What is meant by Km value in enzyme reaction? What does it indicate?

The 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.
Km values are inversely proportional to the substrate specificity of the enzyme.

Substrate concentration governs the reaction only till saturation of the enzyme. So, efficiency of the enzyme would also increase till the substrate concentration reaches saturation. So, increasing Km values increases the efficiency of the enzymes till saturation, before it starts to taper off. Higher Km symbolizes lower affinity for the substrate. This is because, higher the Km value, higher is the substrate concentration required to reach half maximal velocity. So, this would mean the enzyme has low affinity for the substrate.

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