easily, it doesn't matter whether the blocks are close together or far apart, but the individual properties of each block result in some connecting better than others. If this necklace was a mile long, imagine the many strange and complex forms you could build. This isn't precisely how proteins function, but you get the idea. enzymes Since biochemistry explains life from a chemical point of view, it is vital to understand how chemical reactions work, and enzymes are essential to these reactions. Enzymes are pro- teins that act as catalysts--that is, they increase the rate of chemical reactions. An enzyme catalyzes nearly every chemical reaction that occurs in an organism. In a chemical reaction catalyzed by an enzyme, the substance that the enzyme acts upon is called the substrate. The new substance that's formed during the reaction is called the product. The activity of an enzyme is affected by the environment inside the organism (temperature, pH, and other factors), the availability of the substrate, and, in some cases, the concentration of the product. Although almost all enzymes are proteins, it has recently been discovered that a spe- cial type of ribonucleic acid (RNA) can act as a catalyst in certain chemical reactions. This is called an RNA enzyme, or a ribozyme. oxidation-reduction Enzymes are broadly classified into six types, which will be introduced in detail in Chapter 4. Oxidation-reduction is one of the most important enzyme reactions, in which electrons are