You might see the word alloy described as a mixture of metals, but that's a little bit misleading because some alloys contain only one metal and it's mixed in with other substances that are nonmetals (cast iron, for example, is an alloy made of just one metal, iron, mixed with one nonmetal, carbon).
Energy substrates in the human body are either carbohydrates, fat or proteins. Protein degradation yields amino acid residues which can be converted to carbohydrates and used in energy production. Carbohydrates (sugars) and amino acids from proteins give approximately 4 kcal/gram dry weight when burned in the body.
Enzymes operate by binding to a substrate or substrates temporarily, lowering the activation energy necessary for a reaction to take place. The catalytic (active) site is the portion of the enzyme where substrate binding occurs. Each enzyme is specific for one or more substrate types. Two different theories about how enzyme-substrate binding exist. In the induced-fit (key and lock) model (a), the active site of an enzyme has a shape specific to the substrate.