9.4. Proteins

Proteins are polypeptides. They are linear chains of amino acids linked by peptide bonds as shown in Figure 9.3.

Proteins are large molecules made up of smaller units called amino acids. These amino acids are connected by chemical bonds known as peptide bonds, creating a long chain structure called a polypeptide. Understanding that proteins are essentially polypeptides will help you grasp their function and structure in living organisms.

Think of proteins as a necklace made of different colored beads (amino acids). Just as a necklace can vary in color and length based on the types and arrangements of beads, proteins vary in structure and function based on their amino acid composition.

Each protein is a polymer of amino acids. As there are 20 types of amino acids (e.g., alanine, cysteine, proline, tryptophan, lysine, etc.), a protein is a heteropolymer and not a homopolymer.

Proteins are considered heteropolymers because they are composed of various types of amino acids—twenty different kinds in total. Each type has unique properties, contributing to the diverse functions and structures of proteins. In contrast, homopolymers are made from just one type of monomer, such as a polymer made only of glucose.

Imagine making a fruit salad. If you only use apples, you have a simple dish (homopolymer). If you mix apples, bananas, strawberries, and oranges, you create a colorful and varied fruit salad (heteropolymer). Just like the fruit salad, proteins are diverse and complex due to the different amino acids used.

Proteins carry out many functions in living organisms, some transport nutrients across the cell membrane, some fight infectious organisms, some are hormones, and some are enzymes.

Proteins play crucial roles in biological processes. For example, transport proteins help move substances in and out of cells. Enzymes are proteins that catalyze chemical reactions, making them occur faster. Hormones, which are regulatory proteins, help control various processes within the body. Understanding the multifaceted nature of proteins illustrates their importance in maintaining life.

You can think of proteins like a multifunctional toolset. Just as a Swiss Army knife has several tools for different tasks—like cutting, screwing, and opening bottles—proteins serve various roles within the body, each tailored to perform a specific function crucial for survival.

Hence, dietary proteins are the source of essential amino acids. Therefore, amino acids can be essential or non-essential. The latter are those which our body can make, while we get essential amino acids through our diet/food.

Amino acids are classified into two categories: essential and non-essential. Essential amino acids are those that our body cannot synthesize; we must obtain them from our diet. In contrast, non-essential amino acids are those that the body can produce. This distinction emphasizes the importance of a balanced diet to ensure we receive all necessary nutrients.

Consider essential amino acids as special ingredients in a recipe that you can’t substitute or make at home. For example, if you want to bake a specific cake, but you don’t have certain flavorings (essential amino acids) at hand, you can’t achieve the exact flavor profile without going to the store. On the other hand, you can make other ingredients (non-essential amino acids) from basic pantry items.

Collagen is the most abundant protein in the animal world, and Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO) is the most abundant protein in the whole of the biosphere.

Collagen is a structural protein that is abundant in connective tissues, such as skin, tendons, and ligaments, providing strength and support. On the other hand, RuBisCO is an enzyme involved in the first major step of carbon fixation in photosynthesis, essential for plant biology and by extension, all life on Earth. Understanding these proteins highlights their significance in both animal structure and ecological processes.

Imagine collagen as the scaffolding in a building, providing essential structural support. Without it, the building would collapse. Similarly, RuBisCO can be seen as the initial step in a production line, where raw materials are transformed into a final product, in this case, sugars that support life.

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