Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

148 Chapter 3: Proteins
Another example of a protein with a nonprotein portion is hemoglobin (see
Figure 3–19). Each molecule of hemoglobin carries four heme groups, ring-shaped
molecules each with a single central iron atom (Figure 3–53B). Heme gives hemoglobin
(and blood) its red color. By binding reversibly to oxygen gas through its
iron atom, heme enables hemoglobin to pick up oxygen in the lungs and release
it in the tissues.
Sometimes these small molecules are attached covalently and permanently
to their protein, thereby becoming an integral part of the protein molecule itself.
We shall see in Chapter 10 that proteins are often anchored to cell membranes
through covalently attached lipid molecules. And membrane proteins exposed on
the surface of the cell, as well as proteins secreted outside the cell, are often modified
by the covalent addition of sugars and oligosaccharides.
Multienzyme Complexes Help to Increase the Rate of Cell
Metabolism
The efficiency of enzymes in accelerating chemical reactions is crucial to the
maintenance of life. Cells, in effect, must race against the unavoidable processes
of decay, which—if left unattended—cause macromolecules to run downhill
toward greater and greater disorder. If the rates of desirable reactions were not
greater than the rates of competing side reactions, a cell would soon die. We can
get some idea of the rate at which cell metabolism proceeds by measuring the
rate of ATP utilization. A typical mammalian cell “turns over” (i.e., hydrolyzes and
restores by phosphorylation) its entire ATP pool once every 1 or 2 minutes. For
each cell, this turnover represents the utilization of roughly 10 7 molecules of ATP
per second (or, for the human body, about 1 gram of ATP every minute).
The rates of reactions in cells are rapid because enzyme catalysis is so effective.
Some enzymes have become so efficient that there is no possibility of further useful
improvement. The factor that limits the reaction rate is no longer the enzyme’s
intrinsic speed of action; rather, it is the frequency with which the enzyme collides
with its substrate. Such a reaction is said to be diffusion-limited (see Panel 3–2,
pp. 142–143).
The amount of product produced by an enzyme will depend on the concentration
of both the enzyme and its substrate. If a sequence of reactions is to occur
extremely rapidly, each metabolic intermediate and enzyme involved must be
present in high concentration. However, given the enormous number of different
reactions performed by a cell, there are limits to the concentrations that can be
achieved. In fact, most metabolites are present in micromolar (10 –6 M) concentrations,
and most enzyme concentrations are much lower. How is it possible, therefore,
to maintain very fast metabolic rates?
The answer lies in the spatial organization of cell components. The cell can
increase reaction rates without raising substrate concentrations by bringing the
various enzymes involved in a reaction sequence together to form a large protein
assembly known as a multienzyme complex (Figure 3–54). Because this assembly
is organized in a way that allows the product of enzyme A to be passed directly
to enzyme B, and so on, diffusion rates need not be limiting, even when the concentrations
of the substrates in the cell as a whole are very low. It is perhaps not
surprising, therefore, that such enzyme complexes are very common, and they
are involved in nearly all aspects of metabolism—including the central genetic
processes of DNA, RNA, and protein synthesis. In fact, few enzymes in eukaryotic
cells diffuse freely in solution; instead, most seem to have evolved binding sites
that concentrate them with other proteins of related function in particular regions
of the cell, thereby increasing the rate and efficiency of the reactions that they
catalyze (see p. 331).
Eukaryotic cells have yet another way of increasing the rate of metabolic reactions:
using their intracellular membrane systems. These membranes can segregate
particular substrates and the enzymes that act on them into the same membrane-enclosed
compartment, such as the endoplasmic reticulum or the cell
nucleus. If, for example, a compartment occupies a total of 10% of the volume of