9.3: Energy-Requiring Reactions

Energy-requiring reactions use ATP. In many cases, what’s broken apart can be put together again with an input of energy. All the chemical reactions in which big molecules are made from small ones require energy. ATP energy is needed to connect the molecules. Making protein from amino acids and making glycogen from glucose are examples.

Figure 9.2 shows only a very basic outline of the breakdown of the energy-providing nutrients. In fact, these reactions can branch in many directions, and can even change directions in some parts. Examples: The 27-carbon structure of cholesterol is built entirely from acetate (2-carbons). Pyruvate can be made into the amino acid alanine by transferring the amino (-NH₂) to pyruvate.† Thus, we don’t need this amino acid in our diet.

^{†Alanine and pyruvate can be interconverted:}

Protein Synthesis

Protein synthesis is an energy-requiring process in which amino acids are linked together. The topic of protein synthesis is complex and broad (and thus has its own chapter). How does a cell know which amino acids to link together to make a protein? How does protein synthesis relate to our dietary protein requirement? Examples of what can happen when cells make a protein incorrectly (e.g., the hemoglobin in sickle cell anemia) or when cells don’t make enough of a protein (e.g., in lactose intolerance, PKU, familial hypercholesterolemia) have been discussed earlier. Protein synthesis also is relevant to viruses and biotechnology—and even cancer. These aspects will be discussed in upcoming chapters.