CH19: ENZYMES & VITAMINS

OSU BIOPHRM 3311

______ is the energy associated with a chemical reaction

Gibbs Free Energy (G)

_______ is another word for reactants

Substrates

Transitional state is an ______

intermediate state

Reaction occurrence is determined by the _______-

change in G (gibbs free energy)

Activation energy is the _____

input of energy

G_R definition

Free energy of the reactants

G^‡ definition

free energy of the transition state; the peak of the energy curve

G_P definition

Free energy of the products.

Ea = G^‡ - G_R explain

Activation energy; energy required for a reaction to occur.

∆G = G_P - G_R explain

Change in free energy between reactants and products; indicates whether the reaction is spontaneous (ΔG < 0) or non-spontaneous (ΔG > 0) [have to input energy for the rxn to occur!].

Catalyst definition

substance that increases the rate of rxn by lowering the activation energy and is not consumed or changed after catalyzing a rxn

what changes in a catalyzed rxn?

rate increase and G‡ (free energy of the transition state) decrease

what is not changed in a catalyzed rxn?

G_R, G_P, and ∆G and the catalyst

enzymes definition

Are biological catalysts that within the mild conditions of temperature, pH, and pressure of the cells, carry out biochemical reactions at amazing high rates

Nearly all enzymes are _____and a few are RNA called______

• proteins

• ribozymes

enzymes are characterized by a remarkable efficiency and _______, meaning that certain enzyme recognizes ________.

• specificity (substrate and stereoisomerism)

• only certain substances

Some enzymes require non-protein part, called _______ (such as mineral and vitamin) to function.

co-factors

conjugated proteins definition

proteins that incorporate one or more non–amino acid units (co-factor) in its structure (compare to simple proteins)

turnover number definition

the number of substrate molecules converted into product by an enzyme molecule in a unit time - how catalytic activity of an enzyme is measured

the greater the turnover number, the greater the _____

efficiency (more cleaved bonds per second)

papain enzyme reaction catalyzed

hydrolysis of peptide bonds

ribonuclease enzyme reaction catalyzed

hydrolysis of phosphate ester link in ribonucleic acid (RNA)

kinase enzyme reaction catalyzed

transfer of phosphoryl group b/t substrates

acetylcholinesterase enzyme reaction catalyzed

deactivation of the neurotransmitter acetylcholine

carbonic anhydrase enzyme reaction catalyzed

converts CO2 to HCO3-

catalase enzyme reaction catalyzed

decomposition of H2O2 to H2O + O2

enzyme specificity definition

each enzyme works on one or a few substrates only

thrombin enzyme reaction catalyzed

recognizes Leu-Val-Pro-Arg-Gly-Ser peptide sequence and cuts only at the Arg-Gly bond

chymotrypsin enzyme reaction catalyzed

hydrolysis of peptide bonds of a few amino acids: Trp, Tyr, and Phe

carboxypeptidase A enzyme reaction catalyzed

removes the C-terminal amino acid from a wide variety of peptides

Enzymes are specific w/ respects to stereochemistry, which refers to ________

molecules that different only in terms of overall structure shape

If a substrate is chiral, an enzyme usually catalyzes the reaction of ___________

only one of the pair of enantiomers; either L od D but not both

Only one _______ fits the enzyme in such a way that the reaction can occur

enantiomer

An enzyme may require a ______ to function.

cofactor

A _____ is a non-protein part of an enzyme that is essential to the enzyme’s catalytic activity

cofactor

A cofactor can be a ______ or an ________

• metal ion (e.g., Mg²⁺, Zn²⁺)

• organic molecule (e.g. heme, vitamin)

Myoglobin contains an _______ cofactor, which facilitates oxygen binding and transport

iron-containing heme

Most enzyme have their names end with ______

-ase (enzymes w/out this, were discovered prior to this naming rule)

the first part of nomenclature of enzymes IDs the ________

substrate on which the enzyme operates

the second part of the nomenclature of enzymes is the enzyme’s ________

class name that describes the reaction

aldose reductase function

Catalyzes the reduction of aldehyde to alcohol

aldose reductase requires the vitamin-derived coenzyme ______ to function

nicotinamide adenine dinucleotide phosphate (NADPH)

An enzyme provides its specific substrate with an active site to form an ________ that is necessary for its catalyst properties and the formation of the products

enzyme-substrate complex

_______ lowers the activation energy, in order for the reaction to occur

enzyme-substrate complex

Substrates are the substances (called reactants in chemistry) on which _______

enzymes act upon

Active site is the specific region in an enzyme with the ____ and _____ necessary to bind a substrate and catalyze the reaction.

• specific shape

• chemical makeup

The shape of an enzyme's active site is _______ to the shape of its specific substrate or substrates

complementary

enzymes provide proximity effect: _______

Bring substrate(s) and catalytic site together

enzymes provide orientation effect: _____

Hold substrate(s) at the exact distance and orientation necessary for the reaction

enzymes provide catalytic effect: _______

Provide acidic, basic, or other types of groups required for catalysis; often involves transient covalent bond formation and breakage

enzymes provide energy effect: ______

Lower the energy barrier by inducing strain in bonds in the substrate molecule

_______ of enzyme triggers the release of the substrate

Shape change

The three amino acids that form the active site of chymotrypsin are not adjacent in its _______

peptide sequence (primary structure)

Chymotrypsin is a protease consisting of ___ amino acid residues

241

active site of chymotrypsin involves only three amino acids: _______ {numbering indicates the distance in the primary structure}

Asp (102) which is positioned close to His (57) and Ser (195)

chymotrypsin’s substrates are specific amino acid residues with ____ side chains in proteins

aromatic

Chymotrypsin is an enzyme that cleaves peptide bonds on the carboxyl side of _____

Phe, Tyr, and Trp (aromatic)

Enzyme-catalyzed reactions begin with _____ into the active site to form an enzyme-substrate complex

migration of the substrate

first step of how enzymes work

substrate anhors to the active site

second step of how an enzyme works

formation of enzyme-substrate intermediate

in an enzyme-substrate interaction, the _______ hold the substrate in place via weak and non-covalent intermolecular forces, while amino acids that could act as ______ carry out the reaction.

• nonpolar amino acids

• acids or bases

in an enzyme-substrate interaction, it is necessary to ______ by inducing strain in bonds in the unstable intermediate that facilitates the ______.

• lower the energy barrier

• cleavage

in an enzyme-substrate interaction, the enzyme forms an ______ with the substrate at the transitional state but ______ after the reaction.

• intermediate complex

• remains unchanged

Two models were proposed to represent the mechanism by which substrates and enzymes interact: ______

• lock-and-key

• induced fit

in the lock-and-key model, the lock is the ____, and the key is the _____.

• enzyme

• substrate

Only the correctly sized key (substrate) fits into the keyhole (active site) of the lock (enzyme)

In lock-and-key model, the active site is _____; an enzyme only binds the substrates that _______ its active site. The shape _____ upon the binding

• rigid

• exactly fit

• does not change

The_____ assumes that the substrate plays a role in determining the final shape of the enzyme and that the enzyme is partially flexible

induced-fit model

A change in the conformation of an enzyme in response to substrate binding renders _______

the enzyme catalytically active

_____ promotes an enhanced fit at the active site of the enzyme hexokinase, optimizing enzyme-substrate interactions

Glucose

If the amount of the enzyme is kept constant and the substrate concentration is then gradually increased, the reaction velocity will _____

increase until it reaches a maximum

at the point of maximum substrate concentration, increases in substrate concentration will ______

not increase the velocity because all enzymes are in use (plateau occurs).

When the substrate is present in an excess amount increases in _______ increases the rate of reaction with a linear relationship

enzyme concentration

At low temperature, the rate of an enzyme-catalyzed reaction _____ as the temperature is raised

increases

the reaction rate temperature at which enzymes work most optimally within the human body is _____

37C

the reaction rate abruptly declines with further increase of temperature (above 40C) because most enzymes become ________.

denatured

3D structure of proteins is affected by the ______

increasing temperature

The pH range at which the enzyme is most active is known as the _____

optimum pH

Extremely _______ values generally result in complete loss of activity

high or low pH

Changes in pH alter the charges of ______, results in altered shape and function of enzymes

ionizable groups

The _________ varies from one enzyme to another

optimum pH value

substrate concentration change has what type of graph curve?

hyperbolic curve that plateaus

enzyme concentration change has what type of graph curve?

linear curve

temperature change (enzyme activity) has what type of graph curve?

asymmetrical bell-shaped curve

pH change (enzyme activity) has what type of graph curve?

symmetrical bell-shaped curve

Enzymes are divided into six major classes according to the type of reaction catalyzed:

• Oxidoreductases

• Transferases

• Hydrolases

• Lyases

• Isomerases

• Ligases

oxidoreductases (major enzyme class) function

Catalyze oxidation-reduction reactions of substrate molecules, most commonly by addition or removal of oxygen or hydrogen

Because oxidation and reduction must occur together, these oxidoreductases require _____ that are reduced or oxidized as the substrate is oxidized or reduced, respectively.

coenzymes

major oxidoreductases subclasses:

• oxidases

• reductases

• dehydrogenases

examples of oxidoreductases coenzmes

NADPH and NADH

oxidase enzyme function

use oxygen to oxidize its substrates

reductases enzyme function

add hydrogen atoms to substrates and catalyze reduction

dehydrogenases enzyme function

transfers/remove hydrogens from a substrate to electron carriers such as NAD+ and FAD and catalyze oxidation

product type of oxidase reactions

oxidized product

product type of reductase reactions

reduced product

product type of dehydrogenase reaction

oxidized reaction

common cofactors of oxidase

FAD, FMN, Cu2+

common cofactors of reductase

NADH, NADPH

common cofactors of dehydrenase

NAD+, FAD

oxidase’s electron acceptor

O2

reductase’s electron acceptor

various (not O2)