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The variation in a polypeptide's underlying amino acid sequence coupled with its unique folding ensure a wide variety of structures and accompanying functionality across the class of macromolecules we call proteins. One role a protein may play is as a biological catalyst, or enzyme, which assists in accelerating a chemical reaction by lowering the activation energy of the reaction. Enzymes are of incredible biological importance, but the biological utility of proteins also extends into non-enzymatic functions, such as structure (e.g. collagen), transportation (e.g. hemoglobin), regulation (e.g. peptide hormones), movement (e.g. myosin), and immune defense (e.g. antibodies).
A special feature of some proteins is the capability to bind other molecules with non-covalent interactions. Protein binding can be characterized by its affinity and specificity for the binding target. Affinity describes how readily the protein binds its target, and specificity refers to the preferential binding of the target over other entities. A change in the protein's conformation can alter affinity and specificity as seen in the control of voltage-gated ion channels in cell membranes.
The high degree of protein variability allows for a key feature of the adaptive (or acquired) immune system, the production of antibodies. An antibody is a type of protein that has a unique and very specific binding site that will readily bind its target, called an antigen, such that its target is inactivated or tagged for immune response.
A motor protein can perform mechanical work by coupling exergonic (energy releasing) ATP hydrolysis to a conformational change that allows for interaction with the protein's target substrate. Muscle contraction, for example, is achieved through a process of the motor protein myosin binding and releasing its microfilament (actin) substrate. Myosin also acts on microfilaments of the cytoskeleton to generate cellular movement.
Two other types of motor proteins, kinesins and dyneins, act on microtubules and play a role in transport within the cell. Kinesin walks microtubule "tracks" to deliver cellular cargo (e.g. chromosomes during mitosis, vesicles), generally in an antegrade direction (center to periphery). Dynein is used in retrograde cargo transport in the axons of neurons, and is capable of sliding microtubules in relation to one another, generating the movement of cilia and flagella.
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