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

細胞内では 基質濃度などといった 異なる環境条件が 酵素が化学反応を触媒する速度に 大きく影響します。このプロセスは酵素反応速度論と呼ばれ 反応速度をY軸 そしてこの例では 異なる条件での基質濃度を X軸としたグラフで示します。反応速度は 生産物が形成される速度であり 利用可能なすべての酵素が基質に結合したときに Vmaxと呼ばれる最大速度に到達して プラトー状態になります。したがって、反応速度は 結合可能な酵素の濃度によって制限されます。このグラフはまた 反応速度が上昇する速度も示しています。変動はKmで示されています。基質濃度はVmaxの半分であり 酵素における基質結合への親和性 また傾向を示しています。低Kmの酵素は高い親和性を持ちます。なぜなら低濃度の基質で 最大反応速度に達するからです。同様に、高Kmの場合は親和性が低下します。これは、酵素は反応速度を最大にするために 高濃度の基質を必要とするからです。

ジョンソン、ケネスA.「酵素速度論的分析の世紀、1913年から2013年」 FEBSレター587、第17号(2013年9月2日):2753-66。[<a href="https://doi.org/10.1016/j.febslet.2013.07.012">出典</a>]
「酵素フラックス容量と代謝フラックス率の関係:筋肉解糖における非平衡反応」 Proceedings of the National Academy of Sciences of the United States of America 94, no. 13 (June 24, 1997): 7065–69.[<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC21285/。">ソース</a>]

酵素の反応速度論

Enzyme Kinetics

酵素は、反応物の活性化エネルギーを低下させることにより、反応を速めます。酵素が反応物を生成物に変える速さを反応速度と呼びます。反応速度には、利用可能な反応物の数など、いくつかの要因が影響します。酵素速度論とは、酵素が反応速度をどのように変化させるかを研究する学問です。
 
 科学者は通常、試験管の中で一定量の酵素を用いて酵素の反応速度を研究します。一定量の酵素にさらに反応物（基質）を加えると、酵素はより多くの生成物を作ることができるため、反応速度が上がます。その結果、反応速度を基質の濃度に対してグラフ化すると、基質を増やすことで反応速度が向上します。しかし、酵素の活性部位がすべて埋まってしまうと、反応速度は頭打ちになります。この反応速度が最大となる基質濃度をVmaxと呼びます。酵素分子の数がVmaxを制限します。酵素の量を増やせばVmaxは大きくなりますが、基質を増やしても効果はないです。 
 
 反応速度対基質濃度のグラフから、酵素の速度論について他の重要な特性がわかります。反応速度がVmaxの半分になる基質濃度（すなわち、½ Vmax）をミカエリス定数（Km）といいます。Kmは、酵素と基質との親和性を表しています。Kmが低い酵素は、Vmaxに達するために必要な基質の量が少なくて済むため、基質に対する親和性が高くなります。興味深いことに、多くの酵素では、Kmの値は基質の細胞内濃度に非常に近いです。Km付近では、基質濃度のわずかな変化が反応速度に大きな影響を与えるため、細胞内の基質のわずかな変化が生物学的経路全体の機能に影響を与えることになります。
 
 すべての酵素が、ミカエリス・メンテン式として知られる双曲線状の基質速度グラフを描くわけではありません。ミカエリス・メンテン式は、酵素が単一の基質を触媒することを前提としています。アロステリックに制御されている酵素は、複数の活性部位を持ち、反応速度を基質濃度に対してプロットすると、シグモイド型のグラフになる傾向があります。

酵素は、反応物の活性化エネルギーを下げることで反応をスピードアップします。酵素が反応物を生成物に変える速度は、...

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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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enzyme kinetics

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...

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key terms and concepts

Learn how substrate concentration changes enzyme reaction rate, and how V max, K m, and allosteric enzymes shape enzyme kinetics in high school biology.

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Substrate Effects on Enzyme Rates

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