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