Proteins are complex macromolecules constructed from chains of smaller units called amino acids. These chains fold into precise three-dimensional structures, which dictate the specific job the molecule will perform. Proteins are responsible for almost every biological process necessary for life. This article focuses on five major categories of function that highlight their importance in all living systems.
Structural Support
Proteins are the primary components of the biological scaffolding that provides shape, strength, and rigidity to cells, tissues, and organs. This function is largely carried out by fibrous proteins, which are long molecules that assemble into strong fibers.
Collagen is the most abundant protein in mammals, forming the structural framework of connective tissues such as tendons, ligaments, bone, and skin. Its triple helix structure grants it immense tensile strength. Keratin provides structural integrity to outer body parts like hair, nails, and the outermost layer of skin. Within cells, proteins like tubulin and actin form the cytoskeleton, a dynamic internal framework that maintains cell shape.
Enzymatic Catalysis
The most recognized function of proteins is their role as enzymes, which are biological catalysts that dramatically increase the rate of chemical reactions. Enzymes function by lowering the activation energy required for a reaction to proceed, allowing metabolic processes to occur quickly enough to sustain life.
Enzymes possess a specific region called the active site, which accommodates the reactant molecules, known as substrates. The enzyme binds the substrate to form an enzyme-substrate complex, often changing shape slightly to achieve optimal alignment (induced fit). Examples include digestive enzymes like amylase and pepsin, which break down carbohydrates and proteins during digestion.
Transport and Storage
Proteins play a crucial role in moving substances throughout the body and across cell membranes, as well as reserving specific molecules for later use.
Hemoglobin, a globular protein in red blood cells, carries oxygen from the lungs to the body’s tissues. At the cellular level, channel and carrier proteins are embedded within the cell membrane, selectively facilitating the passage of specific ions and molecules like glucose into or out of the cell. Storage proteins act as reserves, such as ferritin, which binds and stores iron atoms within the body to prevent cellular damage and ensure supply.
Movement and Contraction
Dynamic movement, from muscle flexing to cell division, is powered by specific motor and contractile proteins. These proteins convert chemical energy, typically from adenosine triphosphate (ATP), into mechanical force and motion.
The primary proteins involved are actin and myosin, which are fundamental to muscle contraction. In the sliding filament model, myosin heads bind to actin filaments and use ATP energy to cycle through a “power stroke,” pulling the actin filaments past them. This interaction shortens the muscle unit. The same actin-myosin interaction facilitates movement in non-muscle cells, such as cell crawling or the internal transport of vesicles.
Cellular Communication
Proteins are central to the complex system of communication that allows cells to coordinate their activities and respond to environmental changes. This involves proteins acting as both messengers and receivers of signals.
Protein hormones, such as insulin, function as extracellular messengers, traveling through the bloodstream to regulate processes like blood sugar levels in distant cells. Target cells receive these messages via receptor proteins embedded in their membranes that bind specifically to the signal molecule. Binding causes the receptor protein to change shape, initiating a chain of molecular events inside the cell that translates the external signal into a specific cellular response (signal transduction).