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234 Organelles
Lysosomes C. Synthesis and transport of lysosomal
proteins
A. Structure and contents Primary lysosomes arise in the region of the
Golgi apparatus. Lysosomal proteins are syn-
Animal lysosomes are organelles with a diam-
eter of 0.2–2.0 µm with various shapes that thesized in the rER and are glycosylated there
as usual (1; see p. 228). The next steps are
are surrounded by a single membrane. There specific for lysosomal proteins (right part of
are usually several hundred lysosomes per the illustration). In a two-step reaction, ter-
cell. ATP-driven V-type proton pumps are ac- minal mannose residues (Man) are phos-
tive in their membranes (see p. 220). As these
+
accumulate H in the lysosomes, the content phorylated at the C–6 position of the man-
nose. First, N-acetylglucosamine 1-phosphate
of lysosomes with pH values of 4.5–5 is much is transferred to the OH group at C-6 in a
more acidic than the cytoplasm (pH 7–7.3). terminal mannose residue, and N-acetylglu-
The lysosomes are the cell’s “stomach,”
serving to break down various cell compo- cosamine is then cleaved again. Lysosomal
nents. For this purpose, they contain some proteins therefore carry a terminal mannose
6–phosphate (Man6–P; 2).
40 different types of hydrolases,which are
capable of breaking down every type of mac- The membranes of the Golgi apparatus
romolecule. The marker enzyme of lysosomes contain receptor molecules that bind Man
6–P. They recognize lysosomal proproteins
is acid phosphatase. The pH optimum of lyso-
somalenzymesis adjusted to theacid pHvalue by this residue and bind them (3). With the
help of clathrin, the receptors are concen-
and is also in the range of pH 5. At neutral pH, trated locally. This allows the appropriate
as in the cytoplasm, lysosomal enzymes only membrane sections to be pinched off and
have low levels of activity. This appears to be a transported to the endolysosomes with the
mechanism for protecting the cells from di-
gesting themselves in case lysosomal enzymes help of transport vesicles (4), from which pri-
mary lysosomes arise through maturation (5).
enter the cytoplasm at any time. In plants and
fungi, the cell vacuoles (see p. 43) have the Finally, the phosphate groups are removed
from Man 6–P (6).
function of lysosomes.
The Man 6–P receptors arereused. Thefall
in the pH value in the endolysosomes releases
B. Functions the receptors from the bound proteins (7)
which are then transported back to the Golgi
Lysosomes serve for enzymatic degradation of apparatus with the help of transport vesicles.
macromolecules and cell organelles, which
are supplied in various ways. The example
shows the degradationofanoveraged mito- Further information
chondrion by autophagy. To accomplish this, Many hereditary diseases are due to genetic
the lysosome encloses the organelle (1). Dur-
ingthis process,the primary lysosome con- defects in lysosomal enzymes. The metabo-
lism of glycogen ( glycogenoses), lipids (
verts into a secondary lysosome, in which the
hydrolytic degradation takes place (2). Finally, lipidoses), and proteoglycans ( mucopoly-
saccharidoses) is particularly affected. As the
residual bodies contain the indigestible resi- lysosomal enzymes are indispensable for the
dues of the lysosomal degradation process.
Lysosomes are also responsible for the degra- intracellular breakdown of macromolecules,
unmetabolized macromolecules or degrada-
dation of macromolecules and particles taken
up by cells via endocytosis and phagocyto- tion products accumulate in the lysosomes
in these diseases and lead to irreversible cell
sis—e. g., lipoproteins, proteohormones, and damage over time. In the longer term, en-
bacteria (heterophagy). In the process, lyso-
somes fuse with the endosomes (3)in which largement takes place, and in severe cases
there may be failure of the organ affected—
the endocytosed substances are supplied.
e. g., the liver.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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