Chapter 69 – Protein Metabolism
The principal constituents of proteins are amino acids. All 20 amino acids have two features
in common: they have an acidic group (-COOH) and a nitrogen atom attached to the molecule
(-NH2). The amino acids of proteins are aggregated into long chains by means of peptide
linkages; water is released. Some molecules are composed of several protein chains, which
are bound to one another by other linkages such as hydrogen bonding between CO and NH
radicals. This is causing the folded proteins. Amino acids are absorbed from the digestive
tract into the blood (small quantities at a time). The molecules of all amino acids are too large
to diffuse readily through the pores of the cell membranes. Move either inward or outward
through the membranes only by facilitated transport or active transport using carrier
mechanisms. However, in the kidneys the different amino acids can be actively reabsorbed
through the proximal tubular epithelium (nevertheless, upper limit; too much lost in urine).
Almost immediately after entry into tissue cells, amino acids combine with one another by
peptide linkages under the direction of the cell’s messenger RNA and ribosomal system to
form cellular proteins. Storage of large quantities does not occur. However, many of the
intracellular proteins can be rapidly decomposed again into amino acids under the influence of
intracellular lysosomal digestive enzymes. Exceptions: chromosomes in nucleus, and
structural proteins such as collagen and muscle contractile proteins. Liver can store bigger
amount of amino acids. Whenever the plasma amino acid concentrations fall below normal
levels, the required amino acids are transported out of the cells to replenish their supply in the
plasma: there is constant interchange and equilibrium between the plasma amino acids and
labile proteins in virtually all cells of the body. Each cell has an upper limit for the storage of
proteins. After limit has been reached, the excess amino acids in the circulation are degraded
into other products and used for energy or converted to fat or glycogen and stored in these
forms.
The major types of proteins present in the plasma are albumin, globulin and fibrinogen.
Albumin: provides colloid osmotic pressure in the plasma, which prevent plasma loss from
the capillaries. Globulin: performs a number of enzymatic functions in the plasma, but equally
important, they are principally responsible for the body’s both natural and acquired immunity
against invading organisms. Fibrinogen: polymerizes into long fibrin threads during blood
coagulation, thereby forming blood clots that help repair leaks in the circulatory system. Most
are formed in the liver. In cirrhosis of the liver, large amounts of fibrous tissue develop
among the liver parenchymal cells, causing a reduction in their ability to synthesize plasma
proteins. Plasma proteins function as a labile protein storage medium and represent a readily
available source of amino acids whenever a particular
tissue requires them.
Essential amino acids: amino acids that cannot be
synthesized by the body. Synthesis of nonessential
amino acids depends mainly on the formation of
appropriate α-keto acids which are the precursors of
the respective amino acids. By the process of
transamination an amino radical is transferred to the α-
keto acid, and the keto oxygen is transferred to the
donor of the amino radical (e.g. pyruvic acid is
The principal constituents of proteins are amino acids. All 20 amino acids have two features
in common: they have an acidic group (-COOH) and a nitrogen atom attached to the molecule
(-NH2). The amino acids of proteins are aggregated into long chains by means of peptide
linkages; water is released. Some molecules are composed of several protein chains, which
are bound to one another by other linkages such as hydrogen bonding between CO and NH
radicals. This is causing the folded proteins. Amino acids are absorbed from the digestive
tract into the blood (small quantities at a time). The molecules of all amino acids are too large
to diffuse readily through the pores of the cell membranes. Move either inward or outward
through the membranes only by facilitated transport or active transport using carrier
mechanisms. However, in the kidneys the different amino acids can be actively reabsorbed
through the proximal tubular epithelium (nevertheless, upper limit; too much lost in urine).
Almost immediately after entry into tissue cells, amino acids combine with one another by
peptide linkages under the direction of the cell’s messenger RNA and ribosomal system to
form cellular proteins. Storage of large quantities does not occur. However, many of the
intracellular proteins can be rapidly decomposed again into amino acids under the influence of
intracellular lysosomal digestive enzymes. Exceptions: chromosomes in nucleus, and
structural proteins such as collagen and muscle contractile proteins. Liver can store bigger
amount of amino acids. Whenever the plasma amino acid concentrations fall below normal
levels, the required amino acids are transported out of the cells to replenish their supply in the
plasma: there is constant interchange and equilibrium between the plasma amino acids and
labile proteins in virtually all cells of the body. Each cell has an upper limit for the storage of
proteins. After limit has been reached, the excess amino acids in the circulation are degraded
into other products and used for energy or converted to fat or glycogen and stored in these
forms.
The major types of proteins present in the plasma are albumin, globulin and fibrinogen.
Albumin: provides colloid osmotic pressure in the plasma, which prevent plasma loss from
the capillaries. Globulin: performs a number of enzymatic functions in the plasma, but equally
important, they are principally responsible for the body’s both natural and acquired immunity
against invading organisms. Fibrinogen: polymerizes into long fibrin threads during blood
coagulation, thereby forming blood clots that help repair leaks in the circulatory system. Most
are formed in the liver. In cirrhosis of the liver, large amounts of fibrous tissue develop
among the liver parenchymal cells, causing a reduction in their ability to synthesize plasma
proteins. Plasma proteins function as a labile protein storage medium and represent a readily
available source of amino acids whenever a particular
tissue requires them.
Essential amino acids: amino acids that cannot be
synthesized by the body. Synthesis of nonessential
amino acids depends mainly on the formation of
appropriate α-keto acids which are the precursors of
the respective amino acids. By the process of
transamination an amino radical is transferred to the α-
keto acid, and the keto oxygen is transferred to the
donor of the amino radical (e.g. pyruvic acid is
