Amylases Michaelis constants

Estimations of and inhibition studies are, however, more valid, because determinations of absolute rates of bond scission are not necessary. The Michaelis constants for several alpha-amylases have been determined, and are shown in Table XI it may be seen that wide variations occur. For hog-pancreas alpha-amylase, it has been claimed that the value of depends on the degree of hydrolysis of the substrate this is probably true for all alpha-amylases when high degrees of hydrolysis are reached. 

Michaelis Constants of alpha-Amylases from Various Sources... 

Source of alpha-amylase pK s from variation of reaction velocity pK s from variation of Michaelis constant... 

The effect of the addition of methanol on the kinetics of an aqueous enzyme—substrate system has been studied for B. subtilis alpha-amylase acting on soluble starch. The Michaelis constant was found to be unaltered, but the rate of formation of products decreased as the concentration of methanol was increased. 

The Michaelis constant and maximal velocity of a-amylase-free glucoamylase from Rhizopus delemar have been shown to decrease with increasing periodate oxidation of amylose.These kinetic features have been explained on the basis of competitive inhibition by the oxidized non-reducing end of the (1 4)-o -D-... 

It was found that the aryl jS-maltotriosides were preferentially hydrolysed into maltose and aryl jS-D-glucosides by both j8-amylases. The Michaelis constant ATnj and the molecular activity ko were determined for the hydrolyses of these maltotriosides and compared with those of maltotriose and maltotetraose. Aryl jS-maltotriosides were more rapidly hydrolysed than maltotriose by a factor of 30—80, and more slowly hydrolysed than maltotetraose by a factor of 10—30, depending on the kinds of substituents. The rapid hydrolysis of aryl j3-maltotrioside as compared with maltotriose was considered due to the interaction of an aryl group with the subsite of j8-amylase. This is in contrast with Rhizopus niveus glucoamylase-catalysed hydrolysis of phenyl j8-maltoside, whose phenyl group does not interact so much with the subsite of the enzyme. 

In order to understand why the malt a-amylase acts so slowly on a normal hexaose (Fraction PDXII) we have tried to apply the Michaelis-Menten theory and have determined the affinity constants of the enzyme-substrate compounds. If the concentration of starch and dextrin are expressed as moles of maltose per liter we find the affinity constant, K, between malt a-amylase and starch to be about 200 and for /3-amylase about 170. The affinity constant a-amylase to a-dextrin is so small that no values could be obtained. The constant certainly is smaller than 12. Thus, it is clear that the slow action of the malt a-amylase on the dextrin is due to a very low affinity of the enzyme to the substrate. In other words, the enzyme can readily attach itself to a long chain, but only with difficulty to chains shorter than a certain value. In the dextrinization period of the action of the enzyme on starch, a mixture of a-dextrins is produced with an average chain length of about 7 units. It can be assumed, therefore, that the enzyme readily attaches itself to chains that contain more than about 8 units provided that the chains are normal and contain no branchings or other anomalies. When anomalies occur these... 

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