Enzymes [under construction]

*From the pentapeptide, phe-ala-leu-lys-arg, phenylalanine residue is split off by* (A) Trypsin (B) Chymotrypsin (C) Aminopeptidase (D) Carboxypeptidase

C

*In non competitive enzyme activity inhibition, inhibitor* (A) Increases Km (B) Decreases Km (C) Does not effect Km (D) Increases Km

C

*In non-competitive enzyme action* (A) Vmax is increased (B) Apparent km is increased (C) Apparent km is decreased (D) Concentration of active enzyme molecule is reduced

C

*Inactive precursors of enzymes are known as* (A) Apoenzymes (B) Coenzymes (C) Proenzymes (D) Holoenzymes

C

*Kinetics of an allosteric enzyme are explained by* (A) Michaelis-Menten equation (B) Lineweaver-Burk plot (C) Hill plot (D) All of these

C

*Regulation of some enzymes by covalent modification involves addition or removal of* (A) Acetate (B) Sulphate (C) Phosphate (D) Coenzyme

C

*The active site of an enzyme is formed by a few of the enzymes* (A) R groups of the amino acids (B) Amino groups of the amino acids (C) Carboxyl group of the amino acids (D) Exposed sulfur bonds

C

*The shape of an enzyme and consequently its activity can be reversibly altered from moment to moment by* (A) Heat (B) Amino acid substrate (C) Allosteric subunits (D) Sulfur substitutions

C

Enzymes

*Biological catalysts that increase reaction rates by decreasing activation energy* (this does not change the standard free energy of the reaction, however). Enzymes are almost always proteins; however, some types of RNA can act with catalytic activity. These are called ribozymes and are much less commonly encountered than protein catalysts. Protein enzymes are made of a minimum of 51 amino acids. There are thousands of enzymes- each very specific in the substrate it recognizes and the reaction it catalyzes. The vast majority of biochemical reactions are catalyzed by protein enzymes.

Enzyme inhibitor binding is either reversible or irreversible

*Reversible Inhibitors* can bind to enzymes through weak non-covalent interactions such as hydrophobic interactions, dipole-dipole interactions, and hydrogen bonds. *Irreversible Inhibitors* can bind to enzymes by making a covalent bond with some of its amino acid residues.

Factors Affecting Enzyme Activity

*Temperature*: high temperatures denature proteins (and therefore denature enzymes) while cold temperatures slow the reaction process. *pH*: environments outside the enzyme's *optimal pH* can make/break intra- and intermolecular bonds thereby changing the enzyme's shape and effectiveness.

Enzyme Regulation

- Feedback Inhibition - Allosteric Regulation - Phosphorylation - Zymogens

Two theories have been proposed to explain the specificity of enzyme action

- Lock & Key - Induced Fit

Classifications of Enzymes

- Oxidoreductases - Transferases - Hydrolases - Isomerases - Lyases - Ligases

*Allosteric enzymes contain* (A) Multiple subunits (B) Single chain (C) Two chains (D) Three chains

A

*Allosteric enzymes regulate the formation of products by* (A) Feedback inhibition (B) Non-competitive inhibition (C) Competitive inhibition (D) Repression-derepression

A

*An enzyme promotes a chemical reaction by* (A) Lowering the energy of activation (B) Causing the release of heat which acts as a primer (C) Increasing molecular motion (D) Changing the free energy difference between substrate and product

A

*Coenzyme is* (A) Often a vitamin (B) Always an inorganic compound (C) Always a protein (D) Often a metal

A

*Coenzymes are* (A) Heat stable, dialyzable, non protein organic molecules (B) Soluble, colloidal, protein molecules (C) Structural analogue of enzymes (D) Different forms of enzymes

A

*Enzymes are different from catalysts in* (A) Being proteinaceous (B) Not used up in reaction (C) Functional at high temperature (D) Having high rate of diffusion

A

*If the substrate concentration is much below the km of the enzyme, the velocity of the reaction is* (A) Directly proportional to substrate concentration (B) Not affected by enzyme concentration (C) Nearly equal to Vmax (D) Inversely proportional to substrate concentration

A

*In competitive enzyme activity inhibition* (A) The structure of inhibitor generally resembles that of the substrate (B) Inhibitor decreases apparent Km (C) Km remains unaffective (E) Inhibitor decreases Vmax without affecting Km

A

*In enzyme kinetics Vmax reflects* (A) The amount of an active enzyme (B) Substrate concentration (C) Half the substrate concentration (D) Enzyme substrate complex

A

*In enzyme kinetics, Km implies* (A) The substrate concentration that gives one half Vmax (B) The dissocation constant for the enzyme substrate comples (C) Concentration of enzyme (D) Half of the substrate concentration required to achieve Vmax

A

*Isoenzymes are* (A) Chemically, immunologically and electrophoretically different forms of an enzyme (B) Different forms of an enzyme similar in all properties (C) Catalysing different reactions (D) Having the same quaternary structures like the enzymes

A

*Penicillin exerts its effect on bacterial growth through inhibition of cell-wall synthesis. This antibiotic blocks the last step in the formation of the cell wall. Glycopeptide transpeptidase is a bacterial enzyme that catalyzes the cross-linking reaction between peptidoglycan molecules, which are the main constituents of the cell wall. Penicillin inhibits the enzyme by binding covalently to its active site. The penicillin-enzyme complex cannot react further.* This is an example of: A. irreversible inhibition. B. competitive inhibition. C. noncompetitive inhibition. D. uncompetitive inhibition.

A

*The Michaehis-Menten hypothesis* (A) Postulates the formation of an enzyme substrate complex (B) Enables us to calculate the isoelectric point of an enzyme (C) States that the rate of a chemical reaction may be independent of substrate concentration (D) States that the reaction rate is proportional to substrate concentration

A

*The active site of an enzyme is formed by* (A) R group of amino acids (B) NH2 group of amino acids (C) CO group of amino acids (D) Sulphur bonds which are exposed

A

*When [s] is equal to Km, which of the following conditions exist?* (A) Half the enzyme molecules are bound to substrate (B) The velocity of the reaction is equal to Vmax (C) The velocity of the reaction is independent of substrate concentration (D) Enzyme is completely saturated with substrate

A

*Which enzyme is concerned with transfer of electrons?* (A) Desmolase (B) Hydrolase (C) Dehydrogenase (D) Transaminase

A

Phosphorylation

A reversible post-translational modification performed by kinases; the most common mechanism of regulating protein function and transmitting signals throughout the cell In eukaryotes, the most commonly targeted amino acid residues are *serine*, *threonine*, and *tyrosine*.

Apoenzyme

An inactive enzyme without the cofactor

*An enzyme catalyzes the conversion of an aldose sugar to a ketose sugar would be classified as one of the* (A) Transferases (B) Isomerases (C) Oxido reductases (D) Hydrolases

B

*An inorganic ion required for the activity of an enzyme is known as* (A) Activator (B) Cofactor (C) Coenzyme (D) None of these

B

*Combination of apoenzyme and coenzyme produces* (A) Prosthetic group (B) Holoenzyme (C) Enzyme substrate complex (D) Enzyme product complex

B

*Enzymes are one of the key proteins in cells and facilitate chemical reactions. How does a typical enzyme catalyze a chemical reaction?* A. It eliminates the activation energy of a chemical reaction B. It reduces the activation energy of a chemical reaction C. It provides additional energy to the chemical reaction D. It contains a covalently bound substrate, which is needed for the chemical reaction

B

*In Lineweaver-Burk plot, the y-intercept represents* (A) Vmax (B) Km (C) 1/Vmax (D) 1/Km

B

*In competitive enzyme activity inhibition* (A) Apparent Km is decreased (B) Apparent Km is increased (C) Vmax is increased (D) Vmax is decreased

B

*In enzyme assays, all the following are kept constant except* (A) Substrate concentration (B) Enzyme concentration (C) pH (D) Temperature

B

*In reversible non-competitive enzyme activity inhibition* (A) Inhibitor bears structural resemblance to substrate (B) Inhibitor lowers the maximum velocity attainable with a given amount of enzyme (C) Km is increased (D) Km is decreased

B

*In which of the following types of enzyme water may be added to a C—C double bond without breaking the bond?* (A) Hydrolase (B) Hydratase (C) Hydroxylase (D) Esterase

B

*Isoenzymes can be characterized by* (A) Proteins lacking enzymatic activity that are necessary for the activation of enzymes (B) Proteolytic enzymes activated by hydrolysis (C) Enzymes with identical primary structure (D) Similar enzymes that catalyse different reaction

B

*Multiple forms of the same enzymes are known as* (A) Zymogens (B) Isoenzymes (C) Proenzymes (D) Pre-enzymes

B

*The following reaction is characteristic of what type of enzymes?* *2H2O2 → 2H2O + O2* (A) Peroxides (B) Catalase (C) Dehydrogenase (D) Copper containing oxidases

B

*The pH optima of most enzymes is* (A) Between 2 and 4 (B) Between 5 and 9 (C) Between 8 and 12 (D) Above 12

B

*What is the most correct definition of competitive inhibition?* A. Another enzyme completes the same reaction and uses the available reactant B. An inhibitor binds directly to the active site and prevents the reactant from binding C. A metal cofactor prevents the binding of the reactant by binding at the active site D. An inhibitor binds to the enzyme at a binding site, and prevents the enzyme from catalyzing the reaction

B

*Zymogen is a* (A) Vitamin (B) Enzyme precursor (C) Modulator (D) Hormone

B

Mixed Inhibition

Binds either at the enzyme or ES-complex

Uncompetitive Enzyme Inhibitor

Binds the ES-complex, but not the enzyme alone; does not allow the enzyme to move on to the "enzyme & product" stage ↓ Km ↓ Vmax

*'Lock' and 'Key' model of enzyme action proposed by Fisher implies that* (A) The active site is flexible and adjusts to substrate (B) The active site requires removal of PO4 group (C) The active site is complementary in shape to that of the substrate (D) Substrates change conformation prior to active site interaction

C

*Allosteric inhibition is also known as* (A) Competitive inhibition (B) Non-competitive inhibition (C) Feedback inhibition (D) None of these

C

*Allosteric inhibition* (A) Makes active site unifit for substrate (B) Controls excess formation and end product (C) Both (A) and (B) (D) None of these

C

*An allosteric enzyme influences the enzyme activity by* (A) Competiting for the catalytic site with the substrate (B) Changing the specificity of the enzyme for the substrate (C) Changing the conformation of the enzyme by binding to a site other than catalytic site (D) Changing the nature of the products formed

C

*An enzyme is a* (A) Carbohydrate (B) Lipid (C) Protein (D) Nucleic acid

C

*An organic substance bound to an enzyme and essential for the activity of enzyme is called* (A) Holoenzyme (B) Apoenzyme (C) Coenzyme (D) Isoenzyme

C

*Competitive inhibitors* (A) Decrease the Km (B) Decrease the Vmax (C) Increase the Km (D) Increase the Vmax

C

*Different isoenzymes of an enzyme have the same* (A) Amino acid sequence (B) Michaelis constant (C) Catalytic activity (D) All of these

C

*Enzymes accelerate the rate of reactions by* (A) Increasing the equilibrium constant of reactions (B) Increasing the energy of activation (C) Decreasing the energy of activation (D) Decreasing the free energy change of the reaction

C

*Enzymes which catalyse binding of two substrates by covalent bonds are known as* (A) Lyases (B) Hydrolases (C) Ligases (D) Oxidoreductases

C

*Which one of the following statements is not characteristic of allosteric enzymes?* (A) They frequently catalyze a committed step early in a metabolic pathway (B) They are often composed of subunits (C) They follow Michaelis-Menten kinetics (D) They frequently show cooperativity for substrate binding

C

ΔG˚ is the free energy change of a reaction under standard conditions. In which ONE of the following ways does ΔG˚' differ from ΔG˚? A. ΔG˚' is determined when the temperature is 273 degrees Kelvin B. A positive value for ΔG˚' indicates a favorable reaction C. ΔG˚' ignores the concentration of water, and allows it to be treated as a constant D. ΔG˚' is calculated by dividing ∆Go by two E. ΔG˚' is calculated assuming the [H+] = 5 X 10-7 M

C

Isomerases

Change of the molecular form of the substrate to form an isomer For example: Racemase

Holoenzyme

Complete enzyme with cofactor

*A competitive inhibitor of an enzyme has which of the following properties?* (A) It is frequently a feedback inhibitor (B) It becomes covalently attached to an enzyme (C) It decreases the Vmax (D) It interferes with substrate binding to the enzyme

D

*An allosteric enzyme is generally inhibited by* (A) Initial substrate of the pathway (B) Substrate analogues (C) Product of the reaction catalysed by allosteric enzyme (D) Product of the pathway

D

*Covalent modification of an enzyme generally requires a* (A) Hormone (B) cAMP (C) Protein kinase (D) All of these

D

*Enzymes are required in traces because they* (A) Have high turnover number (B) Remain unused at the end of reaction and are re-used (C) Show cascade effect (D) All correct

D

*Factor(s) affecting enzyme activity* (A) Concentration (B) pH (C) Temperature (D) All of these

D

*In competitive inhibition which of the following kinetic effect is true?* (A) Decreases both Km and Vmax (B) Increases both Km and Vmax (C) Decreases Km without affecting Vmax (D) Increases Km without affecting Vmax

D

*In competitive inhibition, the inhibitor* (A) Competes with the enzyme (B) Irreversibly binds with the enzyme (C) Binds with the substrate (D) Competes with the substrate

D

*In reversible non-competitive enzyme activity inhibition* A. Vmax is increased B. Km is increased C. Km is decreased D. Concentration of active enzyme is reduced

D

*Km value of enzyme is substrate concentration at* (A) ½ Vmax (B) 2 Vmax (C) ½ Vmax (D) 4 Vmax

D

*Lineweaver - Burk double reciprocal plot is related to* (A) Substrate concentration (B) Enzyme activity (C) Temperature (D) Both (A) and (B)

D

*Some enzymes experience a decrease in activity proportional to the concentration of their products. This phenomenon could be an example of what process?* A. Feedback inhibition B. Non-competitive inhibition C. Allosteric activation D. Both A and B

D

*Template/lock and key theory of enzyme action is supported by* (A) Enzymes speed up reaction (B) Enzymes occur in living beings and speed up certain reactions (C) Enzymes determine the direction of reaction (D) Compounds similar to substrate inhibit enzyme activity

D

*The Michaelis constant, Km is* (A) Numerically equal to ½ Vmax (B) Dependent on the enzyme concentration (C) Independent of pH (D) Numerically equal to the substrate concentration that gives half maximal velocity

D

*The chemical forces that bind most coenzymes and substrates to enzymes such as LDH are* (A) Hydrogen bonds (B) Peptide bonds (C) Coordinate bonds (D) Covalent bonds

D

*The conversion of threonine to isoleucine is a five-step enzymatic pathway. The end product, isoleucine, fits into the allosteric site of the enzyme at step 1, preventing its normal function. This is an example of:* A. Enzyme specificity. B. Competitive inhibition. C. Enzyme enhancement. D. Feedback inhibition.

D

*Vmax of an enzyme may be affected by* (A) pH (B) Temperature (C) Non-competitive inhibitors (D) All of these

D

*When the velocity of an enzymatic reaction equals Vmax, substrate concentration is* (A) Half of Km (B) Equal to Km (C) Twice the Km (D) Far above the Km

D

Enzyme Kinetics

During an enzymatic reaction, the reaction is first dependent on the concentration of the substrate [a first order reaction]. Over time, however, the reaction reaches saturation and substrate concentration no longer matters [a zero order reaction]. (this is true for most enzymes, but not all)

Allosteric Regulation

Effector molecules bind to enzymes Acts to change the shape of an enzyme and either- allow for substrate binding or prevent substrate binding

Feedback Inhibition

End product blocks later enzyme reactions; allows for pathway regulation

Structure of Enzymes

Enzymes are generally globular proteins, each with a unique tertiary structure that recognizes and binds to specific groups of molecules.

Enzyme Catalyzed Reactions

Enzymes are not destroyed or consumed in reaction E + S ⇌ ES complex → E + P

Hydrolases

Hydrolysis of a substrate; catabolic For example: Phosphatase, Protease, Phosphodiesterase, Chymotrypsin

Zymogen

Inactive enzyme precursors; typically seen in the stomach

Ligases

Joining of two molecules by the formation of new bonds; anabolic For example: DNA Ligase; DNA Polymerase

Competitive Enzyme Inhibitor

Inhibitor molecules bind with the *active site* of an enzyme molecule; prevents the binding of substrates ↑ Km Vmax stays the same

Vmax

Maximum velocity at which the reaction occurs

Non-competitive Enzyme Inhibitor

Molecule binds with *allosteric site* of an enzyme, changing the active site; the substrate cannot bind with the changed active site Km stays the same ↓ Vmax

Cofactors

Non-protein chemical compound or metallic ion that is required for a protein's biological activity to happen; can be considered "helper molecules" that assist in biochemical transformations. Can be subclassified as either inorganic ions or complex organic molecules called *coenzymes*. For example: K+, Mg2+

Lyases

Nonhydrolytic removal or addition of a group to a substrate For example: Decarboxylase

Coenzymes

Organic molecules that are required by certain enzymes to carry out catalysis (a specific type of cofactor); often derivatives of vitamins; typically function as intermediate carriers of electrons, specific atoms, or functional groups that are transferred in the overall reaction For example: NAD+, FAD, coenzyme A, biotin

Michaelis-Menten Enzyme Kinetics

Plot initial reaction velocity vs. substrate concentration

Km

The Michaelis-Menten constant; substrate concentration at which the reaction rate is half of its maximal value [@ half Vmax]; not a direct measurement of affinity

Lock & Key Model

The enzyme active site is complementary in conformation to the substrate, so that enzyme and substrate recognize one another; substrate fits into active site perfectly

Induced Fit Model

The substrate binds to the active site and induces/causes a change in shape of the active site so that it is a complimentary fit; enzyme undergoes conformational changes This model explains why some enzymes can act on more than one substrate; for a number of enzyme reactions, this is more true to what occurs.

Transferases

Transfer of a specific group (phosphate, methyl, etc.) from one substrate to another For example: Kinase

Oxidoreductases

Transfer of hydrogen and oxygen atoms or electrons from one substrate to another; catalyze oxidation/reduction reactions For example: Oxidase, Dehydrogenase

Enzyme Inhibitors

Very specific & work at very low concentrations (so we want a low Km); block enzymes but don't destroy them There are many types of enzyme inhibitors- the two main types are competitive enzyme inhibitors and non-competitive enzyme inhibitors.

Lineweaver-Burk Plots

Widely used to determine important terms in enzyme kinetics, such as Km and Vmax

The progress of a reaction can be depicted in a *Reaction Coordinate Diagram*

ΔG˚ŧ: Free Energy of Activation aka the *activation energy* required to proceed in the reaction ΔG˚: Standard free energy of the reaction; this value allows prediction as to whether a reaction will be spontaneous in the forward or reverse direction (or whether it is at equilibrium). ΔG° > 0 the reaction is endergonic & not spontaneous in the forward direction ΔG° < 0 the reaction is exergonic & spontaneous in the forward direction; if a reaction is spontaneous, it is energetically favorable and does not require the addition of external energy to proceed