Role of Proteins in Insulin Secretion and Glycemic Control - Advances in Food and Nutrition Research - page 6

differentially affect incretin secretion ( Hall, Millward, Long, & Morgan,
2003 ). A large volume of research has conclusively shown the importance
of proteins in glucose homeostasis ( Gannon & Nuttall, 2010 ) and the need
to consider protein quality in its regulation ( Millward, Layman, Tom´,&
Schaafsma, 2008 ).
3. CLASSIFICATION OF PROTEINS AND
PROTEIN QUALITY
Although high-quality dietary proteins are predominantly found in
animal foods, plant foods (legumes, cereals) too contain appreciable
amounts. Vegetable sources are the major protein source in most developing
countries where animal proteins are expensive and are of limited availability.
The quality of a protein depends on its characteristics, the food matrix they
are in, and how well they meet the metabolic needs of individuals ( Millward
et al., 2008 ) . Twenty one amino acids are involved in protein synthesis and
several more occur as metabolic intermediates that are not involved in pro-
tein synthesis. Anatomically, all amino acids are similar in that they contain
both an amino group and a carboxylic acid group attached to an ( a ) carbon
atom. What sets them apart from each other are the characteristics of the side
chain attached to the a -carbon. Based on the chemical characteristics of the
side chain, amino acids can be grouped into distinct categories ( Fig. 1.2 ) .
In addition, the side chains are also characterized by the presence of groups
such as alcohol, thiol, indole, and imidazole ( Reeds, 2000 ). These chemical
characteristics determine how each amino acid behaves in biochemistry.
Proteins are linear polypeptides made of amino acids and are naturally found
folded into secondary and tertiary structures. The secondary and tertiary
structures are formed through the formation of hydrogen bonds and due
to the interactions between side chains of amino acids. Therefore, the phys-
ical and functional characteristics of a protein depend on its amino acid
complement.
Early studies showed that all proteins were not nutritionally equal and
that some were more important toward nitrogen balance than others. Essen-
tial amino acids are those that cannot be synthesized by the human body
( Mercer, Dodds, & Smith, 1989 ) and must be met through the diet. Nine
amino acids are essential for humans (histidine, isoleucine, leucine, lysine,
methionine, phenylalanine, threonine, tryptophan, and valine), while the
remainder could be synthesized frommetabolic intermediates (alanine, argi-
nine, aspartate, asparagines, cysteine, glutamate, glutamine, glycine, proline,