Expert: George Maxwell - 8/13/2007

Question
what are the disaccharides of physiologic importance and why is the rate of an enzymatic reaction always directly dependent on the enzymatic cincentration?

Answer
Hi, and thank you for your question.

Disaccarides (two sugar units joined together) are very common in the human body. The specific importance of disaccharides depends on their individual structures, but generally they perform the role of most sugars: they store chemical energy ready for release  as kinetic energy (e.g. heat) by respiration in cells. In short, they are a form of "fuel".

Sucrose (the "sugar" you buy from the supermarket) is a disaccharide consisting of Glucose and Fructose molecules joined together. Lactose ("milk sugar") comtains glucose and galactose, and is found in the milk of nursing mothers: again, its role is as a fuel for respiration

Enzymes - molecules that catalyse biological processes - are really important. They control the complex sequence of chemical reactions that we call life.

An enzyme works by latching on to specific chemical groups and holding them in place so a reaction can happen: they provide a surface for reactions to take place on with minimal energy input. Enzymes are highly specific, and will only usually catalyse one reaction.

It's not true to say that the rate of an enzyme-catalysed reaction is always directly dependent on enzymatic concentration. Its true that, up to a point, most enzyme-catalysed reactions speed up in the presence of the relevant enzyme, because the enzyme is providing an easy route for the reaction to happen. However, beyond a certain point, adding more enzyme will not result in an increase in the rate of reaction. The process of binding on to reactants, holding them together and releasing the product takes time, and only happens as long as the supply of reactants is maintained. Once more enzyme than is necessary for the reaction to take place is added, it simply sits around doing nothing, and the rate of reaction remains constant until more reactants are added, or until the original reactants are used up.