Chapter 4: Cellular Machinery

Chapter 4: Cellular Machinery

Proteins are a class of organic molecules that are used when the cell needs to take action and perform an energetic task. Proteins are built to do work within the “cellular factory”- if we imagine cells as factories, we can imagine the plasma membrane as the building’s walls, and the proteins are the workers inside. The central room of the factory is the nucleus, which houses the blueprints and instructions (DNA) needed to build the workers (proteins). DNA describes protein construction; proteins do work within their assigned areas. Proteins account for more than 50% of the dry mass of most cells and are needed for almost every dynamic function of a living being. They play roles in structural support, movement, storage of molecules, cell communication, defense, and acceleration of chemical reactions.

Protein structure is divided into 4 levels: primary, secondary, tertiary, and quaternary. The building blocks of proteins are amino acids. Primary structure is a protein’s exact linear sequence of amino acids, from start to finish, much like how your genome is the actual sequence of nucleotides that make up your DNA. An average protein consists of a chain of 300-500 amino acids. As the chain of amino acids exits the ribosome, the organelle responsible for making proteins, it twists and curls back onto itself in random or repeating patterns. These patterns make up the secondary protein structure. The 2 most basic forms are helices and pleated sheets. Folding continues as the secondary structures collapse onto themselves, forming the entire protein. The final 3D shape is the protein’s tertiary structure. In some cases this is the final form, but in many cases, the quaternary structure is needed to complete the protein’s construction. In these cases, multiple proteins will link together to form a single complex, capable of performing more difficult tasks. A great example of a quaternary structure protein is the hemoglobin in blood.

Enzymes are a specific class of proteins. They are catalytic molecules, meaning they catalyze, or reduce the amount of energy needed to start chemical reactions. Because enzymes are a type of protein, they are made of amino acids, fold up, and their structure relates directly to their function. The major characteristic that defines enzymes is the job they perform at an active site. In an active site, specific molecules bind to the enzyme and are either degraded (broken down) or synthesized (linked together). Each enzyme has a specific molecule it can do work on. Without enzymes, chemical reactions like digestion would take months or years. Enzymes ensure that chemical reactions needed for life are fast.

If a molecule’s name ends in -ase, it is almost always an enzyme. A great example is peroxidase, a large protein with an iron ion located at its active site.

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