amine group is removed from amino acids, and the carbon chain is used for carbohydrate, lipid, or

nonessential amino acid synthesis.10

Ten nutritionally essential amino acids must be obtained from dietary intake (Table 2-2). However,

human tissues contain transferases, which convert the α-keto acids of leucine, valine, and isoleucine so

that the corresponding α-keto acids can be used as dietary supplements. The remaining nutritionally

nonessential amino acids can be synthesized in one to three enzyme-catalyzed reactions. Hydroxyproline

and hydroxylysine do not have a corresponding tRNA and arise by posttranslational modification of

proline or lysine by mixed function oxidases. Glutamate, glutamine, and proline are derived from the

citric acid cycle intermediate α-ketoglutarate. Aspartate and asparagine are synthesized from

oxaloacetate. Serine and glycine are synthesized from the glycolysis intermediate 3-phosphoglycerate.

Cysteine and tyrosine are formed from essential amino acids (methionine and phenylalanine,

respectively).11

Table 2-1 Amino Acids Required by Adult Humans

Catabolism of Amino Acid Nitrogen

Ammonia, derived largely from the deamination of amino acids, is toxic to all mammalian cells. The

ammonia formed as a result of the deamination of amino acids is detoxified by one of two routes.12 The

most important pathway involves the conversion of ammonia to urea by enzymes of the Krebs–

Henseleit, or urea cycle, which occurs only in the liver (Fig. 2-10). A second route of ammonia

metabolism involves synthesis of L-glutamine from ammonia and glutamate by renal glutamine

synthetase.

CELLULAR ENERGY GENERATION

Overview and Stage I

5 The citric acid cycle includes a series of mitochondrial enzymes that transform acetyl CoA – itself

derived from pyruvate or fatty acyl CoA – into water, carbon dioxide, and hydrogen-reducing

equivalents. Each molecule of acetyl CoA that enters the citric acid cycle yields 12 molecules of ATP.

The fundamental mechanism by which mammalian cells generate energy is the aerobic conversion of

sugars and fatty acids into ATP. There are four stages with stage I – glycolysis – (see glycolysis above)

beginning in the cytosol, converting glucose into two molecules of pyruvate. Also, cytosolic fatty acids

are converted to fatty acyl CoA. Pyruvate and fatty acyl CoA are transported to the mitochondrial

matrix and converted to acetyl CoA; generating the electron carriers NADH or FADH2 as well as CO2

. In

stage II, mitochondrial acetyl CoA enters the citric acid cycle further generating NADH, FADH2

,

additional CO2

, and GTP. In stage III, oxygen is reduced to water via the electron transport chain using

previously generated molecules of NADH and FADH2 as electron donors. The electron transport chain

causes hydrogen ions to move from the mitochondrial matrix to the intermembrane space generating a

proton motive force. Lastly, in stage IV, ATP synthase uses energy generated by the proton motive force

to generate large amounts of ATP.13

Stage II: The Citric Acid Cycle: Integration of Metabolic Pathways and Oxidation of Acetyl

60

CoA

One major function of the citric acid cycle (also known as the Krebs cycle or the tricarboxylic acid

cycle) is to act as a common pathway for the oxidation of carbohydrate, lipid, and protein and generate

energy in the form of ATP. Conversely, the citric acid cycle is important in gluconeogenesis,

lipogenesis, and amino acid metabolism. In the fed state, a large proportion of ingested energy from

foodstuffs is converted to glycogen or fat. The metabolism of sugars, fats, and proteins, then, allows

adequate fuels for all tissue types under conditions from fed to fasting to starvation. The body

accomplishes production of fuel substrates for organs and regulates intestinally absorbed nutrients for

tissue consumption or storage by integrating three key metabolites: G6P, pyruvate, and acetyl CoA (Fig.

2-11). Each of these three simple chemical molecules can be extensively modified to allow a large

number of metabolites.

Figure 2-10. The urea cycle. Ammonia entering the urea cycle is derived from protein and amino acid degradation in tissues

(endogenous) and the colonic lumen (exogenous).

G6P can be stored as glycogen or converted into glucose, pyruvate, or ribose-5-phosphate (a

nucleotide precursor). Pyruvate can be converted into lactate, alanine (and other amino acids), and

acetyl CoA, or it can enter the tricarboxylic acid cycle by conversion to oxaloacetate. Acetyl CoA is

converted to HMG-CoA (a cholesterol and ketone body precursor) or citrate (for fatty acid and

triglyceride synthesis), or it is degraded to carbon dioxide and water for energy. In humans, acetyl CoA

cannot be converted into pyruvate due to the irreversible reaction of pyruvate dehydrogenase. Thus,

lipids cannot be converted into either carbohydrates or glucogenic amino acids.

Probably the most important citric acid cycle function is to oxidize acetyl CoA into CO2

. The citric

acid cycle is composed of a series of enzyme reactions that occur in the mitochondrial matrix and inner

membrane. Here, acetyl CoA combines with oxaloacetate to form citrate. Through a series of

subsequent enzymatic reactions involving both dehydrogenases and decarboxylases, citrate is

catabolized to result in the generation of hydrogen-reducing equivalents and carbon dioxide (Fig. 2-12).

With each revolution of the citric acid cycle, a molecule of acetyl CoA generates three molecules of the

reduced coenzyme NADH, one molecule of the reduced coenzyme FADH2

, and one molecule of GTP.

The reduced coenzymes NADH and FADH2 are charged with high-energy electrons that subsequently

drive the electron transport chain of stage III to generate a hydrogen ion gradient. These reducing

equivalents are transported to the inner mitochondrial membrane and the electron transport chain to

generate more ATP. Each molecule of NADH is oxidized to yield three molecules of ATP, and each

molecule of FADH2

is oxidized to yield two molecules of ATP. One molecule of ATP is generated at the

substrate level in the conversion of succinyl CoA to succinate; thus, the total molecule of ATP generated

per molecule of acetyl CoA is 12.

61

Stages III and IV: Oxidative Phosphorylation

6 Oxidative phosphorylation converts the energy from NADH and FADH2

into ATP by the electron

transport chain and ATP synthase with a process called the proton motive force. The covalent bond

energy in glucose and fatty acids is transferred into high-energy electrons in stages I (glycolysis) and II

(citric acid cycle). Flow of these high-energy electrons from NADH and FADH2

to oxygen, creating

water, is coupled to transport of protons across the mitochondrial inner membrane from the matrix to

the intermembrane space. Originally called Mitchell’s chemiosmotic hypothesis, many researchers now

refer to this process as the proton motive force. The voltage gradient caused by the transport of these

protons, and the ATP subsequently generated is the process known as oxidative phosphorylation. Below

is an admittedly simplified explication of a complex process, and the stoichiometry of the reactions has

been modified for clarity. Anyone interested in the precise biochemistry should consult the relevant

sources (Fig. 2-13).

Two high-energy electrons carried by NADH or FADH2 pass through three of four major multiprotein

complexes: I – NADH-CoQ reductase; II – succinate-CoQ reductase; III – CoQH2

-cytochrome c reductase;

and, IV – cytochrome c oxidase. The paths of NADH and FADH2 are different initially. NADH electrons

are transferred through complex I – NADH-CoQ reductase – to flavin mononucleotide (FMN), seven

iron–sulfur clusters (Fe-S), and then coenzyme Q (CoQ) to create CoQH2

. Four protons are transported

from the mitochondrial matrix as a result of the actions of complex I. Unlike NADH, FADH2

is oxidized

by complex II – succinate-CoQ reductase. The two FADH2 electrons are transferred to succinate

dehydrogenase-bound FAD when succinate is oxidized to fumarate, then an iron–sulfur cluster, and

finally to CoQ creating CoQH2

.

Figure 2-11. Summation of the key regulatory molecules used by the liver during diverse metabolic functions. Essentially, any

compound found in the body can be synthesized in the liver from glucose-6-phosphate, acetyl coenzyme A, or pyruvate. As a

consequence of the inability of mammalian liver to convert acetyl coenzyme A to pyruvate, fats cannot be converted to

carbohydrates.

62

Figure 2-12. The citric acid cycle. Reduced nicotinamide adenine dinucleotide and reduced flavin adenine dinucleotide, formed in

the citric acid cycle, are subsequently oxidized in mitochondria by means of the electron transport chain to generate ATP. Acetyl

CoA plays a key role.

CoQH2

from NADH or FADH2

is shuttled to complex III – CoQH2

-cytochrome c reductase – within the

inner membrane. The two electrons are further transferred to cytochromes bL and bH, resulting in

pumping of two additional hydrogen ions into the intermembranous space. Electrons are further

transferred to another iron–sulfur cluster, cytochrome c1

, and ultimately the intermembranous space

protein cytochrome c, pumping two additional hydrogen ions. Complex II catalyzes the conversion of

fatty acyl CoA to acetyl CoA for further metabolism through the citric acid cycle. Cytochrome c shuttles

the two electrons through the intermembranous space to complex IV – cytochrome c oxidase –

reoxidizing the cytochrome c molecule, transferring the electrons to copper containing Cua

, the heme

moiety of cytochrome a3

, Cub

, cytochrome a3

, and ultimately to oxygen, yielding water.

The net result of the electron transport chain is the pumping of ten H+ ions into the intermembranous

space for two electrons flowing from NADH to O2

, and six H+ ions for each two electrons from FADH2

to O2

. This generates the proton motive force; a voltage gradient across the inner mitochondrial

membrane that directly provides energy for ATP generation in stage IV. ATP synthase harnesses the

voltage gradient of protons across the inner membrane by interconverting the chemical potential energy

into phosphoanhydride bonds of ATP. ATP synthase is composed of two main complexes: F0,

, consisting

of three types of membrane proteins, and F1

, a five-polypeptide complex protruding into the matrix.

Alternative Fuels

Regardless of the fed state of the human body, there is a requirement for glucose utilization. The

nervous system and erythrocytes have an absolute requirement for glucose. Glucose is a source of

glycerol-3-phosphate for adipose tissue, and most other tissues for integrity of the citric acid cycle. To

maintain adequate glucose for survival, other fuels can be used depending on environmental conditions.

Under conditions of carbohydrate shortage, ketone bodies and free fatty acids are utilized to spare

oxidation of glucose in muscle. These alternate fuels increase intracellular citrate, which inhibits both

phosphofructokinase and pyruvate dehydrogenase. In starvation, fatty acid oxidation results in the

production of glycerol, which, along with gluconeogenesis from amino acids, is the only source of the

63