chain occur in the mitochondria. Most of the energy
derived from carbohydrate,protein, and fat is pro-
duced via the Krebs cycle and the electron transport
system.
Glycogenesisis the conversion of excess glucose to
glycogen.Glycogenolysisis the conversion of glycogen
to glucose (which could occur several hours after a
meal or overnight) in the liver or, in the absence of
glucose-6-phosphate in the muscle, to lactate.Gluco-
neogenesisis the formation of glucose from noncarbo-
hydrate sources, such as certain amino acids and the
glycerol fraction of fats when carbohydrate intake is
limited. Liver is the main site for gluconeogenesis,
except during starvation, when the kidney becomes
important in the process. Disorders of carbohydrate
metabolism include diabetes mellitus, lactose intoler-
ance, and galactosemia.
Protein Metabolism
Proteins contain carbon, hydrogen, oxygen, nitro-
gen, and sometimes other atoms. They form the cellu-
lar structural elements, are biochemical catalysts, and
are important regulators of gene expression. Nitrogen
is essential to the formation of twenty different amino
acids, the building blocks of all body cells. Amino
acids are characterized by the presence of a terminal
carboxyl group and an amino group in the alpha
position, and they are connected by peptide bonds.
Digestion breaks protein down to amino acids. If
amino acids are in excess of the body’s biological
requirements, they are metabolized to glycogen or fat
and subsequently used for energy metabolism. If
amino acids are to be used for energy their carbon
skeletons are converted to acetyl CoA, which enters
the Krebs cycle for oxidation, producing ATP. The
final products of protein catabolism include carbon
dioxide, water, ATP, urea, and ammonia.
Vitamin B 6 is involved in the metabolism (espe-
cially catabolism) of amino acids, as a cofactor in
transamination reactions that transfer the nitrogen
from one keto acid (an acid containing a keto group
‘-CO-’ in addition to the acid group) to another. This is
the last step in the synthesis of nonessential amino
acids and the first step in amino acid catabolism.
Transamination converts amino acids to L-glutamate,
which undergoes oxidative deamination to form
ammonia, used for the synthesis of urea. Urea is trans-
ferred through the blood to the kidneys and excreted
in the urine.
The glucose-alanine cycle is the main pathway by
which amino groups from muscle amino acids are
transported to the liver for conversion to glucose.
The liver is the main site of catabolism for all essential
amino acids, except the branched-chain amino acids,
which are catabolized mainly by muscle and the kid-
neys. Plasma amino-acid levels are affected by dietary
carbohydrate through the action of insulin, which
lowers plasma amino-acid levels (particularly the
branched-chain amino acids) by promoting their
entry into the muscle.
Body proteins are broken down when dietary
supply of energy is inadequate during illness or pro-
longed starvation. The proteins in the liver are utilized
in preference to those of other tissues such as the brain.
The gluconeogenesis pathway is present only in liver
cells and in certain kidney cells.
Disorders of amino acid metabolism include phe-
nylketonuria, albinism, alkaptonuria, type 1 tyrosi-
naemia, nonketotic hyperglycinaemia, histidinaemia,
homocystinuria, and maple syrup urine disease.Fat (Lipid) Metabolism
Fats contain mostly carbon and hydrogen, some
oxygen, and sometimes other atoms. The three main
forms of fat found in food are glycerides (principally
triacylglycerol ‘triglyceride’, the form in which fat is
stored for fuel), the phospholipids, and the sterols
(principally cholesterol). Fats provide 9 kilocalories
per gram (kcal/g), compared with 4 kcal/g for carbo-
hydrate and protein. Triacylglycerol, whether in the
form of chylomicrons (microscopic lipid particles) or
other lipoproteins, is not taken up directly by any
tissue, but must be hydrolyzed outside the cell to
fatty acids and glycerol, which can then enter the cell.
Fatty acids come from the diet, adipocytes (fat
cells), carbohydrate, and some amino acids. After
digestion, most of the fats are carried in the blood as
chylomicrons. The main pathways of lipid metabolism
are lipolysis, betaoxidation, ketosis, and lipogenesis.
Lipolysis (fat breakdown) and beta-oxidation
occurs in the mitochondria. It is a cyclical process in
which two carbons are removed from the fatty acid per
cycle in the form of acetyl CoA, which proceeds
through the Krebs cycle to produce ATP, CO 2 , and
water.
Ketosis occurs when the rate of formation of
ketones by the liver is greater than the ability of tissues
to oxidize them. It occurs during prolonged starvation
and when large amounts of fat are eaten in the absence
of carbohydrate.
Lipogenesis occurs in the cytosol. The main sites
of triglyceride synthesis are the liver, adipose tissue,
and intestinal mucosa. The fatty acids are derivedMetabolism