Essential Cell Biology 5th edition

CHAPTER FOUR
4
Protein Structure and Function
When we look at a cell in a microscope or analyze its electrical or biochemical
activity, we are, in essence, observing the handiwork of proteins.
Proteins are the main building blocks from which cells are assembled,
and they constitute most of the cell’s dry mass. In addition to providing
the cell with shape and structure, proteins also execute nearly all its
myriad functions. Enzymes promote intracellular chemical reactions by
providing intricate molecular surfaces contoured with particular bumps
and crevices that can cradle or exclude specific molecules. Transporters
and channels embedded in the plasma membrane control the passage
of nutrients and other small molecules into and out of the cell. Other
proteins carry messages from one cell to another, or act as signal integrators
that relay information from the plasma membrane to the nucleus
of individual cells. Some proteins act as motors that propel organelles
through the cytosol, and others function as components of tiny molecular
machines with precisely calibrated moving parts. Specialized proteins
also act as antibodies, toxins, hormones, antifreeze molecules, elastic
fibers, or luminescence generators. To understand how muscles contract,
how nerves conduct electricity, how embryos develop, or how our bodies
function, we must first understand how proteins operate.
THE SHAPE AND STRUCTURE
OF PROTEINS
HOW PROTEINS WORK
HOW PROTEINS ARE
CONTROLLED
HOW PROTEINS ARE STUDIED
The multiplicity of functions carried out by these remarkable macromolecules,
a few of which are represented in Panel 4−1, p. 118, arises from
the huge number of different shapes proteins adopt. We therefore begin
our description of proteins by discussing their three-dimensional structures
and the properties that these structures confer. We next look at how
proteins work: how enzymes catalyze chemical reactions, how some
proteins act as molecular switches, and how others generate orderly
movement. We then examine how cells control the activity and location