atoms). Some proteins require a trace element such as
iron or zinc to complete their structure and permit
them to function properly.
Our body proteins have many functions; some of
these are listed in Table 2–5 and will be mentioned
again in later chapters. And though we usually do not
think of protein as an energy food, if our diet includes
more amino acids than are necessary for our protein
synthesis, those excess amino acids will be converted
to simple carbohydrates or even to fat, to be stored as
potential energy. (See Box 2–4: A Protein Mystery:
Prions, for a discussion of disease-causing proteins.)
One very important function of proteins will be dis-
cussed further here: the role of proteins as enzymes.
EnzymesEnzymesare catalysts, which means that they speed
up chemical reactions without the need for an external
source of energy such as heat. The many reactions
that take place within the body are catalyzed by spe-
cific enzymes; all of these reactions must take place at
body temperature.
The way in which enzymes function as catalysts is
called the active site theory, and is based on the
shape of the enzyme and the shapes of the reacting
molecules, called substrates. A simple synthesis reac-
tion is depicted in Fig. 2–9A. Notice that the enzyme
has a specific shape, as do the substrate molecules.38 Some Basic Chemistry
NH 2 C COOHHR- Amino
group
Carboxyl
group
Variable portion
••Peptide bondsIron in hemeA Amino acid
B Polypeptide
C Primary structure
D Secondary structure-
alpha helix
E Tertiary structure-
myoglobinFigure 2–8. Amino acid and protein structure. (A) The structural formula of an amino
acid. The “R” represents the variable portion of the molecule. (B) A polypeptide. Several
amino acids, represented by different shapes, are linked by peptide bonds. (C) The primary
structure of a protein. (D) The secondary structure of a protein. (E) The tertiary structure
of the protein myoglobin. See text for further description.
QUESTION:What mineral is part of myoglobin, and what is its function?