Introduction to Human Nutrition

50 Introduction to Human Nutrition

according to a sequence directed by the base sequence
of the DNA (the genome), and so they serve as the
currency of protein nutrition and metabolism. The
Human Genome Project completed in 2000 revealed
that the human genome consists of only 30 000 genes,
whereas there may be hundreds of thousands of pro-
teins that are responsible for giving a human its par-
ticular characteristics and uniqueness. A new fi eld of
nutrition research has now opened up and is referred
to as “nutrigenomics,” which is the study of how
nutrition and genomics interact to infl uence health.
Proteins and amino acids fulfi ll numerous functions,
many of which are summarized in Table 4.1. Some

amino acids, such as glutamine (Tables 4.2 and 4.3),

play multiple roles. It is not surprising, therefore, that

inappropriate intakes of proteins and/or of specifi c

amino acids can have important consequences for

tissue and organ function, and the maintenance of

health and the well-being of the individual.

This chapter begins with a short historical perspec-

tive and then moves in Sections 4.3 and 4.4 to discuss

the structure, chemistry, and classifi cation of amino

acids. Section 4.5 is concerned with the biology of

protein and amino acid requirements, with Sections

4.6 and 4.7 describing how the requirements are

established and how they may be met, respectively.

Finally, Section 4.8 examines how factors other than

dietary protein can infl uence the requirements for

proteins and amino acids.

4.2 A historical perspective

The early history of protein metabolism and nutrition

is closely tied to the discovery of nitrogen and its

distribution in nature. The reason for this is that pro-

teins, on average, contain about 16% nitrogen by

weight (to convert nitrogen to protein it is necessary

to multiply by 6.25). Daniel Rutherford, in Edinburgh,

can be regarded as the discoverer of nitrogen, which

he called “phlogisticated air” in his Doctorate in

Table 4.1 Some functions of amino acid and proteins

Function Example

Amino acids
Substrates for protein
synthesis

Those for which there is a codon

Regulators of protein
turnover

Leucine; cysteine; arginine; glutamine

Regulators of enzyme
activity (allosteric)

Glutamate and NAG synthase

Phenylalanine and PAH activation
Precursor of signal
transducer

Arginine and nitric oxide

Methylation reactions Methionine
Neurotransmitter Tryptophan (serotonin); glutamine
Ion fl uxes Taurine; glutamate
Precursor of “physiologic”
molecules

Arg (creatinine); Glu-(NH 2 ) purines

Histidine/β-alanine (carnosine)
Cysteine/glycine/glutamate
(glutathione)
Transport of nitrogen Alanine; glu-(NH 2 )
Regulator of gene
transcription

Amino acid depletion and asparagine
synthase gene activation
Regulator of mRNA
translation

Leucine: alters activity of initiation

factor 4E-BP and P70 (6SK) via

mTOR signaling pathway

Proteins
Enzymatic catalysis Branched chain ketoacid
dehydrogenase
Transport B 12 binding proteins; ceruloplasmin;
apolipoproteins; albumin
Messengers/signals Insulin; growth hormone; IGF-1
Movement Kinesin; actin; myosin
Structure Collagens; elastin; actin
Storage/sequestration Ferritin; metallothionein
Immunity Antibodies; cytokine, chemokines
Growth; differentiation;
gene expression

Peptide growth factors; transcription

factors

IGF-1, insulin-like growth factor-1; NAG, N-acetyl glutamate; PAH,
phenylalanine hydroxylase; glu-(NH 2 ), glutamine.

Table 4.2 Multiple functions of an amino acid; glutamine as an

example

Substrate of protein synthesis (codons: CAA, CAG)

Anabolic/trophic substance for muscle; intestine (“competence

factor”)

Controls acid–base balance (renal ammoniagenesis)

Substrate for hepatic ureagenesis

Substrate for hepatic/renal gluconeogenesis

Fuel for intestinal enteroctyes

Fuel and nucleic acid precursor and important for generation of

cytotoxic products in immunocompetent cells

Ammonia scavenger

Substrate for citrulline and arginine synthesis

Nitrogen donor (nucleotides, amino sugars, coenzymes)

Nitrogen transport (1/3 circulating N) (muscle; lung)

Precursor of GABA (via glutamate)

Shuttle for glutamate (CNS)

Preferential substrate for GSH production?

Osmotic signaling mechanism in regulation of protein synthesis?

Stimulates glycogen synthesis

L-Arginine NO metabolism

Taste factor (umami)

CNS, central nervous system; GABA, γ-aminobutyric acid; GSH, growth-

stimulating hormone; NO, nitric oxide.