Thecollection of proteins within a cell determines its health andfunction. Proteins are responsible for nearly every task of cellular life,including cell shape and inner organization, product manufacture and wastecleanup, and routine maintenance. Proteins also receive signals from outsidethe cell and mobilize intracellular response. They are the workhorsemacromolecules of the cell and are as diverse as the functions they serve.
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Phosphorylation can either activate a protein (orange) or inactivate it (green). Kinase is an enzyme that phosphorylates proteins. Phosphatase is an enzyme that dephosphorylates proteins, effectively undoing the action of kinase.
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Proteins can be big or small, mostly hydrophilic or mostly hydrophobic, exist alone or as part of a multi-unit structure, and change shape frequently or remain virtually immobile. All of these differences arise from the unique amino acid sequences that make up proteins. Fully folded proteins also have distinct surface characteristics that determine which other molecules they interact with. When proteins bind with other molecules, their conformation can change in subtle or dramatic ways.
Not surprisingly, protein functions are as diverse as protein structures. For example, structural proteins maintain cell shape, akin to a skeleton, and they compose structural elements in connective tissues like cartilage and bone in vertebrates. Enzymes are another type of protein, and these molecules catalyze the biochemical reactions that occur in cells. Yet other proteins work as monitors, changing their shape and activity in response to metabolic signals or messages from outside the cell. Cells also secrete various proteins that become part of the extracellular matrix or are involved in intercellular communication.
Proteins are sometimes altered after translation and folding are complete. In such cases, so-called transferase enzymes add small modifier groups, such as phosphates or carboxyl groups, to the protein. These modifications often shift protein conformation and act as molecular switches that turn the activity of a protein on or off. Many post-translational modifications are reversible, although different enzymes catalyze the reverse reactions. For example, enzymes called kinases add phosphate groups to proteins, but enzymes called phosphatases are required to remove these phosphate groups (Figure 1).