Negative feedback is a control regulation of the body. When a substrate (this could be an amino acid for example) binds to the active site of an enzyme, there will be a series of reactions that will produce a certain product. This product will bind to the allosteric site of the enzyme. This will cause the active site to change conformation and hence the substrate will no longer be able to bind to the active site.
In positive feedback, the product would increase the rate of product production.
