Enzyme

An enzyme (from Greek énsimo (??????), formed by én = at or in and simo = leaven or yeast) is a protein that catalyzes, or speeds up, a chemical reaction.

Kinetics

In 1913, Leonor Michaelis and Maud Menten proposed a quantitative theory of enzyme kinetics which is still widely used today (usually referred to as Michaelis-Menten kinetics). Enzymes can perform up to several million catalytic reactions per second; to determine the maximum speed of an enzymatic reaction, the substrate concentration is increased until a constant rate of product formation is achieved. This is the maximum velocity (Vmax) of the enzyme. In this state, all enzyme active sites are saturated with substrate. However, Vmax is only one kinetic parameter that biochemists are interested in. The amount of substrate needed to achieve a given rate of reaction is also of interest. This can be expressed by the Michaelis-Menten constant (KM), which is the substrate concentration required for an enzyme to reach one half its maximum velocity. Each enzyme has a characteristic KM for a given substrate. Since Vmax cannot be measured directly, both KM and Vmax are usually determined by extrapolating from a limited data set, using what is known as a double reciprocal, or Lineweaver-Burk plot.

Related Topics:
Leonor Michaelis - Maud Menten - Michaelis-Menten kinetics - Michaelis-Menten constant

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The efficiency of an enzyme can be expressed in terms of kcat/Km. The quantity kcat, also called the turnover number, incorporates the rate constants for all steps in the reaction, and is the product of Vmax and the total enzyme concentration. kcat/Km is a useful quantity for comparing different enzymes against each other, or the same enzyme with different substrates, because it takes both affinity and catalytic ability into consideration. The theoretical maximum for kcat/Km, called diffusion limit, is about 108 to 109 (l mol-1 s-1). At this point, every collision of the enzyme with its substrate will result in catalysis and the rate of product formation is not limited by the reaction rate but by the diffusion rate. Enzymes that reach this kcat/Km value are called catalytically perfect or kinetically perfect. Example of such enzymes are triose-phosphate isomerase, carbonic anhydrase, acetylcholinesterase, catalase, fumarase, ß-lactamase, and superoxide dismutase.

Related Topics:
Triose-phosphate isomerase - Carbonic anhydrase

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