3.1 Synthesis of Biological Macromolecules

Synthesis of Biological Macromolecules

Biological macromolecules are large molecules built from smaller repeating units called monomers. Cells construct these macromolecules through dehydration synthesis and break them apart through hydrolysis. Understanding these two opposing reactions is foundational because they explain how your body builds everything from muscle proteins to DNA, and how it digests food back into usable parts.

Synthesis and Breakdown of Biological Macromolecules

Process of Dehydration Synthesis

Dehydration synthesis (also called a condensation reaction) joins monomers together into larger polymers by removing a water molecule each time a new bond forms. Here's how it works:

1. Two monomers are brought close together.
2. A hydroxyl group ($-OH$) is removed from one monomer, and a hydrogen atom ($-H$) is removed from the other.
3. Those pieces combine to form a water molecule ($H_2O$), and the two monomers form a new covalent bond.
4. This repeats, adding monomers one at a time to build a polymer chain.

Each type of macromolecule uses a different monomer and forms a different type of bond:

• Polysaccharides (starch, cellulose, glycogen) are built from monosaccharides joined by glycosidic bonds
• Proteins are built from amino acids joined by peptide bonds
• Nucleic acids (DNA, RNA) are built from nucleotides joined by phosphodiester bonds

Specific enzymes catalyze each of these reactions by lowering the activation energy required. For example, DNA polymerase catalyzes the linking of nucleotides during DNA synthesis, and glycogen synthase catalyzes glycogen formation from glucose.

Hydrolysis for Polymer Breakdown

Hydrolysis is the reverse of dehydration synthesis. Instead of removing water to build a bond, it adds water to break one.

1. A water molecule is split into $-H$ and $-OH$.
2. The $-H$ attaches to one monomer, and the $-OH$ attaches to the adjacent monomer.
3. The covalent bond between the two monomers breaks.
4. This repeats along the polymer until it's fully broken down into individual monomers.

Enzymes called hydrolases catalyze these reactions, and each type is specific to a particular macromolecule:

• Amylase breaks down starch into monosaccharides
• Peptidases (such as trypsin and pepsin) break down proteins into amino acids
• Nucleases break down nucleic acids into nucleotides
• Lipases break down lipids into fatty acids and glycerol

Hydrolysis is central to several biological processes: digesting food into absorbable monomers, recycling damaged or unneeded cellular components, and releasing stored energy (for example, breaking down glycogen when your cells need glucose).

Enzymes in Macromolecule Reactions

Enzymes are critical to both building and breaking down macromolecules. They work by lowering the activation energy of a reaction, which means the reaction proceeds much faster than it would on its own. Each enzyme is specific to the type of reaction it catalyzes and the substrates it acts on.

Synthesis enzymes (dehydration synthesis):

• DNA polymerase: assembles DNA from nucleotides
• RNA polymerase: assembles RNA from nucleotides
• Ribosomes: catalyze protein formation from amino acids
• Glycogen synthase: assembles glycogen from glucose monomers

Breakdown enzymes (hydrolysis):

• Amylases: break starch and glycogen into monosaccharides
• Peptidases: break proteins into amino acids
• Nucleases: break nucleic acids into nucleotides
• Lipases: break lipids into fatty acids and glycerol

Enzyme activity is tightly regulated to maintain homeostasis. Cells need to build macromolecules when supplies are needed and break them down when energy or raw materials run low. Factors that control enzyme activity include substrate concentration, product concentration, and the presence of inhibitors or activators.

Anabolic and Catabolic Reactions

These two reaction types provide a useful framework for thinking about metabolism as a whole.

Anabolic reactions build complex molecules from simpler ones. They require an input of energy. Dehydration synthesis is anabolic: protein synthesis and DNA replication both consume energy to assemble polymers from monomers.

Catabolic reactions break complex molecules into simpler ones and release energy in the process. Hydrolysis is catabolic: digestion and the breakdown of glycogen both free up monomers and stored energy.