Description of how adult hemoglobin (HbA) is allosteric, having a T conformation with poor affinity for oxygen and an R conformation with much higher affinity for oxygen. With a stronger affinity in the lungs, HbA loads up fully with oxygen; with a weaker affinity in the capillaries, HbA will unload at least half of the four bound oxygens. A theoretical globin with the same p50, but no cooperativity, would deliver at best one oxygen. Therefore, cooperativity doubles the amount of oxygen delivered. Hemoglobin by itself is most stable in the T conformation and can change toward the R conformation as oxygen concentrations rise, as in the lungs. Several activators, denoting a greater need for oxygen, also change HbA’s affinity for oxygen. HbA exemplifies K-type allosterism, meaning that conformational changes alter the KM or the p50. The blood clotting enzymes are examples of V-type allosterism, meaning that effectors alter their maximum velocity. The blood clotting cascade is described in detail. V-type clotting enzymes are synthesized as inactive precursors, which can exist in the blood without having any effect. Activation by other enzymes in this cascade results in cleavage and removal of a small part of the original protein, leading to a conformational change to make the remaining protein active. Activation of the G-protein Ras is described as an example of how a protein can be induced to have an active conformation, for a limited time of some minutes, because it slowly hydrolyzes GTP, the activating factor.