7.8 Regulation of Blood Glucose
Adequate concentrations of glucose in the blood are necessary for brain cells
as they cannot metabolize substances other than glucose and ketone bodies
as energy sources nor can they store or synthesize glucose. After a meal, any
released glucose is absorbed by the gastrointestinal tract (Chapter 11) enters
the bloodstream and is delivered to the peripheral tissues where it may be
metabolized to allow ATP production. Surplus glucose is converted to glycogen
and stored in the liver and skeletal muscles or converted to triacylglycerols
and stored in adipose tissue. During fasting, the liver produces glucose by
glycogenolysis or gluconeogenesis and this is used to maintain blood glucose
concentration (Figure 7.22).
The concentration of glucose in plasma is regulated by the hormones insulin
and glucagon. Insulin is synthesized as preproinsulin in the B cells of the
islets of Langerhans in the pancreas but during its secretion is enzymatically
converted to active insulin (Figures 7.3 (B) and 7.23). Insulin has a number of
functions. It inhibits glycogenolysis, gluconeogenesis, lipolysis, ketogenesis
REGULATION OF BLOOD GLUCOSECZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY &,,
Figure 7.22 Overview of glucose homeostasis.
See text for details.Margin Note 7.2 Sanger and
sequencesSanger determined the complete
sequences of the amino acid
residues, the primary structures, of
both chains of insulin in the early
1950s. This was the first unequivocal
demonstration that proteins have
strictly defined primary structures.
With the techniques of chemistry
and molecular biology available at
that time, this was an incredible
achievement and Sanger was
awarded the 1958 Nobel Prize in
Chemistry. Nowadays, sequencing a
protein as small as insulin would be a
trivial task. The sequences of smallish
peptides are easily determined by
mass spectrometry and it is relatively
easy to determine the base sequence
of a gene (DNA) and then interpret
it in terms of the amino acid order
of the encoded protein. Indeed,
complete genome sequences of
numerous organisms, including those
of human beings and a number of
human pathogens and parasites,
are known and are widely available
in various websites. Incredibly, the
major method for sequencing DNA,
the dideoxy method, was also devised
by Sanger in 1977, when he and
coworkers published the complete
sequence of the genome of the virus,
JX174. Sanger was awarded the
Nobel Prize in Chemistry in 1980 for
this work. Sanger thus belongs to
the tiny elite group of people to have
received two Nobel Prizes.i
TriacylglycerolsSystemic
circulationDietary
carbohydratesGlucoseGlucoseGlucoseGlucoseGlucoseGlucoseATPGastrointestinal
tractLiver
Glycogenolysis GluconeogenesisLactate
Amino acidsLactate
Amino acidsGlycerolGlycerolFat storesGlycogen
storesCO 2 + H 2 OADP+PiGeneral
tissues