Name: substrate effects on enzyme rates
Uploaded: 2019-08-01T09:12:17.000Z
Duration: 1 min 19 s
Channel: JoVE

- [Instructeur] Dans une cellule,des conditions environnementales différentes,comme la concentration du substrat,peuvent grandement influencer la vitesse à laquelleles enzymes catalysent les réactions chimiques. 
Ce processus, appelé cinétique enzymatique,peut être représenté graphiquementavec la vitesse de réaction sur l'axe des yet la condition variable, la concentrationdu substrat dans cet exemple, sur l'axe des x. 
La vitesse de réaction est la vitesseà laquelle le produit se forme etpeut atteindre une vitesse maximale, la V max, lorsquetoutes les enzymes disponiblessont liées aux substrats. 
Par conséquent, la vitesse de réaction est limitée parla concentration des enzymes disponibles pour la liaison. 
Ce graphique peut également révéler la vitesseà laquelle la vitesse de réaction augmente. 
La variable est réfléchie par Km,la concentration du substrat à la moitié de V max,et représente l'affinité,ou la tendance de l'enzyme à se lier au substrat. 
Une enzyme ayant un faible Km a une affinité élevéeparce qu'elle atteint sa vitesse de réaction maximaleà une faible concentration dans le substrat. 
De même, un Km élevé correspond à une faible affinité,car l'enzyme a besoin d'une plus grande concentrationde substrat pour maximiser sa vitesse de réaction.

Johnson, Kenneth A. « Un siècle d’analyse cinétique enzymatique, de 1913 à 2013. » FEBS Letters 587, n° 17 (2 septembre 2013) : 2753-66. [<a href="https://doi.org/10.1016/j.febslet.2013.07.012">Source</a>]
Suarez, R. K., J. F. Staples, J. R. B. Lighton et T. G. West. « Relations entre les capacités de flux enzymatiques et les taux de flux métaboliques : réactions hors équilibre dans la glycolyse musculaire. » Actes de l’Académie nationale des sciences des États-Unis d’Amérique 94, n° 13 (24 juin 1997) : 7065-69. [<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC21285/.">Source</a>]

cinétique enzymatique

Enzyme Kinetics

Les enzymes accélèrent les réactions en abaissant l’énergie d’activation des réactifs. La vitesse à laquelle l’enzyme transforme les réactifs en produits est appelée la vitesse de réaction. Plusieurs facteurs influent sur la vitesse de réaction, y compris le nombre de réactifs disponibles. La cinétique enzymatique est l’étude de la façon dont une enzyme modifie la vitesse d’une réaction.
 
 Les scientifiques étudient généralement la cinétique enzymatique avec une quantité fixe d’enzymes dans l’environnement contrôlé d’un tube à essai. Lorsque plus de réactif, ou de substrat, est ajouté à une quantité fixe d’enzyme, la vitesse de la réaction augmente car l’enzyme peut fabriquer davantage de produit. Par conséquent, lorsque la vitesse de réaction est représentée graphiquement en fonction de la concentration du substrat, l’augmentation du substrat augmente la vitesse de réaction. Cependant, une fois que tous les sites actifs de l’enzyme sont occupés, la vitesse de réaction plafonne. La concentration du substrat à laquelle la vitesse de réaction maximale est atteinte est appelée Vmax. Le nombre de molécules d’enzyme présentes limite Vmax. Si on augmente la quantité d’enzyme, Vmax augmente, mais l’ajout de plus de substrat n’a aucun effet.
 
 Le graphique de la vitesse de réaction par rapport à la concentration du substrat peut révéler d’autres caractéristiques importantes de la cinétique d’une enzyme. La concentration de substrat à laquelle la vitesse de réaction est à mi-chemin de Vmax (c.-à-d. ½ Vmax) est appelée la constante de Michaelis (Km). Km est une représentation de l’affinité entre une enzyme et un substrat. Les enzymes avec un Km inférieur nécessitent moins de substrat pour atteindre Vmax et ont donc une affinité plus élevée pour leur substrat. Fait intéressant, pour de nombreuses enzymes, la valeur de Km est très proche de la concentration cellulaire du substrat. Près de Km, de légères variations dans la concentration du substrat peuvent avoir un impact significatif sur la vitesse de réaction, de sorte que de petites variations dans la disponibilité du substrat cellulaire peuvent avoir un impact sur la fonction d’une voie biologique entière.
 
 Toutes les enzymes ne produisent pas le graphe de vitesse-substrat en forme d’hyperbole connu sous le nom de cinétique de Michaelis Menten. La cinétique de Michaelis Menten suppose que l’enzyme catalyse un seul substrat. Les enzymes qui sont régulées allostériquement ont plusieurs sites actifs et ont tendance à produire un graphique de forme sigmoïde lorsque la vitesse de réaction est tracée par rapport à la concentration du substrat.

Cinétique enzymatique

Les enzymes accélèrent les réactions en diminuant l’énergie d’activation des réactifs. La vitesse à laquelle l’enzyme transforme les réactifs en produits est appelée ...

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substrate effects on enzyme rates

Enzyme kinetics examines how enzymes change the rate of a reaction. Enzymes speed reactions by lowering the activation energy of the reactants. The reaction rate is the speed at which an enzyme turns reactants into products, and it can be affected by several factors, including the number of available reactants.

Scientists often study enzyme kinetics in a test tube with a fixed amount of enzyme. When more substrate, or reactant, is added, the reaction rate increases because the enzyme can make more product. If the reaction rate is graphed against substrate concentration, the curve rises as substrate increases.

The rate does not keep rising forever. Once all of the enzyme's active sites are occupied, the reaction rate levels off. The substrate concentration at which the maximum rate is reached is called V max. V max is limited by the number of enzyme molecules present. If more enzyme is added, V max increases, but more substrate alone has no further effect.

A rate-versus-substrate graph can also show other features of enzyme behavior. The substrate concentration at which the reaction rate is halfway to V max is called the Michaelis constant, or K m. K m shows the affinity between an enzyme and its substrate. A lower K m means less substrate is needed to reach V max, so the enzyme has a higher affinity.

For many enzymes, K m is close to the substrate concentration in cells. Near K m, even small changes in substrate concentration can strongly affect the reaction rate. That means small shifts in cellular substrate availability can influence the activity of an entire biological pathway.

Not all enzymes follow Michaelis Menten kinetics. This model assumes that an enzyme acts on a single substrate. Enzymes regulated allosterically have multiple active sites and usually produce a sigmoid-shaped graph instead of a hyperbolic one when reaction rate is plotted against substrate concentration.

Substrate Effects on Enzyme Rates

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Substrate Effects on Enzyme Rates

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