to product per enzyme molecule per second. Further addition of substrate does not increase the rate which is said to be saturated. The value of the Michaelis constant is numerically equal to the [S] at which the reaction rate is at half-maximum, and is a measure of the substrate's affinity for the enzyme—a small indicates high affinity, meaning that the rate will approach with lower [S] than those reactions with a larger . The constant is not affected by the concentration or purity of an enzyme. The value of is dependent on both the identity of enzyme and that of the substrate, as well as conditions such as temperature and pH. The model is used in a variety of biochemical situations other than enzyme-substrate interaction, including antigen–antibody binding, DNA–DNA hybridization, and protein–protein interaction. It can be used to characterise a generic biochemical reaction, in the same way that the Langmuir equation can be used to model generic adsorption of biomolecular species. When an empirical equation of this form is applied to microbial growth, it is sometimes called a Monod equation. Applications Parameter values vary widely between enzymes: The constant (catalytic efficiency) is a measure of how efficiently an enzyme converts a substrate into product. Diffusion limited enzymes, such as fumarase, work at the theoretical upper limit of , limited by diffusion of substrate into the active site. Michaelis–Menten kinetics have also been applied to a variety of spheres outside of biochemical reactions, including alveolar clearance of dusts, the richness of species pools, clearance of blood alcohol, the photosynthesis-irradiance relationship, and bacterial phage infection. The equation can also be used to describe the relationship between ion channel conductivity and ligand concentration. Biological Oceanography Michaelis Menten can also be applied to limiting nutrients and phytoplankton growth in the global ocean. The relationship is where V is uptake of the nutrient by phytoplankton, S is the concentration of the limiting nutrient, is the maximum uptake rate by phytoplankton, and K is the half saturation constant. The relationship can also be understood for growth rate and K also depend on phytoplankton physiology and genetics. Oligotrophic regions—regions that are nutrient depleted—often have phytoplankton populations characterized by low and low K, which help them to efficiently take up the nutrient that is at low concentrations. These phytoplankton tend to be small, with large area to volume ratios. Phytoplankton in eutrophic regions—nutrient-replete regions—are often characterized by high