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

Dentro de uma célula, diferentes condições ambientais, como a concentração de substrato, podem influenciar muito a taxa em que as enzimas catalisam as reações químicas. Este processo, denominado cinética enzimática, pode ser representado graficamente com a taxa de reação no eixo y, e a condição variável, a concentração de substrato neste exemplo, no eixo x. A taxa da reação é a taxa em que o produto é formado, e pode atingir uma velocidade máxima, conhecida como Vmax, estagnando quando todas as enzimas disponíveis estão ligados a substratos.Portanto, a taxa de reação é limitada pela concentração de enzimas disponíveis para ligação. Este gráfico também pode revelar a velocidade na qual a taxa de reação aumenta. A variável é refletida por Km, a concentração de substrato na metade do Vmax, e representa a afinidade da enzima para, ou tendência para, se ligar ao substrato.Uma enzima com um baixo Km tem uma alta afinidade porque atinge a sua taxa de reação máxima em uma baixa concentração de substrato. Da mesma forma, um Km alto corresponde a uma baixa afinidade porque a enzima requer uma concentração maior de substrato para maximizar sua taxa de reação.

Johnson, Kenneth A. "Um século de análise cinética enzimática, 1913 a 2013." Cartas FEBS 587, nº 17 (2 de setembro de 2013): 2753–66. [<a href="https://doi.org/10.1016/j.febslet.2013.07.012">Fonte</a>]
Suarez, R. K., J. F. Staples, J. R. B. Lighton e T. G. West. "Relações entre Capacidades de Fluxo Enzimático e Taxas de Fluxo Metabólico: Reações de Não Equilíbrio na Glicólise Muscular." Anais da Academia Nacional de Ciências dos Estados Unidos da América 94, nº 13 (24 de junho de 1997): 7065–69. [<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC21285/.">Fonte</a>]

cinética enzimática

Enzyme Kinetics

As enzimas aceleram as reações diminuindo a energia de ativação dos reagentes. A velocidade com que a enzima transforma reagentes em produtos é chamada de taxa de reação. Vários fatores impactam a taxa de reação, incluindo o número de reagentes disponíveis. Cinética enzimática é o estudo de como uma enzima altera a taxa de reação.
 
 Os cientistas normalmente estudam cinética enzimática com uma quantidade fixa de enzima no ambiente controlado de um tubo de ensaio. Quando mais reagente, ou substrato, é adicionado a uma quantidade fixa de enzima, a taxa da reação aumenta à medida que a enzima pode fazer mais produto. Como resultado, quando a taxa de reação é colocada em um gráfico contra a concentração de substrato, o aumento do substrato aumenta a taxa de reação. No entanto, uma vez que todos os locais ativos da enzima são ocupados, a taxa de reação atinge um planalto. A concentração de substrato na qual a taxa máxima de reação é atingida é chamada de Vmax. O número atual de moléculas de enzimas limita a Vmax. Se a quantidade de enzimas for aumentada, a Vmax aumenta, mas adicionar mais substrato não tem efeito.
 
 O gráfico da taxa de reação versus concentração de substrato pode revelar outras características importantes da cinética de uma enzima. A concentração de substrato na qual a taxa de reação está a meio caminho da Vmax (ou seja, ½ Vmax) é chamada de constante de Michaelis (Km). Km é uma representação da afinidade entre uma enzima e um substrato. Enzimas com Km mais baixo requerem menos substrato para alcançar a Vmax e, portanto, têm uma maior afinidade pelo seu substrato. Curiosamente, para muitas enzimas, o valor de Km está muito próximo da concentração celular do substrato. Perto de Km, pequenas mudanças na concentração de substrato podem ter impacto significativo na taxa de reação, de modo que pequenas mudanças na disponibilidade de substrato celular podem ter impacto na função de toda uma via biológica.
 
 Nem todas as enzimas produzem o gráfico de substrato em forma de hipérbole conhecido como cinética de Michaelis Menten. A cinética de Michaelis Menten assume que a enzima catalisa um único substrato. Enzimas que são reguladas alostericamente têm vários locais ativos e tendem a produzir um gráfico em forma de sigmóide quando a taxa de reação é colocada em gráfico contra a concentração de substrato.

Cinética Enzimática

As enzimas aceleram as reações diminuindo a energia de ativação dos reagentes. A velocidade com que a enzima transforma reagentes em produtos é chamada de ...

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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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