Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 1General Principles & Energy Production in Medical Physiology 19

METABOLIC FUNCTIONS

OF AMINO ACIDS

In addition to providing the basic building blocks for proteins,
amino acids also have metabolic functions. Thyroid hor-
mones, catecholamines, histamine, serotonin, melatonin, and
intermediates in the urea cycle are formed from specific ami-
no acids. Methionine and cysteine provide the sulfur con-
tained in proteins, CoA, taurine, and other biologically
important compounds. Methionine is converted into S-ade-
nosylmethionine, which is the active methylating agent in the
synthesis of compounds such as epinephrine.

CARBOHYDRATES

Carbohydrates are organic molecules made of equal amounts
of carbon and H 2 O. The simple sugars, or monosaccharides,
including pentoses (5 carbons; eg, ribose) and hexoses (6 car-
bons; eg, glucose) perform both structural (eg, as part of nu-
cleotides discussed previously) and functional roles (eg,
inositol 1,4,5 trisphosphate acts as a cellular signaling mole-
cules) in the body. Monosaccharides can be linked together to
form disaccharides (eg, sucrose), or polysaccharides (eg, gly-
cogen). The placement of sugar moieties onto proteins (glyco-
proteins) aids in cellular targeting, and in the case of some

FIGURE 1–19 Involvement of the citric acid cycle in transamination and gluconeogenesis. The bold arrows indicate the main pathway
of gluconeogenesis. Note the many entry positions for groups of amino acids into the citric acid cycle. (Reproduced with permission from Murray RK et al:
Harper’s Biochemistry, 26th ed. McGraw-Hill, 2003.)

Transaminase

Transaminase

Transaminase

Phosphoenolpyruvate

carboxykinase

Oxaloacetate

Aspartate

Citrate

α-Ketoglutarate

Succinyl-CoA

Fumarate

Phosphoenolpyruvate

CO 2

CO 2

Alanine Pyruvate Acetyl-CoA

Glutamate

Histidine

Proline

Glutamine

Arginine

Isoleucine

Methionine

Valine

Hydroxyproline

Serine

Cysteine

Threonine

Glycine

Tyrosine

Phenylalanine

Propionate

Glucose

Tryptophan

Lactate

FIGURE 1–20 Urea cycle. The processing of NH 3 to urea for ex-

cretion contains several coordinative steps in both the cytoplasm (Cy-

to) and the mitochondria (Mito). The production of carbamoyl

phosphate and its conversion to citrulline occurs in the mitochondria,

whereas other processes are in the cytoplasm.

NH 2 +

NH 3 +

NH 3 +

NH 4 + NH 3

H 3 N+

Argininosuccinate

H 2 N

C

HN

COO−

COO−

HC

(CH 2 ) 3

(CH 2 ) 3

HC

O ——

NH 3 +

H 2 N

C

Pi

HN

COO−

HC

(CH 2 ) 3

——

Aspartate Fumarate

Citrulline+ NO Arginine

Carbamoyl

phosphate

Urea

Ornithine

O

NH 2

C

NH 2

——

Cyto

Mito