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to be guided along fibronectin-coated pathways in the embryo. Fibronectin
is found in three forms: a plasma form that is involved in blood clotting; a
cell-surface form, which binds to the cell surface transiently; and a matrix
form, which is fibrillar in arrangement. Fibronectin contains a cell-binding
domain (RGD sequence), a collagen-binding domain, and a heparin-binding
domain. Elastin and type III collagen are responsible for elasticity seen in
large arteries and the pinna of the ear (answer a).Cell-cell interactions
involve both transient and more long-term, stable processes. Cell-cell
adhesion is mediated by transmembrane proteins called cell adhesion mol-
ecules which include the calcium or magnesium-dependent selectins, inte-
grins, and cadherins(answers c and d)and the non-calcium-dependent
immunoglobulin (Ig) superfamily. The stable adhesion junction, known as
the zonula adherens, links the cytoskeleton of adjacent cells through cad-
herins (transmembrane linker proteins) to actin filaments inside the cell
[answer e(see feedback for question 199)].


89.The answer is b.(Junqueira, p 357. Kasper, pp 1547–1550. Kumar, pp
719–723.)The patient suffers from alpha-1 antitrypsin (AAT) emphysema.
AAT protects the lung from neutrophil-derived elastase, which breaks
down elastic fibers (answer d),which are composed of a core of elastin
with a surrounding network of fibrillin. Desmosine and isodesmosine are
amino acids unique to elastin and responsible for the covalent binding of
elastin fibers to each other. Lysyl oxidase catalyzes the cross-linking of
tropoelastin (answers a).Microfibrils, composed of fibrillin, facilitate for-
mation of the elastin molecules, but are not directly involved in cross-linking.
Elastase(answer c)is a serine protease that specifically degrades elastin.
Interactions occur between type III collagen and elastic fibers (answer e).
The collagen may serve to limit the stretch of the elastic components. Elas-
ticity is conferred through the highly hydrophobic nature of elastin. One-
third of elastin is composed of the hydrophobic amino acid glycine, which
is randomly distributed throughout the elastin molecule. This is in contrast
to the even distribution of glycine in collagen. The random distribution of
glycine makes elastin hydrophobic. The overall hydrophobicity of elastin
molecules allows for their distensibility and facilitates their capacity to slide
over one another.


90.The answer is c.(Alberts, pp 1098–1100. Junqueira, pp 106–110.)Pro-
lyl and lysyl hydroxylase are the two enzymes that carry out hydroxylation


182 Anatomy, Histology, and Cell Biology

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