Proteins: Collagen, Elastin, General Protein Structure

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Collagen

Collagen, which comprises approximately 70-80% of the dry weight
of the dermis, is primarily responsible for skin’s tensile strength. Each
collagen molecule consists of three polypeptide chains, each containing
about 1,000 amino acids in their primary sequence. In the collagen molecule
the alpha-chains are wrapped around each other to make a triple helical
conformation [29]. In chronologically aged skin, the rate of collagen
synthesis, activity of enzymes that act in the post-translational modification,
collagen solubility, and thickness of collagen fiber bundles in the
skin all decrease [27,30]. Also, the ratio of type III to type I collagen
increases with increasing age [27,31]. In photoaged skin, however, collagen
fibers are fragmented, thickened, and more soluble [27]. It is plausible
that reduced collagen deposition in elderly skin could explain development
of dermal atrophy and might relate to poor wound healing in the
elderly [30].
Histological data, though not quantitative, reveals important information
about orientation and arrangement of collagen fibers in skin. Lavker et al.
compared skin from the upper inner arm of old (ages 70-85) and young
(ages 19-25) individuals using light, transmission electron, and scanning
electron microscopy [32]. Interestingly, they suggested that the upper inner
arm might be an optimal site for analyzing sun-protected skin, because
it is not exposed to the pressure deformations and reformations occurring
in the buttock. They found that in young adults, collagen in the papillary
dermis forms a meshwork of randomly oriented thin fibers and small
bundles. The reticular dermis consists of loosely interwoven, large, wavy,
randomly oriented collagen bundles. However, the collagen within each
bundle is packed together closely [32]. In aged skin the density ofthe collagen
network appears to increase. This likely reflects a decrease in ground
substance that would otherwise form spaces between the collagen [32].
Also, rather than appearing in discrete ropelike bundles of tightly packed
fibers, collagen forms aggregates of loosely woven, mostly straight fibers.
As fibers become straighter in aged skin, there is less room for the skin to

be stretched, so tensile strength increases [32]. Using immuno-electron
microscopy, Vitellaro-Zuccarello et al. found similar age-related trends in
skin collagen. They also noted greater intensity of collagen III staining in
subjects over 70 years of age [33]. Hence histological and more recent
methods are in agreement, revealing that increased age is associated with
decreased collagen content and straightening of collagen fibers, forming
looser bundles, an increased type III:type I collagen ratio, and decreased
ground substance.

Elastin

The skin’s intact elastic fiber network, which occupies approximately 2-4%
of the dermis by volume, provides resilience and suppleness. This network
shows definite changes associated with aging, especially between the ages
of 30 and 70. In sun-exposed skin, an excessive accumulation of elastoic
material occurs. Accumulation of new elastin in response to photoaging is
also apparent from upregulation of the elastin promoter activity and
increased abundance of elastin mRNA [34,35]. Bernstein et al. compared
photo aged to intrinsically aged skin, and found a 2.6-fold increase in elastin
mRNA, a 5.3-fold increase in elastin expression, and a 5-fold increase in
elastin promoter activity in photodamaged skin [34]. However, these apparent
increases in elastin synthesis do not account for the massive accumulation
of elastoic material seen histologically in photoaged skin [34]. Some
attribute this to elastin degradation being slower than synthesis, leading
to an accumulation of partially degraded fibers. In purified skin elastin,
the amount of racemized aspartic acid increases rapidly and is highly correlated
with age (r = 0.98) [35). This indicates that skin’s elastin, like
elastin in the aorta and lung, is long-lived and accumulates damage over
time [36,37].
In innate aging, fragmentation of elastic fibers results in their decreased
number and diameter. Computerized image analysis of elastin-stained skin
biopsies from the buttock and upper inner arm reveal an age-related increase
in mean elastin fiber length and percentage surface area coverage in the
dermis, but these fibers are thought to be abnormally enriched in polar
amino acids, carbohydrates, lipids, and calcium [36]. Through different
mechanisms, photoaging and intrinsic aging ultimately result in a deficiency
of functional, structurally intact elastic fibers [30]. The finer oxytaIan
fibers in the papillary dermis are depleted or lost altogether; eluanic and
elastic fibers become progressively abnormal. These alterations largely

account for the widely recognized decrease in skin’s physiological elasticity
with increased age [36].
Examination of intrinsically aged skin elastin and fibrillin with immunohistochemical
staining revealed that elastin was located in the papillary
dermis just below the basement membrane, as small fibers mostly oriented
perpendicular to the epidermis. In the deeper dermis, fibers were thicker
and oriented differently. Areas surrounding adnexal structures and larger
vessels in the deep dermis were also intensely stained [34]. Photoaged skin
demonstrated similar small-diameter fibers just below the basement membrane
within a zone lacking excessive staining, which was of variable
thickness [34]. This may correspond to the subepidermal low echogenic
band seen in ultrasound imaging. Beneath this area of relatively sparse
staining was a region of poorly formed, clumped, thick fibers. This staining
pattern occupied the superficial to mid-dermis, below which staining
again resumed its well-defined pattern as seen in sun protected skin [34].
Elastin, therefore, exhibits numerous age-related changes, including slow
degradation and accumulation of damage in existing elastin with intrinisic
aging, increased synthesis of apparently abnormal elastin in photoexposed
areas, and abnormal localization of elastin in the upper dermis of photodamaged
skin. These factors lead to the histologically evident elastoic
accumulation and contribute to characteristic changes in ultrasound images
of aged skin.

General Protein Structure

Through Raman spectroscopy, little difference is seen between photoexposed
and protected areas in young individuals; the majority of proteins in
young skin are in helical conformation. Intrinsically aged skin shows
slightly altered protein structure, and photoaged skin reveals markedly
altered protein conformation, with increased folding and less exposure of
aliphatic residues to water [38,39]. Amino acid composition of proteins and
free amino acids in aged skin also differ significantly from that of young
skin, including an increase in overall hydrophobicity of amino acid fractions
from the elderly [40]. Because free amino acids are believed to playa
key role in stratum corneum water binding, this shift in their composition,
combined with the evidence of altered tertiary protein structure, provides
insight into the increased incidence of xerosis in aged individuals.

Author : kaabinet

kaabinet

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