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19.1 Biological Catalysts19.2 Names and Classification of Enzymes19.3 Enzymes as Catalysts19.4 Factors Affecting Enzyme Activity

Chapter 19Enzymes and Vitamins

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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 three-dimensional shapes that fit the shapes of reactants.

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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 D-lactate.

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The specificity of an enzyme The specificity of an enzyme for one of two for one of two enantiomersenantiomersis a matter of fit. One is a matter of fit. One enantiomerenantiomer fits better into fits better into the active site of the enzyme the active site of the enzyme than the other than the other enantiomerenantiomer. . Enzyme catalyzes reaction Enzyme catalyzes reaction of the of the enantiomerenantiomer that fits that fits better into the active site of better into the active site of the enzyme.the enzyme.

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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.

Names of Enzymes

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19.2 Enzyme CofactorsPlease see section 19.8 of these slides.

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Enzymes are classified according to the reaction they catalyze.

Class Reactions catalyzedOxidoreductases Oxidation-reductionTransferases Transfer groups of atomsHydrolases HydrolysisLyases Add atoms/remove atoms

to/from a double bondIsomerases Rearrange atomsLigases Use ATP to combine

molecules

19.3 Enzyme Classification

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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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Match the type of reaction with an enzyme:1) aminase 2) dehydrogenase3) isomerase 4) synthetase

A. Converts a cis-fatty acid to a trans-fatty acid.B. Removes 2 H atoms to form double bond.C. Combine two molecules using ATP.D. Adds NH3.

Learning Check

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Match the type of reaction with an enzyme:1) aminase 2) dehydrogenase3) isomerase 4) synthetaseA. 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.

Solution

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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.

19.4 How Enzymes Work

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Hydrolysis of a peptide bond by Chymotripsin

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Enzymes may recognize and catalyze:A single substrate.A group of similar substrates.A particular type of bond.

Enzyme Specificity

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Lock-and-Key ModelIn 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 ModelIn 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 ESWithin the ES complex, the reaction occurs to convert substrate to product (P).

ES E + PThe 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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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.

Learning Check

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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.

Solution

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Diagnostic Enzymes The levels of diagnostic enzymes determine the amount of damage in tissues.

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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.

19.6 Temperature and Enzyme Action

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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.

pH and Enzyme Action

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Optimum pH ValuesMost 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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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 sucroseB. Changing the pH to 4C. Running the reaction at 70°C

Learning Check

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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 reaction1) no change 2) increase 3) decrease A. 2 Increasing the concentration of sucraseB. 3 Changing the pH to 4C. 3 Running the reaction at 70°C

Solution

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Inhibitors: Are molecules that cause a loss of catalytic activity.Prevent substrates from fitting into the active sites.E + S ES E + PE + I EI no P

19.8 -19.9Enzyme Regulation: Inhibition

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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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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.

Noncompetitive, irreversible inhibition Inhibition

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Malonate and SuccinateDehydrogenase

Malonate:Is a competitive inhibitor of succinatedehydrogenase.Has a structure that is similar to succinate.Inhibition is reversed by adding succinate.

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Identify each description as an inhibitor that is: 1) Competitive 2) NoncompetitiveA. 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.

Learning Check

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Identify each description as an inhibitor that is: 1) Competitive 2) NoncompetitiveA. 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.

Solution

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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 EnzymesDigestive enzymes are:

Produced as zymogens in one organ and transported to anotherwhen needed.

Activated by removing small peptide sections.

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Allosteric Regulator EnzymesAn 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 changes the activity site so that the enzyme becomes less effective so that the enzyme becomes less effective catalyst and rate slows down.catalyst and rate slows down.

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A positive regulator A positive regulator changes the activity changes the activity site so that the enzyme site so that the enzyme becomes a better becomes a better catalyst and rate catalyst and rate accelerates.accelerates.

A negative regulator A negative regulator changes the activity changes the activity site so that the enzyme site so that the enzyme becomes less effective becomes less effective catalyst and rate slows catalyst and rate slows down.down.

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Feedback ControlIn 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 CheckIdentify each statement as:1) zymogen 2) allosteric enzyme3) positive regulator 4) feedback controlA. 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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SolutionIdentify each statement as:1) zymogen 2) allosteric enzyme3) positive regulator 4) feedback controlA. 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 CoenzymesA 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 cooordinatebond 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 enzyme2) An enzyme that required a cofactor

A. Requires Mg2+ for hydrolysis of phosphateesters.

B. Requires vitamin B3 to transfer an acetyl group.C. Is active with four polypeptide subunits.

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SolutionIdentify each enzyme as: 1) A simple enzyme2) An enzyme that required a cofactor

A. 2 Requires Mg2+ for hydrolysis of phosphateesters.

B. 2 Requires vitamin B3 to transfer an acetyl group.C. 1 Is active with four polypeptide subunits.

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Function of CoenzymesA coenzyme prepares the active site for catalytic activity.

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19.10 Water-Soluble VitaminsWater-soluble vitamins are:Soluble in aqueous solutions.Used as cofactors by many enzymes.Not stored in the body.

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Fat-Soluble VitaminsFat-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 CheckIdentify each compound as a:1) water-soluble vitamin 2) fat-soluble vitaminA. Folic acidB. Retinol (Vitamin A)C. Vitamin CD. Vitamin EE. Niacin

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SolutionIdentify each compound as a:1) water-soluble vitamin 2) fat-soluble vitaminA. 1 Folic acidB. 2 Retinol (Vitamin A)C. 1 Vitamin CD. 2 Vitamin EE. 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.

N

N N

NHH3C

H3C

CH2 CH CH CH CH2 OH

OHOHOH

O

O

D-Ribitol

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Niacin (Vitamin B3)Niacin:

Is part of the coenzyme nicotinamide adenine dinucleotide (NAD+) involved in oxidation-reduction reactions.Deficiency can result in dermatitis, muscle fatigue, and loss of appetite.Is found in meats, rice, and whole grains.

N

C

O

OH

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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.

HO CH2 C CH C N CH2 CH2 C OH

O

H

OH OCH3

CH3

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Pyridoxine (Vitamin B6)Pyridoxine and pyridoxal are two forms of vitamin B6, which are converted to the coenzyme pyridoxalphosphate (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 pyrrolerings 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.

O CHOH

CH2OH

OHHO

O

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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 AVitamin A is obtained from meats and beta-carotenes in plants.Beta-carotenes are converted by liver enzymes to vitamin A (retinol).

H3C CH3

CH3

CH3 CH3

CH3 CH3H3C CH3

H3C

CH3 CH3

CH2OHH3C CH3

CH3

Beta-carotene

Retinol (vitamin A)

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Vitamin DVitamin 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 EVitamin E:

Is an antioxidant in cells.May prevent the oxidation of unsaturated fatty acids.Is found in vegetable oils, whole grains, and vegetables.

O

CH3

HO

H3C

CH3

CH3

CH3

CH3 CH3 CH3

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Vitamin KVitamin 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.

3 n

CH3

CH3

O

O CH3 CH3

Vitamin K2 (menaquinone)

CH3

CH3

O

O CH3 CH3

Vitamin K1 (phylloquinone)

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Learning CheckIdentify the vitamin associated with each:1) Thiamin (B1) 2) Vitamin A 3) Vitamin K 4) Vitamin D5) Ascorbic Acid

A. Collagen formationB. BeriberiC. Absorption of phosphorus and calcium in bone D. VisionE. Blood clotting

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SolutionIdentify the vitamin associated with each: 1) Thiamin (B1) 2) Vitamin A 3) Vitamin K 4) Vitamin D5) Ascorbic Acid

A. 5 Collagen formationB. 1 BeriberiC. 4 Absorption of phosphorus and calcium in bone D. 2 VisionE. 3 Blood clotting

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Chapter SummaryEnzymes 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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TermsCarcinomaIn 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.

Ricketsis 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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TermspH 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.Thrombinis 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 endocrineglands.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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TermsThrombin: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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TermsVitamin B12,: available as food supplementFolic Acid: found in food

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Vitamin K1Phylloquinone is a polycyclic aromatic ketone, based on 1,4-naphthoquinone, 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