Final answer:
The kinetics of a thrombin-catalyzed reaction are decreased in the presence of antithrombin, particularly when heparin is also involved, as it enhances antithrombin's inhibitory effect on thrombin.
Step-by-step explanation:
When considering antithrombin in the presence of heparin and its effects on the kinetics of a thrombin-catalyzed reaction, it is important to understand the role that antithrombin plays in the regulation of thrombin, an enzyme majorly involved in the blood clotting process. Antithrombin is a natural inhibitor of several enzymes in the clotting system, including thrombin. When heparin is present, it binds to antithrombin and induces a conformational change that significantly enhances the inhibitory effect of antithrombin on thrombin. Consequently, the presence of antithrombin with heparin results in a decreased activity of thrombin, thus leading to the inhibition of the reaction it would normally catalyze. In summary, the kinetics of a thrombin-catalyzed reaction are decreased by the presence of antithrombin, especially when heparin is involved.