Page 5 - 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 101

of glucose by P i is an unfavorable reaction, characterized
by a G of about 4 kcal/mol, at pH 7.0 and 25 C. (Note

◦
0
that the biochemist’s standard state differs from that as
usually defined in that the activity of the hydrogen ion is
taken as 10 −7 M, or pH 7.0, rather than 1 M, or pH 0.0.
pH 7.0 is much closer to the pH in most cells.) This prob-
lem is neatly solved in cells by using ATP, rather than P i ,
as the phosphoryl donor:
Glucose + ATP ←→ Glucose 6-phosphate + ADP.
The G for this reaction, which is catalyzed by the en-

0
zyme hexokinase, is approximately −4 kcal/mol. Thus the
phosphorylation of glucose by ATP is an energetically fa-
vorable reaction and is one example of how the chemical
energy of ATP may be used to drive otherwise unfavorable
reactions.
Glucose 6-phosphate is then isomerized to form fruc-
tose 6-phosphate, which in turn is phosphorylated by ATP
at the 1-position to form fructose 1,6-bisphosphate. It
FIGURE 2 Some important reactions in metabolism. Shown are seems odd that a metabolic pathway invests 2 mol of ATP
+
the phosphorylation of ADP to ATP, NAD , NADH, FAD, FADH 2 in the initial steps of the pathway when ATP is an im-
acetate, CoA, and acetyl CoA. For clarity, just the parts of the
portant product of the pathway. However, this investment
+
larger molecules that undergo reaction are shown. NAD , nicoti-
pays off in later steps.
namide adenine dinucleotide; NADH, nicotinamide adenine dinu-
cleotide (reduced form); FAD, flavin adenine dinucleotide; FADH 2 , Fructose 1,6-bisphosphate is cleaved to form two triose
flavin adenine dinucleotide (reduced form); CoA, coenzyme A; phosphates that are readily interconvertible. Note that the
AMP, adenosine monophosphate. oxidation–reduction state of the triose phosphates is the
same as that of glucose 6-phosphate and the fructose phos-
phates. All molecules are phosphorylated sugars. In the
from carbon dioxide, water, and inorganic nutrients. The next step of glycolysis, glyceraldehyde 3-phosphate is ox-
discussion of the complicated topic of metabolism is idized and phosphorylated to form a sugar acid that con-
somewhat simplified by separation of the subject into tains a phosphoryl group at positions 1 and 3. The oxidiz-
two areas—catabolic and anabolic metabolism. Catabolic ing agent, nicotinamide adenine dinucleotide (NAD ), is
+
metabolism is degradative and is generally exergonic. ATP a weak oxidant (E , at pH 7.0 of −340 mV). The oxida-

0
is a product of catabolic metabolism. In contrast, an- tion of the aldehyde group of glyceraldehyde 3-phosphate
abolic metabolism is synthetic and requires ATP. Fortu- to a carboxylate is a favorable reaction that drives both
nately, there are relatively few major pathways of energy the oxidation and the phosphorylation. This is the only
metabolism. oxidation–reduction reaction in glycolysis.
The hydrolysis of acyl phosphates, such as that of
position 1 of 1,3-bisphosphoglycerate, is characterized
A. Glycolysis and Fermentation

by strongly negative G values. That for 1,3-bisphos-
0
Carbohydrates are a major source of energy for organisms. phoglycerate is approximately −10 kcal/mol, which is

The major pathway by which carbohydrates are degraded significantly more negative than the G for the hydrol-
0
is called glycolysis. Starch, glycogen, and other carbohy- ysis of ATP to ADP and P i . Thus, the transfer of the acyl
drates are converted to the sugar glucose by pathways that phosphate from 1,3-bisphosphoglycerate to ADP to form
will not be considered here. In glycolysis, glucose, a six- 3-phosphoglycerate and ATP is a spontaneous reaction.
carbon sugar, is oxidized and cleaved by enzymes in the Since two sugar acid bisphosphates are formed per glu-
cytoplasm of cells to form two molecules of pyruvate, a cose metabolized, the two ATP invested in the beginning
three-carbon compound (see Figs. 3 and 4). The overall of the pathway have been recovered.
reaction is exergonic and some of the energy released is In the next steps of glycolysis, the phosphate on the
conserved by coupling the synthesis of ATP to glycolysis. 3-position of the 3-phosphoglycerate is transferred to the
Before it may be metabolized, glucose must first be hydroxyl residue at position 2. Removal of the elements
phosphorylated on the hydroxyl residue at position 6. of water from 2-phosphoglycerate results in the formation
Under intracellular conditions, the direct phosphorylation of an enolic phosphate compound, phospho(enol)pyruvate