Ganong's Review of Medical Physiology, 23rd Edition

Hydroxyproline
Serine
Cysteine
Threonine
Glycine
Isoleucine
Methionine
Valine
Tryptophan
Glucose
Tyrosine
Phenylalanine
Propionate
METABOLIC FUNCTIONS
OF AMINO ACIDS
In addition to providing the basic building blocks for proteins,
amino acids also have metabolic functions. Thyroid hormones,
catecholamines, histamine, serotonin, melatonin, and
intermediates in the urea cycle are formed from specific amino
acids. Methionine and cysteine provide the sulfur contained
in proteins, CoA, taurine, and other biologically
important compounds. Methionine is converted into S-adenosylmethionine,
which is the active methylating agent in the
synthesis of compounds such as epinephrine.
CARBOHYDRATES
Histidine
Proline
Glutamine
Arginine
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 carbons;
eg, glucose) perform both structural (eg, as part of nucleotides
discussed previously) and functional roles (eg,
inositol 1,4,5 trisphosphate acts as a cellular signaling molecules)
in the body. Monosaccharides can be linked together to
form disaccharides (eg, sucrose), or polysaccharides (eg, glycogen).
The placement of sugar moieties onto proteins (glycoproteins)
aids in cellular targeting, and in the case of some
CHAPTER 1 General Principles & Energy Production in Medical Physiology 19
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.)
Alanine
Transaminase
Phosphoenolpyruvate
carboxykinase
Phosphoenolpyruvate
Oxaloacetate
Fumarate
Succinyl-CoA
CO 2
Aspartate
H 2 N
C — O
HC
COO −
Pi
Lactate
Pyruvate
Aspartate
NH 3 +
Transaminase
α-Ketoglutarate
Cyto
Glutamate
NH 4 + NH 3
Acetyl-CoA
Citrate
CO 2
Transaminase
Argininosuccinate
H3N +
Ornithine
(CH 2 ) 3
COO −
NH 3 +
H 2 N
NH +
C —
2
HN
HN
(CH2 ) 3
Citrulline + NO Arginine
(CH2 ) 3
Carbamoyl
phosphate
Mito
COO −
NH 3 +
FIGURE 1–20 Urea cycle. The processing of NH 3 to urea for excretion
contains several coordinative steps in both the cytoplasm (Cyto)
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.
HC
Fumarate
HC
Urea
NH 2
C — O
NH 2