Protein Structure And Function

Proteins, the working molecules of a cell, carry out the program of activities encoded by genes. This program requires the coordinated effort of many different types of proteins, which first evolved as rudimentary molecules that facilitated a limited number of chemical reactions. Gradually, many of these primitive proteins evolved into a wide array of enzymes capable of catalyzing an incredible range of intracellular and extracellular chemical reactions, with a speed and specificity that is nearly impossible to attain in a test tube. With the passage of time, other proteins acquired specialized abilities and can be grouped into several broad functional classes: structural proteins, which provide structural rigidity to the cell; transport proteins, which control the flow of materials across cellular membranes; regulatory proteins, which act as sensors and switches to control protein activity and gene function; signaling proteins, including cell-surface receptors and other proteins that transmit external signals to the cell interior; and motor proteins, which cause motion.

A key to understanding the functional design of proteins is the realization that many have "moving" parts and are capable of transmitting various forces and energy in an orderly fashion. However, several critical and complex cell processes—synthesis of nucleic acids and proteins, signal transduction, and photosynthesis—are carried out by huge

Electron density map of the F1-ATPase associated with a ring of 10 c-subunits from the F0 domain of ATP synthase, a molecular machine that carries out the synthesis of ATP in eubacteria, chloroplasts, and mitochondria. [Courtesy of Andrew Leslie, MRC Laboratory of Molecular Biology, Cambridge, UK.]

macromolecular assemblies sometimes referred to as molecular machines.

A fundamental goal of molecular cell biologists is to understand how cells carry out various processes essential for life. A major contribution toward achieving this goal is the identification of all of an organism's proteins—that is, a list of the parts that compose the cellular machinery. The compilation of such lists has become feasible in recent years with the sequencing of entire genomes—complete sets of genes— of more and more organisms. From a computer analysis of

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