Page 18 - 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

114 Bioenergetics

this same CO 2 is released. This seemingly perplexing phe- Muscle contraction is regulated by a Ca -binding pro-
2+
nomenon may readily be explained on an energetic basis. tein in the thin filaments. Ca 2+ is required for muscle con-
Acetyl CoA is carboxylated by using bicarbonate as the traction. During rest, the concentration of Ca 2+ in muscle
source of CO 2 and ATP hydrolysis as the source of energy: cells is kept low by the operation of two Ca -ATPases,
2+
oneintheplasmamembraneandtheotherininternalmem-
−
acetyl CoA + ATP + HCO → malonyl CoA
3
branes called the sarcoplasmic reticulum. The release of
+ ADP + P i . (12) Ca 2+ triggers muscle contraction, and its uptake into the
lumen of the sarcoplasmic reticulum causes relaxation.
The enzyme that catalyzes this reaction, acetyl CoA car-
boxylase, contains biotin, one of the B vitamins. Sev-
+
eral other vitamins, including niacin (part of NAD and V. CONCLUDING STATEMENTS
NADP ) and riboflavin (part of FAD), are essential play-
+
ers in metabolism.
There are two aspects of bioenergetics that we want to
The carboxylation of acetyl CoA without the hydrol-
emphasize at the end of this article. These are the depen-
ysis of ATP is energetically unfavorable. The exergonic
dence of life on photosynthesis and the diversity of energy
hydrolysis of ATP pulls the reaction toward malonyl CoA
interconversions in living systems.
synthesis. But why bother to carboxylate acetyl CoA?
Photosynthesis is the only major biological process
All the carbon atoms in synthesized fatty acids are de-
that uses a source of energy, sunlight, from outside the
rived from the acetyl group of acetyl CoA. In principle,
earth’s environment to convert inorganic molecules to or-
fatty acids could be made by condensation of acetyl units
ganic molecules, including carbohydrates, proteins, nu-
and subsequent reduction. However, the condensation of
cleic acids, lipids, and pigments. Green plants and algae
two acetyl CoA molecules is energetically unfavorable.
are autotrophs; they make their own food. Actually, plants
The release of CO 2 as part of a reaction helps to drive a
synthesize all the thousands of compounds that they con-
reaction to completion. The oxidative decarboxylation re-
tain from CO 2 ,H 2 O, and inorganic nitrogen and sulfur
actions of the citric acid cycle illustrate this fact. The loss
compounds absorbed through the roots. The only source
of CO 2 from the malonyl group as it condenses with the
of carbon is CO 2 , which is assimilated through photosyn-
acetyl group bound to the fatty acid synthetase drives the
thesis. Most other organisms are heterotrophs; they must
condensation reaction. The resulting β-keto compound is
take up and catabolize carbohydrates and fats to provide
reduced to the level of a hydrocarbon by NADPH.
the energy to sustain life. The ultimate source of these
ATP hydrolysis provided the energy for the carboxy-
compounds is photosynthesis, and the source of energy
lation of acetyl CoA. The immediate energy source for
for their synthesis, sunlight. All heterotrophic organisms
the condensation reaction was the loss of the same CO 2
are dependent upon photosynthesis for their existence.
moleculeaddedtotheacetylCoA.ItisclearthatCO 2 plays
Animals also depend on plants for essential organic
a catalytic but essential role in fatty acid biosynthesis.
molecules that they are unable to make. We call some
of these molecules vitamins. Several vitamins, including
C. ATP and Motility
niacin, riboflavin, pyridoxine, and biotin, are key players
At macroscopic and microscopic levels, ATP hydroly- in catabolic and anabolic metabolism, and deficiencies in
sis results in movements. The most familiar of these these vitamins have severe effects. Also, animals are in-
movements are those caused by muscle contraction. Mus- capable of synthesizing polyunsaturated fatty acids (fatty
cle contraction is an example of the conversion of the acids with more than one double bond). Polyunsaturated
phosphate bond (chemical) energy of ATP to mechanical fatty acids are essential components of membrane lipids
energy. Vertebrate muscle is composed of two types of and must be obtained in the diet. So, the next time you
filaments, thick and thin. The protein myosin is the major have a salad, pay a tribute to photosynthesis.
component of the thick filaments, whereas actin and other In photosynthesis, the electromagnetic energy of light
proteins make up the thin filaments. The thick and the thin is converted to chemical energy in the form of organic
filaments are interdigitated. Muscle contraction is thought molecules. The primary photochemical reactions are elec-
to take place by a sliding of the thin filaments relative to tron transfer reactions that create oxidized chlorophylls
the thick filaments. Myosin has ATPase activity. The cat- and reduced acceptors. The reaction center chlorophylls
alytic site in myosin is located on a part of the molecule and the acceptors are arranged within the photosynthetic
(the head) that interacts with the actin filaments. ATP hy- membrane so that the electrons are transferred at least part-
drolysis is thought to cause changes in the interactions of way across the membrane. Thus, the membrane is charged
the myosin head with the actin filaments such that the head by the primary electron transport, and electrical work has
moves along the actin filament in one direction. been done. The electron transport that follows the primary