Multiple enzyme reactions

The Michaelis constant (Km) is a means of characterising an enzyme's affinity for a substrate. The Km in an enzymatic reaction is the substrate concentration at which the reaction rate is half its maximum speed.
Thus, a low Km value means that the enzyme has a high affinity for the substrate (as a "little" substrate is enough to run the reaction at half its max speed). This is only true for reactions where substrate is limiting and the enzyme is NOT allosteric. That being said, even though a chemical like ATP and NADH may be limiting, those enzymes that have a higher affinity for them will get them more than those that do not.