The major circadian clock seems to be located in the nuclei of the hypothalamus.

suprachiasmatic

Explanation:

The hypothalamus is inferior to the thalamus. Hypothalamus has the suprachiasmatic nucleus that regulates circadian rhythms and also functions as the body’s internal biological clock. The suprachiasmatic nucleus of the hypothalamus regulates the melatonin secretion. Melatonin promotes sleep as more melatonin is released during darkness than in light.

In response to visual input from the eyes, the suprachiasmatic nucleus stimulates sympathetic postganglionic neurons of the superior cervical ganglion. This, in turn, stimulates the pineal gland to secrete melatonin. Low level of melatonin during the day and significantly higher levels at night maintain the circadian clock.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products