Page 235 - Color Atlas of Biochemistry
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226 Organelles
ER: structure and function will take and whether its transport will be
constitutive or regulated depends on the sig-
The endoplasmic reticulum (ER) is an exten- nal sequences or signal structures that pro-
sive closed membrane system consisting of teins carry with them like address labels (see
tubular and saccular structures. In the area p. 228). In addition to proteins, the Golgi ap-
of the nucleus, the ER turns into the external paratus also transports membrane lipids to
nuclear membrane. Morphologically, a dis- their targets.
tinction is made between the rough ER (rER)
and the smooth ER (sER). Large numbers of
ribosomes are found on the membranes of the B. Smooth endoplasmic reticulum
rER, whichare lackingonthe sER. On the Regions of the ER that have no bound ribo-
other hand, the sER is rich in membrane- somesare known asthe smooth endoplasmic
bound enzymes, which catalyze partial reac- reticulum (sER). In most cells, the proportion
tions in the lipid metabolism as well as bio- representedbythe sER is small. AmarkedsER
transformations. is seen in cells that have an active lipid me-
tabolism, such as hepatocytes and Leydig
cells. The sER is usually made up of branching,
A. Rough endoplasmic reticulum and the
closed tubules.
Golgi apparatus
Membrane-located enzymes in the sER
The rER (1)is a site of active protein catalyze lipid synthesis. Phospholipid synthe-
biosynthesis. This is where proteins destined sis (see p. 170) is located in the sER, for exam-
for membranes, lysosomes, and export from ple, and several steps in cholesterol biosyn-
the cell are synthesized. The remaining pro- thesis (see p. 172) also take place there. In
teins are produced in the cytoplasm on ribo- endocrine cells that form steroid hormones, a
somes that are not bound to membranes. large proportion of the reaction steps in-
Proteins synthesized at the rER (1)are volved also take place in the sER (see p. 376).
folded and modified after translation (protein In the liver’s hepatocytes, the proportion
maturation; see p. 230). They remain either in represented by the sER is particularly high.
the rER as membrane proteins, or pass with It contains enzymes that catalyze so-called
the help of transport vesicles (2)to the Golgi biotransformations. These are reactions in
apparatus (3). Transport vesicles are formed which apolar foreign substances, as well as
by buddingfrom existingmembranes,and endogenous substances—e. g., steroid hormo-
they disappear again by fusing with them nes—are chemically altered in order to inacti-
(see p. 228). vate them and/or prepare them for conjuga-
The Golgi apparatus (3)is a complexnet- tion with polar substances (phase I reactions;
work, also enclosed, consisting of flattened see p. 316). Numerous cytochrome P450
membrane saccules (“cisterns”), which are enzymes are involved in these conversions
stacked on top of each other in layers. Pro- (see p. 318) and can therefore be regarded
teins maturehereand aresorted and packed. as the major molecules of the sER.
A distinction is made between the cis, medial, The sER also functions as an intracellular
and trans Golgi regions, aswell asa trans calcium store, which normally keeps the Ca 2+
Golgi network (tGN). The post–translational level in the cytoplasm low. This function is
modification of proteins, which starts in the particularly marked in the sarcoplasmic retic-
ER, continues in these sections. ulum, a specialized form of the sER in muscle
From the Golgi apparatus, the proteins are cells (see p. 334). For release and uptake of
2+
transported by vesicles to various targets in Ca , the membranes of the sER contain sig-
the cells—e. g., to lysosomes (4), the plasma nal-controlled Ca 2+ channels and energy-de-
membrane (6), and secretory vesicles (5)that pendent Ca 2+ ATPases (seep.220). In thelu-
release their contents into the extracellular men of the sER, the high Ca 2+ concentration is
2+
space by fusion with the plasma membrane buffered by Ca -binding proteins.
(exocytosis; see p. 228). Protein transport can
either proceed continuously (constitutive), or
it can be regulated by chemical signals. The
decision regarding which pathway a protein
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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