Page 17 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

P1: FYD Revised Pages
Encyclopedia of Physical Science and Technology EN002H-54 May 17, 2001 20:22

Bioenergetics 113

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Na . Since the extracellular Na concentration is higher Inhibitors of the enzyme responsible for the acidi-
than that in the cytoplasm and the membrane potential is fication of the stomach are well known and equally
negative, the Na flows from outside to inside the cell. As- well-advertised alleviators of “heartburn.” This enzyme
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suming a membrane potential of −50 mV and a 100-fold is present in the parietal cells of the stomach and re-
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Na concentration gradient, the flow of Na would lib- sembles the Na /K -ATPase. Instead of catalyzing the
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erate about 3.8 kcal/mol at 25 C. This exergonic flow of ATP-dependent exchange of Na and K , the stomach
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Na provides the energy needed for the active transport acid pump excretes H into the lumen of the stomach in
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of the amino acid or carbohydrate. Although Na flux is exchange for K .
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the immediate source of energy for the active transport in
Na -linked transporters, it is important to keep in mind
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B. Biosynthetic Use of ATP
that the ultimate energy source is ATP hydrolysis by the
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Na /K -ATPase. The input of energy in the form of the hydrolysis of ATP
Plants, yeasts, and fungi do not contain a Na /K - to either ADP and P i or to adenosine monophosphate
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ATPase in their plasma membranes. Instead, they contain (AMP) and pyrophosphate powers the synthesis of biolog-
aH -ATPase that is the generator of the plasma membrane ical molecules, including, as we have seen, carbohydrates
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potential. The H -ATPase is structurally and mechanisti- in photosynthesis, proteins, DNA, RNA, and fatty acids.
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cally related to the Na /K -ATPase but translocates only To delve into the role of ATP in biosynthesis in depth
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H . The H -ATPase is capable of generating large elec- is not possible in this brief article. Aspects of fatty acid
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trochemical proton gradients. The imbalance in the Na + biosynthesis, however, reveal interesting principles of the
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and K concentrations between the inside and the outside energetics of biosynthetic pathways.
of the plant cell is maintained by other mechanisms that Fatty acids are oxidized completely to CO 2 and water
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include exchange transport of Na for H . by β-oxidation and the citric acid cycle. Acetyl CoA is the
The active transport of some organic molecules across end product of β-oxidation of fatty acids and is the source
the plasma membrane of plants, yeasts, and fungi is linked of carbon for fatty acid biosynthesis. Yet, the pathways for
to the cotransport of H down its eletrochemical gradi- fatty acid degradation and synthesis are so very different
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ent into the cell. An important example of proton-linked that they even occur within different compartments within
transport is that of sucrose loading into the vascular ele- cells. Fatty acid synthesis takes place in the cytoplasm of
ment, the phloem, that transports sucrose from the leaves animal cells and in the plastids of plant cells, whereas β-
to the remainder of a plant. The concentration of sucrose oxidation is located in mitochondria in both animal and
in phloem cells near leaves that are actively carrying out plant cells.
photosynthesis can be 0.5 M or higher, whereas that in Often, the pathway for the synthesis of a compound
the intracellular space, just 0.001 M. The energy cost of differs significantly from that for its degradation. Among
generating this gradient is 3.7 kcal/mol at 25 C. The im- the reasons that the separation of synthetic and degrada-
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mediate source of energy is proton flow, and the ultimate tive pathways evolved are energetics and regulation. The
source, ATP hydrolysis by the H -ATPase. oxidation of fatty acids to acetyl CoA is very exergonic.
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The concentration of free Ca 2+ (meaning that unbound It is not feasible on energetic grounds to make fatty acids
to proteins and membrane lipids) in the cytoplasm of cells from acetyl CoA by reversing β-oxidation. Metabolism
is normally maintained at a very low level. Under cer- of carbohydrates and fats is regulated in mammals by
tain circumstances, however, transient increases in the a number of hormones, including insulin, glucagon, and
cytoplasmic Ca 2+ concentration are triggered. Ca 2+ is a epinephrine (adrenaline). Having separate pathways for
major player in the transmission of some hormonally in- the degradation and the biosynthesis makes it possible to
duced signals in plants and animals. Muscle contraction is turn off one pathway while up-regulating another. For ex-
also induced by release of Ca 2+ from internal membranes ample, glucagon and epinephrine selectively stimulate the
within muscle cells. breakdown of fats and fatty acids, whereas insulin has the
The plasma membrane contains an enzyme that cat- opposite effect. The fine control of fatty acid metabolism
alyzes the export of Ca 2+ from the cytoplasm at the ex- that has evolved would clearly not be possible without
pense of ATP hydrolysis. The Ca -ATPase has features the existence of separate pathways for biosynthesis and
2+
that place it in the category of plasma membrane en- catabolism.
zymes that also includes the Na /K -ATPase and the CO 2 is required for the synthesis of fatty acids. Yet,
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H -ATPase. The Ca -ATPase functions to keep the cy- when fatty acid synthesis is carried out in the presence of
2+
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2+
tosolic Ca concentration low (<1 µM). It is not a major radioactive CO 2 , the fatty acid made is devoid of radioac-
contributor to the generation of the membrane potential or tivity. ATP is used to add CO 2 to a precursor, and in a
to the energetics of the transport of bioorganic molecules. subsequent step in the pathway of fatty acid biosynthesis,