Names of Enzymes

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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 hydrolysis of sucrose.

• Describes the function of the enzyme. For
  example, oxidases catalyze oxidation.

• Can be a common name, particularly for the
• digestive enzymes, such as pepsin and trypsin.

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Enzyme ClassificationEnzymes are classified by
the type of reaction they catalyze.
Oxidoreductases Catalyze oxidation-reduction
reactions
Transferases Catalyze transfer of groups
between compounds
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Enzyme Classification
Hydrolyases Catalyze hydrolysis reactions
Lyases Catalyze addition or removal of groups
without hydrolysis or oxidation
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Enzyme Classification
Isomerases Catalyze rearrangement reactions
Ligases Catalyze reactions that connect
molecules - requires energy input
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Enzyme Cofactors

• 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.

• A holoenzyme is the enzyme cofactor, an
  apoenzyme is the enzyme cofactor.

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Metal Ions as Cofactors

• Many active enzymes
• require a metal ion.
• Zn2, a cofactor for carboxypeptidase,
  stabilizes the carbonyl oxygen during the
  hydrolysis of a peptide bond.

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Water-Soluble Vitamins

• Water-soluble vitamins are
• Soluble in aqueous solutions.
• Cofactors for many enzymes.
• Not stored in the body.

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Thiamine (Vitamin B1)

• Coenzyme form - thiamine pyrophosphate

Coenzyme for decarboxylation reactions
Nutritional sources milk, grains, green
vegetables
Deficiency disease beriberi
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Riboflavin (Vitamin B2)

• Coenzyme form - FAD

Coenzyme for oxidation-reduction reactions
Nutritional sources milk, grains, green
vegetables
Deficiency symptoms dermatitis, tongue
inflamation
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Niacin (Vitamin B3)

• Coenzyme forms - NAD and NADP

Coenzyme for oxidation-reduction reactions
Nutritional sources milk, grains, green
vegetables
Deficiency disease pellegra (rough skin)
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Pantothenic Acid (Vitamin B5)

• Coenzyme form - coenzyme A

Coenzyme for transfer of acyl groups
Nutritional sources most foods
Deficiency symptoms fatigue, apathy, muscle
cramps
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Pyridoxine (Vitamin B6)

• Coenzyme form - pyridoxal pyrophosphate

Coenzyme for amino group transfer reactions
Nutritional sources milk, grains, green
vegetables
Deficiency symptom dermatitis, anemia
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Biotin (Vitamin B7)

• Coenzyme form - biotin

Coenzyme for carboxylation reactions
Nutritional sources milk, liver, also produced
by intestinal
bacteria
Deficiency symptom dermatitis
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Folic Acid (Vitamin B9)

• Coenzyme form - tetrahydrofolate

Coenzyme for intermolecular one-carbon transfers
Nutritional sources green leafy vegetables,
yeast, and produced by
intestinal bacteria
Deficiency symptoms anemia, diarrhea
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Vitamin B12

• Coenzyme form - 5deoxyadenosylcobalamin

Coenzyme for intramolecular rearrangements
Nutritional sources milk, eggs, other animal
products
Deficiency disease pernicious anemia
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Vitamin C

• Coenzyme form - vitamin C

Coenzyme for hydroxylation reactions
Nutritional sources citrus fruits, tomatoes,
broccoli
Deficiency disease scurvy (weakened collagen)
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ZymogenInactive precursor form of an enzyme or
other protein
Digestive enzymes
Blood-clotting proteins
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Active Site

• The active site
• Is a region within an enzyme that fits the shape
  of the reacting molecule called a substrate.

• Contains amino acid R groups that bind the
  substrate.

• Releases products when the reaction is complete.

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Enzyme Catalyzed Reaction Basic Mechanism
The proper fit of a substrate (S) in an active
site forms an enzyme-substrate (ES) complex.
Within the ES complex, the reaction occurs to
convert substrate to product (P).
The products, which are no longer attracted to
the active site, are released.
Overall, substrate is converted to product.
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Enzyme Catalyzed Reaction

• In an enzyme-catalyzed
• reaction
• A substrate attaches to the active site.

• An enzyme-substrate (ES) complex forms.

• Reaction occurs and products are released.

• An enzyme is used over and over.

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Enzymes are Biological Catalysts

• Enzymes are proteins that
• Catalyze nearly all the chemical reactions taking
  place in the cells of the body.
• Increase the rate of reaction by providing
  pathway with a lower the energy of activation.

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Lock and Key Model

• In the lock-and-key model of enzyme action,
• The active site has a rigid shape.

• An enzyme only binds substrates that exactly fit
  the active site.

• Only substrates with the matching shape can fit.

• The substrate is the key that fits that lock.

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Induced-fit Model

• In the induced-fit model of enzyme action,
• Enzyme structure is flexible, not rigid.

• Enzyme and substrate adjust the shape of the
  active site to bind substrate.

• Shape changes improve catalysis during reaction.

• The range of substrate specificity increases.

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Enzyme Specificity

• Enzymes may recognize and catalyze

• A single substrate. (absolute specificity)
• Example urease

• A specific stereoisomer (stereochemical
  specificity)
• Example D-amino acid oxidase

• A group of similar substrates.(group specificity)
• Example alcohol dehydrogenase

• A particular type of bond.(linkage specificity)
• Example lipase

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Substrate Concentration

• An increase in substrate
• concentration

• Increases the rate of reaction (at constant
  enzyme concentration).

• Eventually saturates
• an enzyme with substrate to give maximum
  activity.

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Enzyme Concentration

• An increase in enzyme
• concentration

• Increases the rate of reaction (at constant
  substrate concentration).

• Binds more substrate with enzyme.

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Temperature and Enzyme Action

• Enzymes
• Are most active at an optimum temperature
  (usually 37C 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 charges change.

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Optimum pH Values

• Enzymes in
• The body have an optimum pH of about 7.4.
• Certain organs, enzymes operate at lower and
  higher optimum pH values.

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Feedback Control (Feedback Inhibition)

• In feedback control
• A product acts as a negative regulator.

• As the concentration of the end product
  increases, the end product binds with the first
  enzyme (E1) in a sequence decreasing the rate of
  the first reaction.

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Specific Example of Feedback Inhibition Biosynthes
is of Threonine
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Allosteric Enzymes

• An allosteric enzyme is
• An enzyme in a reaction sequence that binds a
  regulator substance.

• A positive regulator is one that enhances the
  binding of substrate and accelerates the rate of
  reaction.

• A negative regulator when it prevents the binding
  of the substrate to the active site and slows
  down the rate of reaction.

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Irreversible Inhibition

• In irreversible inhibition, a substance
• Bonds with R groups at the active site.
• Destroys enzyme activity.

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Competitive Inhibition

• A competitive inhibitor
• Has a structure that is similar to that of the
  substrate.

• Competes with the substrate for the active site.

• Has its effect reversed by increasing substrate
  concentration.

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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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Noncompetitive Inhibition

• A noncompetitive inhibitor
• Binds to a site other than the active site.

• Alters the arrangement of groups in the active
  site.

• Doesnt prevent binding of the substrate, but
  blocks activity.

• Cannot have its effect reversed by adding more
  substrate.

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Some Medical Uses of Inhibitors
Cancer chemotherapy Methotrexate
(amethopterin) - inhibits synthesis of
thymine (part of DNA) 5-Fluorouracil -
inhibits DNA synthesis
Tetracyclines - inhibit protein synthesis
Penicillin - inhibits bacterial cell wall
synthesis
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Neural Transmission using Acetylcholine
Release inhibited by toxin from Clostridium
botulinum
Binding inhibited by curare
Breakdown inhibition of acetylcholine esterase
Inhibitors nerve gases (sarin, tabun),
insecticides (malathion, parathion)
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Isoenzymes

• Isoenzymes
• Catalyze the same reaction in different tissues
  in the body.

• Such as lactate dehydrogenase (LDH), which
  converts lactate to pyruvate, consists of five
  isoenzymes.

• Can be used to identify the organ or tissue
  involved in damage or disease.

• Such as LDH have one form more prevalent in heart
  muscle and another form in skeletal muscle and
  liver.

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Isoenzymes of Lactate Dehydrogenase

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Diagnostic Enzymes

• Levels of enzymes
• CK, LDH, and AST
• Are elevated following a heart attack.
• Are used to determine the severity of the attack.

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Diagnostic Enzymes

• Diagnostic enzymes
• Determine the amount of damage in tissues.
• That are elevated may indicate damage or disease
  in a particular organ.

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Enzyme Classification
isomerase
transferase
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Enzyme Classification
oxidoreductase
lyase
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Enzyme Classification
ligase
hydrolase