
• Each amino acid has a unique degradation pathway

Section 26.2
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Degradation Pathways
• The amino nitrogen atom is removed and excreted from
the body as urea
• The remaining carbon skeleton is converted to pyruvate,
acetyl CoA, or a citric acid cycle intermediate

Section 26.2
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Amino acids produced during protein digestion
enter the _____ of the body.
a. energy production pool
b. amino acid pool
c. protein synthesis pool
d. nitrogen balance pool

Section 26.3
Transamination and Oxidative Deamination
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• Degradation of an amino acid takes place in two stages
̶ Removal of the α-amino group
̶ Degradation of the remaining carbon skeleton
• Removal of amino groups requires:
– Transamination: A biochemical reaction characterized by
the interchange of the amino group in an α-amino acid
with the keto group in an α-keto acid
– Oxidative deamination

Section 26.3
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Glutamate Production via Transamination
• Glutamate is produced through transamination when α-
ketoglutarate is the amino group acceptor

Section 26.3
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Aspartate Production via Transamination
• This occurs when glutamate is the reacting acid and
oxaloacetate is the reacting keto acid

Section 26.3
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Ammonium Production via Oxidative Deamination
• Oxidative deamination is a biochemical reaction in which
an α-amino acid is converted to an α-keto acid with
release of an ammonium ion
– Occurs in the mitochondria of the liver and kidney

Section 26.3
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Practice Exercise
Indicate whether each of the following reaction characteristics is
associated with the process of transamination or with the
process of oxidative deamination:
a. One of the reactants is a keto acid and one of the products is a keto acid.
b. Enzymes with a specificity toward α-ketoglutarate are often active.
c. NAD is used as an oxidizing agent.
d. An aminotransferase enzyme is active.

Section 26.3
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Practice Exercise
Indicate whether each of the following reaction characteristics is
associated with the process of transamination or with the
process of oxidative deamination:
a. One of the reactants is a keto acid and one of the products is a keto acid.
b. Enzymes with a specificity toward α-ketoglutarate are often active.
c. NAD is used as an oxidizing agent.
d. An aminotransferase enzyme is active.
Answers:
a. Transamination
b. Transamination
c. Oxidative deamination
d. Transamination

Section 26.3
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What two types of biochemical reactions are
required for the removal of the amino group from
most amino acids?
a. Amination and reductive deamination
b. Amination and oxidative deamination
c. Transamination and reductive deamination
d. Transamination and oxidative deamination

Section 26.4
The Urea Cycle
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• The net effect of transamination and deamination
reactions is the production of ammonium ions and
aspartate
• Urea cycle: A series of biochemical reactions in which
urea is produced from ammonium ions and aspartate as
nitrogen sources
• Urea produced in the liver is transported via blood to the
kidneys and eliminated from the body in urine
• Urea is an odorless white solid with a salty taste, has a
melting point of 133oC, and is soluble in water

Section 26.4
The Urea Cycle
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Carbamoyl Phosphate
• One of the sources of fuel for the urea cycle
• Two ATP molecules are expended in the formation of
one carbamoyl phosphate molecule
• It contains a high-energy phosphate bond
• It is formed in the mitochondrial matrix

Section 26.4
The Urea Cycle
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Steps of the Urea Cycle
• Stage 1 - Carbamoyl group transfer
– The carbamoyl group of carbamoyl phosphate is
transferred to ornithine to form citrulline
• Stage 2 - Citrulline–aspartate condensation
– Citrulline is transported into the cytosol and reacts with
aspartate to produce argininosuccinate synthetase,
utilizing ATP
• Stage 3 - Argininosuccinate cleavage
– Argininosuccinate is cleaved to arginine and fumarate by
the enzyme argininosuccinate lyase

Section 26.4
The Urea Cycle
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Steps of the Urea Cycle
• Stage 4 - Urea from arginine hydrolysis
– Hydrolysis of arginine produces urea and regenerates
ornithine under the influence of arginase
– The oxygen atom present in urea comes from water
– Ornithine is transported back to mitochondria to be used
in the urea cycle

Section 26.4
The Urea Cycle
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Urea Cycle Net Reaction
• The equivalent of four ATP molecules is expended in the
production of one urea molecule
– Two molecules of ATP are consumed in the
production of carbamoyl phosphate
– The equivalent of two ATP molecules is consumed in
step two of the urea cycle to give AMP and the PPi

Section 26.4
The Urea Cycle
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Figure 26.6 - Conversion of Carbamoyl Phosphate to
Urea

Section 26.4
The Urea Cycle
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Linkage Between the Urea and Citric Acid Cycles
• Fumarate produced is ultimately converted to asparte
• Aspartate re-enters the urea cycle at step two

Section 26.4
The Urea Cycle
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The net effect of amino acid degradation is the
production of the ammonium ion, which is toxic.
How is the ammonium ion eliminated from the
body?
a. It is biosynthesized for the production of nonessential
amino acids.
b. It is recycled in the production of amino acids.
c. It is converted to urea in the urea cycle and excreted in
the urine.
d. Both (b) and (c).

Section 26.5
Amino Acid Carbon Skeletons
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• Transamination/oxidative deamination removes the
amino group from an amino acid
– An α-keto acid that contains the skeleton of the amino acid
is produced
• Each of the 20 amino acids undergo a different
degradation process
– Products formed are among a group of seven
intermediates
• Four products are intermediates in the citric acid cycle
• Three products are pyruvate, acetyl CoA, and acetoacetyl
CoA

Section 26.5
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• The amino acids converted to citric acid cycle
intermediates can serve as glucose precursors
– Glucogenic amino acid: An amino acid that has a
carbon-containing degradation product that can be used to
produce glucose via gluconeogenesis
• The amino acids converted to acetyl CoA or acetoacetyl
CoA can contribute to the formation of fatty acids
– Ketogenic amino acid: An amino acid that has a carbon-
containing degradation product that can be used to
produce ketone bodies

Section 26.5
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Figure 26.9 - Fates of Carbon Skeletons of Amino Acids

Section 26.5
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What are the four intermediates that contain the
carbon skeletons from amino acid degradation in
the citric acid cycle?
a. Citric acid, α-ketoglutarate, acetyl CoA, and fumarate
b. α-Ketoglutarate, succinyl CoA, fumarate, and
oxaloacetate
c. α-Ketoglutarate, acetyl CoA, succinyl CoA, and
fumarate
d. Citric acid, succinyl CoA, fumarate, and oxaloacetate

Section 26.6
Amino Acid Biosynthesis
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• Nonessential amino acids are synthesized in fewer steps
than essential amino acids
• The primary source of essential amino acids for humans
and animals is plants

Section 26.6
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Figure 26.10 - Summary of the Starting Materials for the
Biosynthesis of the 11 Nonessential Amino Acids

Section 26.6
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Which of the following statements is/are true of
amino acids?
a. Nonessential amino acids are synthesized in fewer
steps than essential amino acids.
b. Most bacteria and plants synthesize all amino acids via
pathways that are not present in humans.
c. Plants are the major source of the essential amino acids
in humans and animals.
d. All the above.

Section 26.7
Hemoglobin Catabolism
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Red Blood Cells
• They are highly specialized cells whose primary function
is to deliver oxygen to cells and remove carbon dioxide
from body tissues
• Mature red blood cells have no nucleus or DNA
– Filled with hemoglobin
• Red blood cells are formed in the bone marrow
– Approximately 200 billion new red blood cells are formed
daily
• The life span of a red blood cell is approximately four
months

Section 26.7
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• Hemoglobin is a conjugated protein with two
components
– Globin - The protein portion
– Heme - The prosthetic group
• Iron atom present in heme interacts with oxygen
– A reversible complex is formed

Section 26.7
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• Old RBCs are broken down in the spleen and liver
• Degradation of hemoglobin
– Globin protein part is converted to amino acids, which
become part of the amino acid pool
– The iron atom becomes part of ferritin
• An iron-storage protein
– The tetrapyrrole carbon arrangement of heme is degraded
to bile pigments
• Eliminated in feces or urine

Section 26.7
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Bile Pigments
• Colored tetrapyrrole degradation products present in bile
• Types of bile pigments
– Biliverdin - Green in color
– Bilirubin - Reddish orange in color
– Stercobilin - Brownish in color
– Urobilin - Yellow in color

Section 26.7
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Bile Pigments
• Daily normal excretion of bile pigments
– 1–2 mg in urine
– 250–350 mg in feces
• Jaundice
– Caused by an imbalance between the formation and
removal of bilirubin
– Gives the skin and white of the eye the characteristic
yellow tint of the illness

Section 26.7
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Degradation of heme from hemolysis produces the
product _____, which is converted to _____.
a. bilirubin; biliverdin
b. biliverdin; bilirubin
c. bilirubin; urobilin
d. stercobilin; urobilin

Section 26.8
Proteins and the Element Sulfur
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Biodegradation of Cysteine
• Occurs in two steps
– A transamination reaction
– Release of —SH

Section 26.8
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Biosynthesis of Cysteine
• Serine is the precursor
• Serine is converted to cysteine in two steps
– Activation of serine by an acetyl CoA molecule
– Sulfhydration with hydrogen sulphide
• Hydrogen sulphide is produced by sulfate assimilation

Section 26.8
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Figure 26.13 (a) - Steps 1 and 2 of Sulfate Assimilation

Section 26.8
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Figure 26.13 (b) - Steps 3 and 4 of Sulfate Assimilation

Section 26.8
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Hydrogen Sulfide as a Biochemical Signalling Agent
• It regulates vascular blood flow and blood pressure
– Acts as a smooth muscle relaxant and vasodilator
• It influences brain function
– Brain levels of H2S are lower than normal in cases of
Alzheimer’s disease
• It influences insulin levels in type I diabetes
– Excess of H2S leads to reduced insulin production

Section 26.8
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In degradation of the sulfur-containing amino acid
cysteine, the sulfur is released in the
form of:
a. hydrogen sulfide.
b. sulfate ion.
c. sulfur dioxide.
d. none of the above.

Section 26.8
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In degradation of the sulfur-containing amino acid
cysteine, the sulfur is released in the
form of:
a. hydrogen sulfide.
b. sulfate ion.
c. sulfur dioxide.

Section 26.9
Interrelationships Among Metabolic Pathways
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• The metabolic pathways of carbohydrates, lipids, and
proteins are integrally linked to one another
− A change in one pathway can affect many other pathways
• Examples
− Feasting - Over-eating
− Causes the body to store a limited amount as glycogen and
the rest as fat
− Fasting - Food is not ingested
− The body uses its stored glycogen and fat for energy
− Starvation - Prolonged fasting
− Body protein is broken down to amino acids to synthesize
glucose

Section 26.9
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During starvation, what is used as a source of
energy after the glycogen stores have been
depleted?
a. Amino acids of degraded proteins which are used to
synthesize glucose
b. Body fats which are converted to ketone bodies and
used as a source of brain energy
c. Glycogen stores are never depleted
d. Both (a) and (b)

Section 26.10
B Vitamins and Protein Metabolism
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• All eight B vitamins participate in various pathways of
protein metabolism
– Niacin
• Oxidative deamination reactions
– PLP
• Transamination reactions

Section 26.10
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Figure 26.15 - Involvement of B Vitamins in Protein
Metabolism

Section 26.10
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Transamination reactions require the cofactor PLP,
which involves:
a. folate.
b. riboflavin.
c. vitamin B6.

Chapter 26
What best describes what happens to the protein
after eating a high-protein meal?
a. Protein digestion begins in the stomach and then moves to the
small intestine where complete digestion occurs. The free amino
acids are stored in the amino acid pool.
b. Proteins are denatured in the stomach and are then moved to the
small intestine for complete digestion.
c. Protein digestion begins in the mouth and then moves to the
stomach for complete digestion by the enzyme pepsin. The free
amino acids are then moved to the small intestine and stored in the
amino acid pool.
d. Protein digestion begins in the mouth, is continued in the stomach,
and is completed in the small intestine.

Chapter 26
In the early 1990s, nitric oxide (NO) was
discovered in the body as a gaseous chemical
messenger. What effect does nitric oxide have in
the body?
a. It stimulates the urea cycle to ensure proper functioning in the
removal of the toxic ammonium ion.
b. It plays a part in maintaining blood pressure and is found in the
brain where it may play a part in long-term memory.
c. It is rapidly converted to an amino group which is used in amino
acid synthesis.
d. It carries messages into the mitochondria to signal the production
of large amounts of energy when called for by the hormone
epinephrine.

Protein-Metabolism.pptx

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Protein-Metabolism.pptx