Enzymes turnover number

In sections A to C, we shall discuss four types of techniques. The first is rapid % mixing. This is extremely useful "since it is possible to mix two solutions in a fraction of a millisecond, and the majority of enzyme turnover numbers are less I than 1000 s 1. Rapid mixing techniques are now standard laboratory practice because of their ease and their wide range of application. 

It was once thought that the rate of equilibrium of the catalytic acid and basic groups on an enzyme with the solvent limited the rates of acid- and base-catalyzed reactions to turnover numbers of 103 s 1 or less. This is because the rate constants for the transfer of a proton from the imidazolium ion to water and from water to imidazole are about 2 X 103 s 1. However, protons are transferred between imidazole or imidazolium ion and buffer species in solution with rate constants that are many times higher than this. For example, the rate constants with ATP, which has a pKa similar to imidazole s, are about I0 J s 1 M-1, and the ATP concentration is about 2 mM in the cell. Similarly, several other metabolites that are present at millimolar concentrations have acidic and basic groups that allow catalytic groups on an enzyme to equilibrate with the solvent at 107 to 108 s-1 or faster. Enzyme turnover numbers are usually considerably lower than this, in the range of 10 to 103 s-1, although carbonic anhydrase and catalase have turnover numbers of 106 and 4 X 107 s 1, respectively. 

The total activity of a particular enzyme in vivo is determined primarily by three factors (1) the inherent catalytic competence of the enzyme (turnover number moles of substrate transformed per mole of enzyme per unit time) (2) the level of that enzyme which is expressed in relevant tissues (3) the possible presence of agents that inhibit enzyme activity by competitive or non-competitive actions on the enzyme protein. 

Compare the enzyme turnover number with a typical hydrogen-ion turnover number. What does this tell you about the efficiency of enzymes Compare the activation energy for the enzyme-catalyzed reaction with that for the acid-catalyzed reaction. Does the difference in a values account completely for the ratio of turnover numbers for the enzyme and H+ ... 

The enzyme AChE, found at the receptor sites of tissue innervated by the cholinergic nervous system, hydrolyzes ACh very rapidly it has one of the highest enzyme turnover numbers (number of molecules of substrate that it turns over per unit time) known.11 A similar enzyme having ACh as its preferred substrate is found in or on erythrocytes (red blood cells, RBCs) and is known as erythrocyte, or true, ChE (RBC-ChE). Butyrocholinesterase (BuChE, also known as serum or plasma cholinesterase, and as pseudocholinesterase), another enzyme of the ChE family, has butyrylcholine as its preferred substrate. Butyrylcholine is present in plasma or serum and in some tissues. BuChE and RBC-ChE are discussed in the Blood Cholinesterases section below. 

Enzyme Turnover Number (per minute) Reaction Catalyzed... 

Even under these conditions, the value of k should increase linearly with the donor concentration and no maximum value of k exists as it does for the simpler mechanism of equations (3) and (4). Experimentally, maximal values of k are often quoted in various papers, but they may be attributed to insufficient substrate concentration (the inequality A)Xo kiOo is violated), or to enzyme inactivation due to the excess peroxide concentration. For example, catalase inactivation can be caused by the formation of the inactive catalase complex II. In these cases it is desirable to use a lower value of substrate concentration and a smaller enzyme turnover number in order to avoid the inactivation. 

---