Download Amino Acids, Proteins, and Enzymes

Chapter 19
Enzymes and Vitamins
19.1
19.2
19.3
19.4
Biological Catalysts
Names and Classification of Enzymes
Enzymes as Catalysts
Factors Affecting Enzyme Activity
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19.1 Catalysis by Enzymes
Enzymes are proteins that:
„ Increase the rate of reaction
by lowering the energy of
activation.
„ Catalyze nearly all the
chemical reactions taking
place in the cells of the
body.
„ Have unique threedimensional shapes that fit
the shapes of reactants.
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2
„
Enzymes are specific with respect to
stereochemistry – catalyze reaction of
only one of the pair of enantiomers. For
example,
the
enzyme
lactate
dehydrogenase catalyzes the removal of
hydrogen from L-lactate but not from Dlactate.
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The specificity of an enzyme
for one of two enantiomers
is a matter of fit. One
enantiomer fits better into
the active site of the enzyme
than the other enantiomer.
Enzyme catalyzes reaction
of the enantiomer that fits
better into the active site of
the enzyme.
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Names of Enzymes
The name of an enzyme:
„ Usually ends in –ase.
„ Identifies the reacting substance. For example,
sucrase catalyzes the reaction of sucrose.
„ Describes the function of the enzyme. For
example, oxidases catalyze oxidation.
„ Could be a common name, particularly for the
digestion enzymes such as pepsin and trypsin.
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19.2 Enzyme Cofactors
„
Please see section 19.8 of these slides.
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19.3 Enzyme Classification
„
Enzymes are classified according to the reaction
they catalyze.
Class
Reactions catalyzed
ƒ Oxidoreductases Oxidation-reduction
ƒ Transferases
Transfer groups of atoms
ƒ Hydrolases
Hydrolysis
ƒ Lyases
Add atoms/remove atoms
to/from a double bond
ƒ Isomerases
Rearrange atoms
ƒ Ligases
Use ATP to combine
molecules
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Classification of Enzymes:
Oxidoreductases and Transferases
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Classification: Hydrolases and
Lyases
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Classification: Isomerases and
Ligases
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Learning Check
Match the type of reaction with an enzyme:
1) aminase
2) dehydrogenase
3) isomerase
4) synthetase
A.
B.
C.
D.
Converts a cis-fatty acid to a trans-fatty acid.
Removes 2 H atoms to form double bond.
Combine two molecules using ATP.
Adds NH3.
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Solution
Match the type of reaction with an enzyme:
1) aminase
2) dehydrogenase
3) isomerase
4) synthetase
A. 3 Converts a cis-fatty acid to a trans-fatty acid.
B. 2 Removes 2 H atoms to form double bond.
C. 4 Combine two molecules using ATP.
D. 1 Adds NH3.
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19.4 How Enzymes Work
The active site:
„ Is a region within an
enzyme that fits the
shape of molecules
called substrates.
„ Contains amino acid R
groups that align and
bind the substrate.
„ Releases products
when the reaction is
complete.
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13
Hydrolysis of a peptide bond by Chymotripsin
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Enzyme Specificity
Enzymes may recognize and catalyze:
„ A single substrate.
„ A group of similar substrates.
„ A particular type of bond.
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Lock-and-Key Model
In the lock-and-key model of enzyme action:
„ The active site has a rigid shape.
„ Only substrates with the matching shape can fit.
„ The substrate is a key that fits the lock of the
active site.
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Induced-fit Model
In the induced-fit model of enzyme action:
„ The active site is flexible, not rigid.
„ The shapes of the enzyme, active site, and substrate
adjust to maximize the fit, which improves
catalysis.
„ There is a greater range of substrate specificity.
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Enzyme Catalyzed Reaction
„
„
„
„
The proper fit of a substrate (S) in an active site
forms an enzyme-substrate (ES) complex.
E+S
ES
Within the ES complex, the reaction occurs to
convert substrate to product (P).
ES
E+P
The products, which are no longer attracted to the
active site, are released.
Overall, substrate is converted to product.
E+S
ES
E+P
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Example of An Enzyme Catalyzed
Reaction
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Learning Check
A. The active site is:
1) The enzyme shape.
2) A section of the enzyme.
3) The substrate.
B. In the induced-fit model, the shape of the
enzyme when substrate binds:
1) Stays the same.
2) Adapts to the shape of the substrate.
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Solution
A. The active site is:
2) A section of the enzyme.
B. In the induced-fit model, the shape of the
enzyme when substrate binds:
2) Adapts to the shape of the substrate.
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Diagnostic Enzymes
„
The levels of diagnostic enzymes determine the
amount of damage in tissues.
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19.6 Temperature and Enzyme
Action
Enzymes:
„ Are most active at an
optimum temperature
(usually 37°C in
humans).
„ Show little activity at
low temperatures.
„ Lose activity at high
temperatures as
denaturation occurs.
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pH and Enzyme Action
Enzymes:
„ Are most active at
optimum pH.
„ Contain R groups of
amino acids with
proper charges at
optimum pH.
„ Lose activity in low or
high pH as tertiary
structure is disrupted.
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Optimum pH Values
„
„
Most enzymes of the body have an optimum pH of
about 7.4.
In certain organs, enzymes operate at lower and
higher optimum pH values.
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Optimum pH Values
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Enzyme Concentration
„
„
The rate of reaction
increases as enzyme
concentration
increases (at
constant substrate
concentration).
At higher enzyme
concentrations,
more substrate
binds with enzyme.
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Substrate Concentration
„
„
The rate of reaction
increases as substrate
concentration
increases (at constant
enzyme
concentration).
Maximum activity
occurs when the
enzyme is saturated.
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Learning Check
Sucrase has an optimum temperature of 37°C
and an optimum pH of 6.2. Determine the effect
of the following on its rate of reaction.
1) no change
2) increase
3) decrease
A. Increasing the concentration of sucrose
B. Changing the pH to 4
C. Running the reaction at 70°C
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Solution
Sucrase has an optimum temperature of 37°C
and an optimum pH of 6.2. Determine the effect
of the following on its rate of reaction
1) no change
2) increase
3) decrease
A. 2 Increasing the concentration of sucrase
B. 3 Changing the pH to 4
C. 3 Running the reaction at 70°C
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19.8 -19.9
Enzyme Regulation: Inhibition
Inhibitors:
„ Are molecules that cause a loss of catalytic
activity.
„ Prevent substrates from fitting into the
active sites.
E+S
ES
E+P
E+I
EI
no P
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Reversible Competitive Inhibition
A competitive inhibitor:
„ Has a structure like
the substrate.
„ Competes with the
substrate for the active
site.
„ Has its effect reversed
by increasing
substrate
concentration.
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Noncompetitive, irreversible
inhibition Inhibition
A noncompetitive inhibitor:
„ Has a structure different
than the substrate.
„ Distorts the shape of the
enzyme, which alters the
shape of the active site.
„ Prevents the binding of the
substrate.
„ Cannot have its effect
reversed by adding more
substrate.
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Malonate and Succinate
Dehydrogenase
Malonate:
„ Is a competitive
inhibitor of
succinate
dehydrogenase.
„ Has a structure that
is similar to
succinate.
„ Inhibition is
reversed by adding
succinate.
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Learning Check
Identify each description as an inhibitor that is:
1) Competitive 2) Noncompetitive
A. Increasing substrate reverses inhibition.
B. Binds to enzyme surface, but not to the
active site.
C. Structure is similar to substrate.
D. Inhibition is not reversed by adding more
substrate.
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Solution
Identify each description as an inhibitor that is:
1) Competitive 2) Noncompetitive
A. 1 Increasing substrate reverses inhibition.
B. 2 Binds to enzyme surface, but not to the
active site.
C. 1 Structure is similar to substrate.
D. 2 Inhibition is not reversed by adding more
substrate.
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Zymogens
Zymogens
(proenzymes):
„ Are inactive forms of
enzymes.
„ Are activated when
one or more peptides
are removed.
„ Such as proinsulin is
converted to insulin
by removing a small
peptide chain.
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Digestive Enzymes
Digestive enzymes are:
„ Produced as zymogens in one organ and
transported to another
„ when needed.
„ Activated by removing small peptide sections.
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Allosteric Regulator Enzymes
„
„
„
An allosteric enzyme is an enzyme That
regulates the activity of an enzyme
A positive regulator enhances the binding
of substrate and the enzyme therefore, it
accelerates the rate of reaction.
A negative regulator changes the activity site
so that the enzyme becomes less effective
catalyst and rate slows down.
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A positive regulator
changes the activity
site so that the enzyme
becomes
a
better
catalyst
and
rate
accelerates.
A negative regulator
changes the activity
site so that the enzyme
becomes less effective
catalyst and rate slows
down.
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Feedback Control
In feedback control:
„ A product acts as a negative regulator.
„ An end product binds with the first enzyme (E1)
in a sequence, when sufficient product is present.
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Learning Check
Identify each statement as:
1) zymogen
2) allosteric enzyme
3) positive regulator
4) feedback control
A. An enzyme in a pathway that controls the rate
of the reaction.
B. Speeds up a reaction by combining with an
enzyme in the pathway.
C. Removal of a peptide activates the enzyme.
D. Some product binds to the first enzyme to
limit the production of product.
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Solution
Identify each statement as:
1) zymogen
2) allosteric enzyme
3) positive regulator
4) feedback control
A. 2 An enzyme in a pathway that controls the rate
of the reaction.
B. 3 Speeds up a reaction by combining with an
enzyme in the pathway.
C. 1 Removal of a peptide activates the enzyme.
D. 4 Some product binds to the first enzyme to
limit the production of product.
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Enzyme Cofactors and Coenzymes
„
„
„
A simple enzyme is an active enzyme that
consists only of protein.
Many enzymes are active only when they
combine with cofactors such as metal ions
or small molecules.
A coenzyme is a cofactor that is a small
organic molecule such as a vitamin.
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Enzyme Cofactors
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Metal Ions as Cofactors
„
„
Many active enzymes
require a metal ion.
Zn2+, a cofactor for
carboxypeptidase,
makes a cooordinate
bond with the carbonyl
oxygen and catalyzes
the hydrolysis of a
peptide bond.
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Some Enzymes and Their
Cofactors
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Learning Check
Identify each enzyme as:
1) A simple enzyme
2) An enzyme that required a cofactor
A. Requires Mg2+ for hydrolysis of phosphate
esters.
B. Requires vitamin B3 to transfer an acetyl group.
C. Is active with four polypeptide subunits.
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Solution
Identify each enzyme as:
1) A simple enzyme
2) An enzyme that required a cofactor
A. 2 Requires Mg2+ for hydrolysis of phosphate
esters.
B. 2 Requires vitamin B3 to transfer an acetyl group.
C. 1 Is active with four polypeptide subunits.
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Function of Coenzymes
„
A coenzyme prepares the active site for catalytic
activity.
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19.10 Water-Soluble Vitamins
„
„
„
Water-soluble
vitamins are:
Soluble in
aqueous
solutions.
Used as
cofactors by
many
enzymes.
Not stored in
the body.
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Fat-Soluble Vitamins
Fat-soluble vitamins:
„ Are A, D, E, and K.
„ Soluble in lipids, but not in aqueous solutions.
„ Important in vision, bone formation,
antioxidants, and blood clotting.
„ Stored in the body.
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Learning Check
Identify each compound as a:
1) water-soluble vitamin 2) fat-soluble vitamin
A. Folic acid
B. Retinol (Vitamin A)
C. Vitamin C
D. Vitamin E
E. Niacin
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Solution
Identify each compound as a:
1) water-soluble vitamin 2) fat-soluble vitamin
A. 1 Folic acid
B. 2 Retinol (Vitamin A)
C. 1 Vitamin C
D. 2 Vitamin E
E. 1 Niacin
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Thiamin (Vitamin B1)
Thiamin:
„ Was the first B vitamin identified.
„ Is part of the coenzyme thiamin pyrophosphate.
„
„
TPP coenzyme is required by enzymes in the
decarboxylation of α-keto carboxylic acids.
Deficiency results in beriberi (fatigue, weight loss, and
nerve degeneration).
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Riboflavin (Vitamin B2)
Riboflavin is:
„ Made of ribitol and flavin.
„ Part of the coenzymes flavin adenine dinucleotide
(FAD) and flavin mononucleotide (FMN).
„ Needed for good vision and healthy skin.
O
H3C
N
H3C
N
N
N
H
O
D-Ribitol
CH2 CH CH CH CH2 OH
OH OH OH
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Niacin (Vitamin B3)
Niacin:
„ Is part of the coenzyme
nicotinamide adenine
dinucleotide (NAD+)
involved in oxidationreduction reactions.
„ Deficiency can result in
dermatitis, muscle fatigue,
and loss of appetite.
„ Is found in meats, rice, and
whole grains.
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O
C
OH
N
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Pantothenic Acid (Vitamin B5)
Pantothenic acid:
„ Is part of coenzyme A needed for energy
production as well as glucose and cholesterol
synthesis.
„ Deficiency can result in fatigue, retarded
growth, cramps, and anemia.
„ Is found in salmon, meat, eggs, whole grains,
and vegetables.
CH3 OH O
HO CH2 C
CH3
O
CH C N CH2
CH2 C OH
H
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Pyridoxine (Vitamin B6)
„
Pyridoxine and pyridoxal are two forms of vitamin
B6, which are converted to the coenzyme pyridoxal
phosphate (PLP).
„
PLP is required in the transamination of amino acids
and decarboxylation of carboxylic acids.
Deficiency of pyridoxine may lead to dermatitis,
fatigue, and anemia.
„
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Cobalamin (Vitamin B12)
Cobalamin:
„ Consists of four pyrrole
rings with a Co2+.
„ Is a coenzyme for
enzymes that transfer
methyl groups and
produce red blood cells.
„ Deficiency can lead to
pernicious anemia and
nerve damage.
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Ascorbic Acid (Vitamin C)
Vitamin C:
„ Is required in collagen
synthesis.
„ Deficiency can lead to
weakened connective tissue,
slow-healing wounds, and
anemia.
„ Is found in blueberries,
citrus fruits, tomatoes,
broccoli, red and green
vegetables.
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CH2OH
O
HO
O
CHOH
OH
64
Folic Acid (Folate)
Folic acid (folate):
„ Consists of pyrimidine,
p-aminobenzoic acid,
and glutamate.
„ Forms the coenzyme
THF used in the transfer
of carbon groups and the
synthesis of nucleic acids.
„ Deficiency can lead to abnormal red blood cells,
anemia, and poor growth.
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Vitamin A
„
„
Vitamin A is obtained from meats and betacarotenes in plants.
Beta-carotenes are converted by liver enzymes
to vitamin A (retinol).
H3C
Beta-carotene
CH3
CH3
CH3
CH3
CH3
H3C
H3C
CH3
CH3
CH3
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CH3
H3C
CH3
CH3
CH2OH
Retinol (vitamin A)
66
Vitamin D
Vitamin D (D3):
„ Is synthesized in skin
exposed to sunlight.
„ Regulates the absorption of
phosphorus and calcium
during bone growth.
„ Deficiency can result in
weakened bones.
„ Sources include cod liver oil,
egg yolk, and enriched milk.
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Vitamin E
Vitamin E:
„ Is an antioxidant in cells.
„ May prevent the oxidation of unsaturated fatty
acids.
„ Is found in vegetable oils, whole grains, and
vegetables.
CH3
HO
CH3
H3C
CH3
O
CH3
CH3
CH3
CH3
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Vitamin K
„
„
„
Vitamin K1 in plants has a saturated side chain.
Vitamin K2 in animals has a long unsaturated
side chain.
Vitamin K2 is needed for the synthesis of
zymogens for blood clotting.
O
O
CH3
CH3
CH3
CH3
3
O
CH3
CH3
Vitamin K1 (phylloquinone)
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n
O
CH3
CH3
Vitamin K2 (menaquinone)
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Learning Check
Identify the vitamin associated with each:
1) Thiamin (B1)
2) Vitamin A
3) Vitamin K
4) Vitamin D
5) Ascorbic Acid
A.
B.
C.
D.
E.
Collagen formation
Beriberi
Absorption of phosphorus and calcium in bone
Vision
Blood clotting
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Solution
Identify the vitamin associated with each:
1) Thiamin (B1)
2) Vitamin A
3) Vitamin K
4) Vitamin D
5) Ascorbic Acid
A.
B.
C.
D.
E.
5
1
4
2
3
Collagen formation
Beriberi
Absorption of phosphorus and calcium in bone
Vision
Blood clotting
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Chapter Summary
„
„
„
„
„
Enzymes are catalysts for biochemical reactions.
Enzymes are mostly water soluble and globular.
Many enzyme require a co-factor that are metal ions
or non-protein organic molecules known as
coenzymes.
There are six major classes of reactions catalyzed by
enzymes.
Enzymes draws substrates into its active site and
hold them in its active site by non-covalent
interactions to produce enzyme-substrate complex.
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Chapter Summary Contd.
„
„
„
„
Reactions take place within the enzyme-substrate
complexes.
When the reaction is over, product is released and
the enzyme returns to its original conditions.
With increasing temperature, rate of enzyme
catalyzed reactions increases to a maximum and then
starts to decrease as the enzyme protein denatures at
high temperature.
Enzyme catalyzed reaction rate is maximal at an
optimum pH.
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Chapter Summary Contd.
„
„
The effectiveness of enzymes is controlled by a
variety of activation and inhibition mechanisms.
Vitamins are organic molecules required in small
amounts in the diet because our body can not
synthesize them.
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Terms
„
Carcinoma
In medicine, carcinoma is any cancer that arises from epithelial cells. It is malignant by
definition: carcinomas invade surrounding tissues and organs, and may spread to lymph
nodes and distal sites (metastasis). Carcinoma in situ (CIS) is a pre-malignant condition,
in which cytological signs of malignancy are present, but there is no histological
evidence of invasion through the epithelial basement membrane.
„
Rickets
is a disorder most commonly caused by Vitamin D deficiency. This results in
insufficient calcium uptake by bones in developing children. The lack of
calcium being absorbed into the body,causes fragile or malformed bones,
which are unable to support the weight of a growing body. Calcium or
phosphorus deficiency in children is termed rickets, while that of adults is
termed osteomalacia.
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Terms
„
pH of small intestine:
Each digestive enzyme works best at an optimum pH. The stomach has a pH of about 2, while the
small intestine has a pH of between 7 and 8
„
.
Thrombin
is a coagulation protein that has many effects in the coagulation cascade. It is a serine
protease that converts soluble fibrinogen into insoluble strands of fibrin .
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Epithelial Cells:
„
„
In zootomy, epithelium is a tissue composed of a layer of
cells. Epithelium lines both the outside (skin) and the
inside (e.g. intestine) of organisms. The outermost layer of
our skin is composed of dead squamous epithelial cells, as
are the mucous membranes lining the inside of mouths and
body cavities. Other epithelial cells line the insides of the
lungs, the gastrointestinal tract, the reproductive and
urinary tracts, and make up the exocrine and endocrine
glands.
Functions of epithelial cells include secretion, absorption,
protection, transcellular transport, sensation detection, and
selective permeability. Endothelium (the inner lining of
blood vessels) is not related to epithelium except by name.
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Terms
„
„
„
Thrombin:is a coagulation protein that has many effects in
the coagulation cascade. It is a serine protease that
converts soluble fibrinogen into insoluble strands of fibrin.
Anemia or anaemia, which literally means "without
blood," is a deficiency of red blood cells and/or
hemoglobin. This results in a reduced ability of blood to
transfer oxygen to the tissues, and this causes hypoxia;
since all human cells depend on oxygen for survival,
varying degrees of anemia can have a wide range of
clinical consequences
Pyrrole
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Terms
„
„
Vitamin B12,: available as food supplement
Folic Acid: found in food
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Vitamin K1
„
„
„
Phylloquinone is a polycyclic aromatic ketone, based on 1,4naphthoquinone, with 2-methyl and 3-phytyl substituents. It is often
called vitamin K1.
It is a fat-soluble vitamin that is stable to air and moisture but
decomposes in sunlight. It is found naturally in a wide variety of green
plants. Vitamin K denotes a group of 2-methilo-naphthoquinone
derivatives. They are human vitamins, lipophilic (i.e., soluble in lipids)
and therefore hydrophobic (i.e., poorly soluble in water). They are
needed for the posttranslational modification of certain proteins,
mostly required for blood coagulation.
Vitamin K2 (menaquinone, menatetrenone) is normally produced by
bacteria in the intestines, and dietary deficiency is extremely rare
unless the intestines are heavily damaged.
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„
End of Chapter Nineteen
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