Catalytic perfection

But k must always be greater than or equal to k h / (A i + kf). That is, the reaction can go no faster than the rate at which E and S come together. Thus, k sets the upper limit for A ,. In other words, the catalytic effieiency of an enzyme cannot exceed the diffusion-eontroUed rate of combination of E and S to form ES. In HgO, the rate constant for such diffusion is approximately (P/M - sec. Those enzymes that are most efficient in their catalysis have A , ratios approaching this value. Their catalytic velocity is limited only by the rate at which they encounter S enzymes this efficient have achieved so-called catalytic perfection. All E and S encounters lead to reaction because such catalytically perfect enzymes can channel S to the active site, regardless of where S hits E. Table 14.5 lists the kinetic parameters of several enzymes in this category. Note that and A , both show a substantial range of variation in this table, even though their ratio falls around 10 /M sec. 

Acetylcholine serves as a neurotransmitter. Removal of acetylcholine within the time limits of the synaptic transmission is accomplished by acetylcholinesterase (AChE). The time required for hydrolysis of acetylcholine at the neuromuscular junction is less than a millisecond (turnover time is 150 ps) such that one molecule of AChE can hydrolyze 6 105 acetylcholine molecules per minute. The Km of AChE for acetylcholine is approximately 50-100 pM. AChE is one of the most efficient enzymes known. It works at a rate close to catalytic perfection where substrate diffusion becomes rate limiting. AChE is expressed in cholinergic neurons and muscle cells where it is found attached to the outer surface of the cell membrane. 

Answer Catalytic perfection A/An reaction involves two molecules. 

Speed and efficiency mailer. Acetylcholine is rapidly de-Stroyetl l v the en/yme acetylcholinesterase. T his enzyme, which has a turnover number of 25,000 per second, has attained catalytic perfection with a of 2 X 10 M s V Why is the... 

Because such enzymes convert substrate to product virtually every time the substrate diffuses into the active site, they are said to have achieved catalytic perfection. Living organisms overcome the diffusion control limit for the enzymes in biochemical pathways that do not achieve this high degree of catalytic efficiency by organizing them into multienzyme complexes. In these complexes, the active sites of the enzymes are in such close proximity to each other that diffusion is not a factor in the transfer of substrate and product molecules. 

The fecat/J M ratios of the enzymes superoxide dismutase, acetylcholinesterase, and triosephosphate isomerase are between 10 and lO" s Enzymes such as these that have fecat/ M ratios at the upper limits have attained kinetic perfection. Their catalytic velocity is restricted only by the rate at which they encounter substrate in the solution (Table 8.8). Any further gain in catalytic rate can come only by decreasing the time for diffusion. Remember that the active site is only a small part of the total enzyme structure. Yet, for catalytically perfect enzymes, every encounter between enzyme and substrate is productive. In these cases, there may be attractive electrostatic forces on the enzyme that entice the substrate to the active site. These forces are sometimes referred to poetically as Circe effects. 

There is an upper limit for the value of 2/ M which is contingent upon the rate of diffusion-controlled encounters of enzyme and substrate molecules, i.e., the rate of product formation is no longer limited by the reaction rate but by the diffusion rate. In an aqueous solution, this limiting value lies between 10 and 10 molL s (compare Sect. 20.2). Enzymes such as catalase that exhibit a value of k 2jK of this order of magnitude are considered to be (almost) catalytically perfect because (almost) every contact between enzyme and substrate leads to a reaction. 

RNase A-Catalyzed RNA Cleavage - M -lndependent Catalytic Perfection Early Attempts at Expanding the Catalytic Repertoire of Nucleic Acids... 

DNAz5mies, namely DNAzyme 8-17 (Dz8-17) and DNAzyme 10-23 (DzlO-23). Under multiple turnover conditions, DzlO-23 appeared to achieve limits of catalytic perfection at 10-25 mM Mg, impressive fecat ( 4min ) and fecat/lfm (10 M" min" ) values are routinely observed. 

RNase A-Catalyzed RNA Cleavage -M -lndependeiit Catalytic Perfection... 

Whereas the devdopment of highly active M -independent RNA-deaving DNAzjunes with three modifications provided unprecedented self-deavage rates under specific M -free conditions where other unmodified catalysts were inactive, fecai/ftm values, while similar to those found for the Mg -dependent DzlO-23 (when measured at 0.5 mM Mg ), stiU fdl short of the catalytic perfection seen for DzlO-23 at lOmM Mg. ... 

Enzymes accelerate the attainment of equilibrium of chemical reactions by a factor of 10 -10 as compared with uncatalyzed reactions. Several enzymes, e.g., catalase, acetylcholinesterase, and fumarase, have achieved virtual catalytic perfection approaching the diffusion-controlled limit. Thus, the splitting of H2O2 is accelerated by a factor of 3 x 10 in the presence of catalase. 

The catalytic efficiency reaches its maximum value of k when kt, fc. Because k is the rate constant for the formation of a complex from two species that are diffusing freely in solution, the maximum efficiency is related to the maximum rate of diffusion of E and S in solution (Section 7.5). This limit leads to rate constants of about 10 -10 dm mol s for molecules as large as enzymes at room temperature. The enzyme catalase has 77 = 4.0 x 10 dm mol s and is said to have attained catalytic perfection in the sense that the rate of the reaction it catalyzes is essentially diffusion controlled it acts as soon as a substrate makes contact. 

Finally, it is useful to note that the catalytic efficiency reaches a maximum when the formation of the enzyme-substrate complex ES is rate-determining. This corresponds to the situation where k2> > k i, so that every ES which is formed is converted to product, none re-dissociates to E + S. However, the rate cannot exceed the rate of the collisions of E and S, and this upper limit is determined by the rate of diffusion in the solution. Experiments on the kinetics of enzyme-catalysed reactions demonstrate that a number of them achieve this state of catalytic perfection [2, p. 372],... 

---