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# Effects of Enzymes on Chemical Reaction Rates

 ✅ Paper Type: Free Essay ✅ Subject: Biology ✅ Wordcount: 1018 words ✅ Published: 24th May 2018

Enzymes increases the rates of chemical reactions by providing a reaction pathway with a transition state free energy barrier lower than the uncatalyzed reaction (i.e. by lowering the free energy barrier that separates the reactants and products). The diagram below shows the reaction coordinate.

(a)

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An enzyme activity is often articulated by the initial rate (V0) of the reaction been catalyzed. V0 are µmol min-1; and can also be represented by the enzyme unit (U) or Katal (Kat), where 1µmol min-1 equals 1unite (U) equals 16.67 nanokatal (Kat). An enzyme unit is the sum of the enzyme that catalyzed the transformation of 1µmole of substrate per minute under specified conditions of temperature, pH and substrate concentration. The Michaelis-Menten model of enzyme catalysis:

E + S â‡‹K1K2 ES fiK3 E + P

Where the rate constants K1; K2; and K3 describe association rates with each catalytic step processes. At low [S], V0 is directly proportional to [S], while at high [S] the velocity tends towards the maximum velocity (Vmax). The Michaelis-Menten equation Vo = (Vmax. [S])/ (Km + [S]), and this describe the hyperbolic curve on the graph shown in fig 2.

Km is defined by Michaelis and Menten as; a sum of stability of the ES complex equals the sum of the rates of the products [ES] over its rate of formation.

Km = (K2 +K3)/K1

For many enzymes, K2 is much greater than K3. Under these situations, Km is taking as a measure of affinity of enzyme’s substrate as its value is dependent of the relative values of K1 and K2 for ES formation and dissociation, correspondingly. Km is determined experimentally by substrate concentration as this equivalent to Km value at which the velocity is equal to half of Vmax. The Lineweaver-Burk plot:

As Vmax and Km is impossible to estimate them due to their values being achieved at infinite substrate concentration, they can be experimentally determined by measuring V0 at different substrate concentrations; (see fig 2). Although, the Linewearver-Burk plot is a derivation of Michaelis-Menten equation:

1/V0 = (1/Vmax + Km)/ (Vmax .1/[S])

Which gives a straight line, with the interception on the Y-axis equals 1/Vmax and that of the X-axis equals -1/Km, it is also useful in determining how an inhibitor binds to enzyme. The Km and Vmax can also be determined using Eadie-Hofstee plot of V0/[S] against V0, where intercept on X-axis equals Vmax and the slop of the line are equal to -1/Km. Question (C):

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Inhibitors lower the catalytic rate of an enzyme activity. There are two main types of inhibitors: Irreversible or reversible. Reversible inhibitors can be subdivided into competitive and non-competitive. An irreversible inhibitor covalently binds tightly to amino acid residues at the active site of the enzyme and permanently in-activate the enzyme active site. A competitive inhibitor caused a conformation change to enzyme by binding on to enzyme other than its active site and therefore, decreases enzymes catalytic rate. This can be determined using Lineweaver-Burk plot where non-competitive inhibitors were seen to decrease Vmax without any change to Km. Whereby competitive inhibitors decrease the enzyme catalytic rate by binding onto the enzyme active site and overcome substrates. At high substrate concentration competitive inhibitors can be overcome. The Lineweaver-Burk plot of competitive inhibitors shows an increase of Km without any change to Vmax. So by using Lineweaver-Burk plots, the effects of inhibitors of catalytic activity can be determined. For example:

Non-competitive inhibitor effects cannot be overcome by increasing substrates, as this does not bind to the active site.

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