Which of the following statements about dissociation constant (Kd) is true?

The dissociation constant (Kd) is a critical measure that quantifies the binding affinity between a receptor and its ligand (or between an enzyme and its substrate). When Kd is low, this indicates a strong affinity, meaning the receptor and ligand or enzyme and substrate bind tightly and are less likely to dissociate. Conversely, a high Kd value implies weaker binding.

This concept directly relates to the efficiency of receptor-ligand interactions in biological systems. By understanding Kd, we can infer how effectively a ligand binds to its receptor, which is essential for processes such as signal transduction and metabolic pathways.

Understanding Kd is key to interpreting how well molecular interactions occur under physiological conditions, making it an important concept in biochemistry and pharmacology. The other options fail to capture the true nature of Kd in the context of enzyme kinetics or receptor interactions. For instance, Kd is not universally constant across all enzymes, does not indicate substrate concentration at maximum velocity, and is certainly relevant to enzyme kinetics, as it provides insights into the behavior of enzyme-substrate interactions.