Name: enzyme action and inhibition
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 34 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.1

Enzymes

Gli enzimi sono catalizzatori biologici che accelerano la velocità delle reazioni biochimiche. La maggior parte degli enzimi sono proteine, costituite da amminoacidi;mentre alcuni sono molecole di RNA, note come ribozimi. Gli enzimi agiscono abbassando l'energia di attivazione di una reazione, aumentando così la velocità di reazione.

Possono catalizzare sia la sintesi che la rottura dei legami chimici, ma non influenzano la direzione o l'equilibrio della reazione. Ogni enzima si lega ad uno specifico reagente, detto substrato, e catalizza una particolare reazione. Il substrato si lega ad una regione catalitica distinta dell'enzima, detta sito attivo, attraverso interazioni, come forze intermolecolari e legami covalenti transitori, risultando in un complesso enzima-substrato.

Questo legame è altamente specifico a causa della complementarità conformazionale richiesta fra l'enzima e il suo substrato. Pertanto, un particolare enzima può solo catalizzare reazioni specifiche in base alla sua conformazione. Un modo semplice per comprendere la formazione complessa è il modello chiave-e-serratura, il quale ipotizza che il substrato si adatti al sito attivo dell'enzima, come una chiave"che si inserisce nella corrispondente serratura.

Tuttavia, un altro modello, quello di adattamento indotto, tiene conto della natura dinamica del complesso. Questo modello afferma che, quando un substrato si lega, induce piccoli cambiamenti conformazionali, risultando in un adattamento più stretto, che favorisce la reazione. L'energia di attivazione per una reazione, può essere ridotta con vari metodi.

I meccanismi comuni includono l'induzione di cambiamenti conformazionali nel substrato, i quali consentono di rompere più facilmente un legame o di portare in stretta vicinanza i gruppi reattivi di due substrati, promuovendo così la formazione del legame. L'attività enzimatica può essere soppressa temporaneamente o permanentemente da molecole naturali o sintetiche dette inibitori. Per esempio, un inibitore competitivo compete con il substrato per legarsi al sito attivo dell'enzima, prevenendo così il legame del substrato.

D'altra parte, un inibitore non competitivo si lega ad un'altra posizione sull'enzima, e questo causa un cambiamento conformazionale nel sito attivo, riducendo l'attività catalitica dell'enzima.

enzimi

All'interno degli organismi viventi, gli enzimi agiscono come catalizzatori per molte reazioni biochimiche coinvolte nel metabolismo cellulare. Il ruolo degli enzimi è ridurre le energie di attivazione delle reazioni biochimiche formando complessi con i suoi substrati. L'abbassamento delle energie di attivazione favorisce un aumento della velocità delle reazioni biochimiche.

Le carenze enzimatiche possono spesso tradursi in malattie potenzialmente letali. Ad esempio, un'anomalia genetica che porta alla carenza dell'enzima G6PD (glucosio-6-fosfato deidrogenasi) influisce negativamente sulla via metabolica che fornisce NADPH alle cellule.

Un’interruzione in questo percorso metabolico può ridurre il glutatione nei globuli rossi causando danni ad altri enzimi e proteine come l’emoglobina. L'eccessiva metabolizzazione dell'emoglobina aumenta il livello di bilirubina, il che porta all'ittero, una condizione che può diventare grave. Pertanto, le persone che soffrono di carenza di G6PD devono evitare determinati alimenti e medicinali contenenti sostanze chimiche che potrebbero provocare danni ai globuli rossi carenti di glutatione.

Funzione e struttura dell'enzima

Gli enzimi sono raggruppati in diverse classi in base alla specifica funzione che svolgono. Ad esempio, le ossidoreduttasi sono coinvolte nelle reazioni redox, mentre le transferasi catalizzano il trasferimento di gruppi funzionali. La formazione del legame con l'idrolisi dell'ATP richiede ligasi, mentre le reazioni di idrolisi e la formazione di doppi legami sono catalizzate rispettivamente da idrolasi e liasi. Gli enzimi isomerasi solitamente catalizzano le reazioni di isomerizzazione.

Gli enzimi generalmente possiedono siti attivi. Si tratta di regioni specifiche sulla molecola con una conformazione che favorisce il legame dell'enzima con un substrato specifico (una molecola reagente) per formare un complesso enzima-substrato o l'intermedio di reazione.

Due modelli, il modello lock-and-key e il modello di adattamento indotto, tentano di spiegare il funzionamento di un sito attivo (Figura 1). L'ipotesi più semplicistica del lock-and-key suggerisce che il sito attivo e la forma molecolare del substrato sono complementari, incastrandosi come una chiave in una serratura (Figura 1a). D'altra parte, l'ipotesi dell'adattamento indotto suggerisce che la molecola dell'enzima è flessibile e cambia forma per accogliere un legame con il substrato (Figura 1b).

Tuttavia, sia il modello lock-and-key che il modello di adattamento indotto tengono conto del fatto che gli enzimi possono legarsi solo con substrati specifici e catalizzare solo una particolare reazione.

<img alt="Eq1" src="/files/ftp_upload/11384/11384_Image_1.png" style="text-align: center; height: 300px;" />

Figura 1 (a) Secondo il modello lock-and-key, la forma del sito attivo di un enzima si adatta perfettamente al substrato. (b) Secondo il modello di adattamento indotto, il sito attivo è alquanto flessibile e può cambiare forma per legarsi al substrato.

Inibitori enzimatici

L'attività degli enzimi può anche essere interrotta dal processo di inibizione enzimatica. Esistono diversi tipi comuni di inibizione enzimatica.

Durante l'inibizione competitiva, una molecola (naturale o sintetica) diversa dal substrato si lega direttamente al sito attivo dell'enzima. La somiglianza strutturale e chimica dell'inibitore con il substrato facilita il suo legame al sito attivo. Tali inibitori competitivi, quindi, competono con i substrati, impedendo loro di legarsi all'enzima. Molto spesso, l’aumento della concentrazione del substrato può sopprimere gli effetti dell’inibizione competitiva.

Nell'inibizione non competitiva, una molecola (naturale o sintetica) si lega a una regione allosterica (altra) dell'enzima, diversa dal suo sito attivo. Il legame dell'inibitore provoca un cambiamento conformazionale nel sito attivo dell'enzima, con conseguente diminuzione della capacità dell'enzima di catalizzare la reazione. A differenza dell’inibizione competitiva, un aumento della concentrazione del substrato non mitiga gli effetti inibitori dell’inibizione non competitiva.

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/12-7-catalysis">Openstax, Chemistry 2e, Section 12.7: Catalysis.</a>

Enzimi

All'interno degli organismi viventi, gli enzimi agiscono come catalizzatori per molte reazioni biochimiche coinvolte nel metabolismo cellulare. Il ruolo degli enzimi è quello di ridurre il ...

Education

Didattica

JoVE Core

Chimica

Chemical Kinetics

Cinetica Chimica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

enzyme action and inhibition

Enzymes are biological catalysts that speed up many reactions in living organisms. They help control cellular metabolism by lowering the activation energy needed for biochemical reactions. When activation energy drops, reaction rates can increase.

Enzyme problems can also lead to serious disease. A genetic abnormality that causes G6PD, or glucose-6-phosphate dehydrogenase, deficiency can disrupt the pathway that supplies NADPH to cells. In red blood cells, this can reduce glutathione and damage other enzymes and proteins such as hemoglobin. As hemoglobin is broken down too much, bilirubin builds up and can cause jaundice. People with G6PD deficiency often need to avoid certain foods and medicines that may trigger damage to their red blood cells.

Enzymes are grouped by the reactions they carry out. Oxidoreductases take part in redox reactions. Transferases move functional groups from one molecule to another. Ligases form bonds and use ATP hydrolysis, while hydrolases drive hydrolysis reactions and lyases help form double bonds. Isomerases catalyze isomerization reactions.

Most enzymes have an active site, which is the region that binds a specific substrate, or reactant molecule. The enzyme and substrate form an enzyme-substrate complex, which acts as the reaction intermediate. Two models explain how this binding works. In the lock-and-key model, the active site and substrate fit together like a key in a lock. In the induced-fit model, the enzyme changes shape to fit the substrate. Both models explain enzyme specificity, meaning an enzyme usually binds only certain substrates and catalyzes only one particular reaction.

Enzyme activity can also be blocked by inhibitors. In competitive inhibition, an inhibitor that is similar to the substrate binds directly to the active site and competes with the substrate. Increasing substrate concentration can often reduce this effect. In non-competitive inhibition, the inhibitor binds to an allosteric site, which is a different region of the enzyme. This binding changes the shape of the active site and lowers enzyme activity. Unlike competitive inhibition, adding more substrate does not fix non-competitive inhibition.

Enzyme Action and Inhibition

reaction rate

Measuring Reaction Rates

Measuring reaction rates shows how fast reactants are used up or products are formed over time. The rate of reaction is the change in the amount of a reactant or product per unit time. Because of that, scientists measure how a property changes as the reaction proceeds and connect that change to reactant or product amounts. For reactions that make or use a gas, the rate can be followed by changes in volume or pressure. A rate expression is the mathematical way to write how concentration changes...

measuring reaction rates

Tracking Reaction Kinetics with Light and Pressure

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concentration and rate law

Reaction Order from Concentration

Reaction order from concentration shows how reactant amounts affect reaction rate. Rate laws, also called differential rate laws or rate equations, are mathematical expressions that connect reaction rate with the concentrations of reactants. For a generic reaction a A + b B → products, the rate law can be written using the reactant concentrations [A] and [B] and a rate constant k. In a rate law, k is the rate constant for one specific reaction at one specific temperature. The exponents m and n...

determining order of reaction

Finding Rate Law from Initial Rates

Rate laws connect the speed of a chemical reaction to the concentration of its reactants. The rate constant, k, and the reaction orders are not taken from the chemical equation. They are found experimentally by watching how the rate changes when the reactant concentrations change. One common way to do this is the method of initial rates. In this method, several trials are run with different starting concentrations. The measured rates are then compared to determine the reaction orders and,...

the integrated rate law: the dependence of concentration on time

the-integrated-rate-law-the-dependence-of-concentration-on-time

Reading Reaction Progress with Integrated Laws

Integrated rate laws connect reaction concentration to time. They help show how much reactant or product is present after a certain period. They can also help estimate how long a reaction must run to reach a given extent. A common use is finding how long radioactive material must be stored before its radioactivity drops to a safe level. These equations come from the differential rate law after it is integrated with respect to time. For a first-order reaction, where rate = k[A], the integrated...

half-life of a reaction

Reaction Half-Life in Kinetics

Reaction half-life is the time needed for half of a reactant to be consumed. After each half-life, half of the remaining reactant is used up. A hydrogen peroxide decomposition example shows this pattern clearly. The concentration drops from 1.000 M to 0.500 M in the first 6.00 hours, then to 0.250 M in the next 6.00 hours, and then to 0.125 M in the third period. Each later 6.00-hour interval removes half of what is left. For first-order reactions, the half-life does not depend on the reactant...

temperature dependence on reaction rate

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arrhenius plots

Determining Activation Energy from Temperature

The Arrhenius equation links activation energy and the rate constant, k, for chemical reactions. It is written as k = Ae−Ea/RT. In this equation, R is the ideal gas constant with a value of 8.314 J/mol·K, T is the temperature on the kelvin scale, Ea is the activation energy in J/mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor depends on collision frequency and the orientation of reacting molecules. Experimental kinetic data can be used to find the activation...

reaction mechanisms

Elementary Steps in Reaction Mechanisms

Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps. Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced...

rate-determining steps

Reaction Bottlenecks in Chemical Kinetics

Reaction bottlenecks help explain rate-determining steps in multistep reaction mechanisms. In a mechanism with several elementary steps, one step is often much slower than the others. That slowest step is called the rate-limiting step, or rate-determining step. A reaction cannot move faster than its slowest step, so this step controls the overall reaction rate. The idea is like traffic on a four-lane freeway with one lane closed. Cars can only move as fast as the bottleneck allows. In the same...

catalysis

Catalyst Action in Reaction Rates

Catalysts change the rate of a chemical reaction by giving it a faster path. A catalyst increases reaction rate without being consumed during the process. Its effect is often explained with reaction mechanisms and energy diagrams. For an endothermic reaction, the reactants and products start and end at the same energies in both cases. The uncatalyzed path is a one-step mechanism with one transition state. The catalyzed path follows a different mechanism with two transition states and a lower...

enzymes

Enzymes are biological catalysts that speed up many reactions in living organisms. They help control cellular metabolism by lowering the activation energy needed for biochemical reactions. When activation energy drops, reaction rates can increase. Enzyme problems can also lead to serious disease. A genetic abnormality that causes G6PD, or glucose-6-phosphate dehydrogenase, deficiency can disrupt the pathway that supplies NADPH to cells. In red blood cells, this can reduce glutathione and...

key terms and concepts

Explore how enzymes speed reactions, how active sites work, and how competitive and non-competitive inhibitors affect enzyme activity in high school biology.

Enzyme Action and Inhibition | JoVE K12

chemistry

JoVE Core: Chimica illustra concetti chiave di chimica attraverso video animazioni concise e di immediata comprensione. Inoltre, i video della sezione "scienziati in azione" mostrano procedure metodologiche realizzate da ricercatori provenienti dai migliori laboratori internazionali.

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

3.2.9-12.E

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.f

Arizona Science Standards

High School

HS.P1

HS.P1U1.3

Next Generation Science Standards

Matter and Its Interactions

HS-PS1-4

HS-PS1-5

Florida Department of Education

SC.912.P.12

SC.912.P.12.12

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Enzyme Action and Inhibition

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