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Bioinorganic Chemistry 127

anotherpotentialgradient,thistimeacrossamitochondrial system I, specifically to reduce P + , where it rests until
700
membrane. This potential gradient across a mitochondrial P 700 is excited by energy transferred from light by the
membrane is also used to generate ATP from ADP. chlorophyll surrounding photosystem I.
The electron is then transferred through the membrane
via chlorophyll, a quinone, and iron–sulfur clusters to a
1. Photosynthesis
ferridoxin on the inside of the chloroplast. The electron
Photosynthesis is the process of converting solar radiation is then used to generate NADPH, an organic proton and
into chemical energy. This occurs in plants, algae, and electron carrier, which carries out many chemical trans-
photosynthetic bacteria. Cyclic photosynthesis (nonoxy- formations inside the chloroplast.
genic) only uses photosystem I to capture light, whereas The energy to make ATP is generated by a proton
noncyclic (oxygenic) photosynthesis couples the oxida- gradient across the membrane, a result of water oxida-
tion of water to oxygen with photon capture using both tion (which produces four protons per oxygen molecule
photosystem I and photosystem II reaction centers. Cyclic formed). The protein ATP synthase converts the energy
photosynthesis is a less efficient light-harvesting scheme from this gradient to chemical energy through formation
used by bacteria. The chlorophyll is excited by light, and of ATP. The electrons of photosynthesis are used to fix
electrons flow through a series of iron–sulfur clusters. The carbon dioxide and produce sugars such as glucose.
+
electrons are used to reduce NADP . Electrons eventu-
ally flow back to reduce the reaction-center chlorophyll
2. Respiration
through flavoproteins and the heme proteins cytochromes
bc 1 and c 2 . Oxidation of glucose by oxygen to carbon dioxide and
In higher organisms, the light reactions of photosyn- water is the overall reaction in respiration:
thesis take place in the membrane of chloroplasts. A
schematic view of the enzymatic machinery for oxygenic C 6 H 12 O 6 + 6O 2 −→ 6CO 2 + 6H 2 O. (5)
photosynthesis from spinach is shown in Fig. 7a. The pho- This process yields a substantial amount of energy and
tosynthetic machinery lies in the membrane of chloro- is harnessed to synthesize 38 ATP molecules from ADP.
plasts. By coupling the oxidation of water to oxygen in In eukaryotes, the mitochondria are the site of oxidative
noncyclic photosynthesis, higher plants efficiently cap- metabolism. As with photosynthesis, inorganic elements
ture solar energy. As shown, many inorganic elements play essential roles in respiration. In the initial stages
are included in this process. The light vibrationally ex- of respiration, glucose is broken down into two pyru-
cites magnesium-containing modified porphyrins called vate molecules, C 3 H 3 O , in a process termed glycolysis.
−
3
chlorophylls. The vibrational energy is then funneled from This process requires enzymes that contain functional in-
these “antenna chlorophyll” to a special pair of chloro- organic elements (Table II). Each pyruvate is oxidized by
phylls, P 680 . An electron within the special pair is excited NAD to form acetyl CoA, CH 3 CO–SCoA, and carbon
+
and transferred through a series of organic cofactors to dioxide, producing two equivalents of NADH. The acetyl
+
−
a dissociable quinone, Q B . This quinone passes the elec- group, CH 3 CO , is then oxidized by three NAD and one
trons to PSI via cytochrome b 6 f and plastocyanin (see FAD to produce two carbon dioxide molecules and three
below). The electron removed from P 680 is replenished by NADH and one FADH 2 in a series of reactions known as
a cluster of four manganese ions that form the catalytic the citric acid cycle. The citric acid cycle contains more
center of the oxygen-evolving complex (OEC). The OEC metalloenzymes. The NADH molecules produced by gly-
is responsible for water oxidation. The manganese cluster colysis and the citric acid cycle are oxidized by oxygen
accumulates four oxidizing equivalents before converting with a mechanism that produces a total of 34 ATPs per
water to oxygen as given by molecule of glucose.
The energy necessary to generate ATP is extracted from
+
−
2H 2 O −→ 4H + 4e + O 2 . (4)
the oxidation of NADH and FADH 2 by the electron trans-
The structure of the manganese cluster has been one of port chain, a series of four protein complexes, denoted
the most controversial areas of bioinorganic chemistry; it Complexes I–IV (Fig. 7b). NADH is oxidized by Com-
is expected that this issue will soon be resolved through plex I; FADH 2 is oxidized by Complex II. Each complex
X-ray crystallographic analysis. contains multiple redox centers: several iron–sulfur pro-
Cytochrome b 6 f is an electron transfer protein that con- teins and flavin mononucleotide in Complex I, and three
tains several iron sites including hemes and iron–sulfur iron–sulfur centers and a heme in Complex II. The elec-
clusters. This complex migrates through the membrane trons are then passed to coenzyme Q, which contains an
and transfers the electron to plastocyanin, a blue copper organic redox center. Coenzyme Q transfers the electrons
protein. Plastocyanin then transfers the electron to photo- to Complex III. Complex III contains three hemes and