Download Amino acids and prot..

II- Globins (Histones): They are basic proteins rich in histidine amino acid.
They are present in :
a - combined with DNA
b - combined with heme to form hemoglobin of RBCs.
III- Scleroproteins: They are structural proteins, not digested.
include: keratin, collagen and elastin.
a- α-keratin: protein found in hair, nails, enamel of teeth and outer layer
of skin.
• It is α-helical polypeptide chain, rich in cysteine and hydrophobic
(non polar) amino acids so it is water insoluble.
Keratin is an extremely strong protein that is a major component in
skin, hair, nails, hooves, horns, and teeth. The amino acids which
combine to form it have several unique properties and, depending on
the levels of the various amino acids, it can be inflexible and hard, like
hooves, or soft, as is the case with skin.
It is difficult to dissolve keratin because it contains non polar amino
acids plus cysteine, which means that it is able to form disulfide
bridges. These bridges create a thread shape that is extremely strong,
as sulfur atoms bond to each other, creating a fibrous matrix that is not
readily soluble.
Depending on how much cysteine disulfide keratin contains, the bond
can be extremely strong to make hard cells like those found in hooves,
or it can be softer to make flexible tissue like hair and skin. Because of
the high levels of sulfur in this protein, it gives a distinct sulfurous
odor when burned or heated.
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• b- collagens: protein of connective tissues found in bone,
teeth, cartilage, tendons, skin and blood vessels.
• Collagen may be present as gel e.g. in extracellular matrix
or in vitreous humor of the eye.
• Collagens are the most important protein in mammals.
They form about 30% of total body proteins.
• There are more than 20 types of collagens, the most
common type is collagen I which constitutes about 90%
of cell collagens.
• Structure of collagen: three helical polypeptide chains
(trimer) twisted around each other forming triplet-helix
molecule.
• ⅓ of structure is glycine, 10% proline, 10%
hydroxyproline and 1% hydroxylysine. Glycine is found
in every third position of the chain. The repeating
sequence –Gly-X-Y-, where X is frequently proline and Y
is often hydroxyproline and can be hydroxylysine.
Solubility: collagen is insoluble in all solvents and not digested.
•
When collagen is heated with water or dil. HCl it will be converted into gelatin
which is soluble , digestible and used as diet (as jelly). Gelatin is classified as
derived protein.
• Steps in Biosynthesis of Collagen:
1.
2.
3.
4.
5.
6.
Translation on ribosomes to form 3 polypeptide chains called α chains. This is
the preprocollagen
Hydroxylation of some of proline and lysine into hydroxyproline and
hydroxylysine. This step needs enzymes lysyl hydroxylase and prolyl
hydroxylase in the presence of vitamin C .
Release of α chains into from ribosome to endoplasmic reticulum and addition
of glucose and galactost to hydroxy group of some lysine forming procollagen.
Combination of the 3 chains and formation of triple helix (also is a procollagen)
Release into extracellular space and removal of segments from N- and Cterminal to form collagen or called tropocollagen
Cross linkage of collagen molecules to form mature collagen fibres (or collagen
network). This cross linkage is essential for achieving strength necessary for
proper function of connective tissues.
Note that,
Hydroxylation reaction to form hydroxy proline and hydroxylysine
are important for the following:
a- attachment of sugars (glucose and galactose) to OH groups of
hydroxy lysine and hydroxy proline
b- hydroxylation is very important to organize the three chains in the
form a triple helix. The three chains are linked together by
hydrogen bonding between OH groups in the three chains.
c- OH groups of hydroxylysine is tneeded to permit the cross-linking
of the triple helices into the fibers and networks of the tissues.
cross-linking (aldol reaction) by lysyl oxidase which links
hydroxylysine and lysine residues. Multiple collagen fibrils form
into collagen fibers.
Collagen biosynthesis (Fig. 2)
Collagen biosynthesis (Fig. 3)
Some collagen diseases:
1- Scurvy: disease due to deficiency of vitamin C which is
important coenzyme for conversion of proline into hydroxyproline
and lysine into hydroxylysine. Thus, synthesis of collagen is
decreased leading to abnormal bone development, bleeding,
loosing of teeth and swollen gum.
2- Osteogenesis Imperfecta (OI):
Inherited disease resulting
from genetic deficiency or mutation in gene that synthesizes
collagen type I leading to abnormal bone formation in babies and
frequent bone fracture in children. It may be lethal.
OI Type I
OI type I is the most common type of OI. People with OI type I have an average of
20 to 40 fractures before puberty. Fewer fractures occur after puberty. Prenatal
diagnosis often is possible. People with OI type I have one or more of the following
features:
-fragile bones
-- hearing loss beginning in the teens, twenties, or thirties
-curvature of the spine
-thin, smooth skin
In other severe types of OI:
Person have 20 or more fractures during the first three years of life and more than
100 fractures by puberty. Loose joints and poor muscle development are common.
Characteristic features include:
- soft bones that not only break but also bend easily
-misshapen arms and legs. severe curvature of the spine
-severely misshapen chest that may cause problems with breathing
- poor development of the teeth, causing teeth to be discolored and to break easily
- short stature (some people may only grow to be three feet tall).
C- Elastin: present in walls of large blood vessels (such as aorta).
It is very important in lungs, elastic ligaments, skin, cartilage, ..
It is elastic fiber that can be stretched to several times as its normal
length. Elastin is a protein in connective tissue that is elastic and
allows many tissues in the body to resume their shape after
stretching or contracting.
Structure: composed of 4 polypeptide chains (tetramer), similar to
collagen being having 33% glycine and rich in proline and lysine but
differs in that it has low hydroxyproline and absence of hydroxy
lysine.
Emphysema: Emphysema is a long-term, progressive disease of the
lungs that primarily causes shortness of breath due to over-inflation
of the alveoli (air sacs in the lung). In people with emphysema, the
lung tissue involved in exchange of gases (oxygen and carbon
dioxide) is impaired or destroyed. Emphysema is included in a group
of diseases called chronic obstructive pulmonary disease .
Emphysema is called an obstructive lung disease because airflow
on exhalation is slowed or stopped because over-inflated alveoli
do not exchange gases when a person breaths due to little or no
movement of gases out of the alveoli which are badly damaged or
collapsed.
Normal lung tissue looks like a new sponge. Emphysematous lung
looks like an old used sponge, with large holes and a dramatic loss
of elasticity. When the lung is stretched during inflation
(inhalation), elastin will help stretched tissue to relax to its resting
state. In emphysema, this elastic function is impaired, resulting in
air trapping in the lungs.
Causes of emphysema
Elastin is a lung protein. Chemicals particularly cigarette smoke
(Over years of chronic exposure to cigarette smoke), increase the
number and activity of lung neutrophils and macrophages and
stimulate enzyme called elastase to be secreted form neutrophils.
Elastase cause destruction of elastin of lung.
Role of α1-antitrypsin (A1A)
α1-antitrypsin
is an enzyme
(secreted from liver) and inhibit
elastase and prevent destruction
of elastin. So α1-antitrypsin
deficiency especially in smokers
leads to degradation of elastin and
destruction of lung (loss of
elasticity of lung, a disease called
emphysema.
Symptoms: shortness of breath ,
chronic cough.
Treatment:
stop
smoking,
bronchdilator
(to
open
bronchioles, supplemental oxygen
Elastin and skin
Elastin is best known as the component of
skin that, along with collagen, imparts
suppleness and elasticity, and the lack of
which gives us wrinkles. But elastin plays
an important role behind the scenes as
well, conferring on blood vessels their
rubbery resilience and the digestive organs
their ability to accommodate a meal.
Elastin also allows ligaments and tendons
to stretch and retract, enabling movement.
The biosynthesis of Elastin begins with the embryonic period and
continues till we reach adulthood. Our body stops producing
Elastin around that time in our lives. Our body will depend on the
preexisting elastin.
With aging, elastin progressively degenerate and break down. Our
natural Elastin is no longer renewed. The skin progressively loses
its elasticity and lines and wrinkles start appearing.
IV-Conjugated proteins
i.e. On hydrolysis, give protein part and non protein part and
subclassified into:
1- Phosphoproteins: These are proteins conjugated with phosphate
group. Phosphorus is attached to OH group of serine or threonine.
e.g. Casein of milk and vitellin of yolk.
2- Lipoproteins:
These are proteins conjugated with lipids.
Functions: a- help lipids to transport in blood
b- Enter in cell membrane structure helping lipid soluble substances
to pass through cell membranes.
3- Glycoproteins:
proteins conjugated with sugar (carbohydrate)
e.g.
- Some hormones such as erythropoeitin
- present in cell membrane structure
- blood groups.
4- Nucleoproteins: These are basic proteins ( e.g. histones)
conjugated with nucleic acid (DNA or RNA).
e.g. a- chromosomes: are proteins conjugated with DNA
b- Ribosomes: are proteins conjugated with RNA
5- Metalloproteins: These are proteins conjugated with metal like
iron, copper, zinc, ……
a- Iron-containing proteins:
- hemoglobin (Hb),
- myoglobin (protein of skeletal muscles and cardiac muscle),
NB: Hb and myoglobin are hemeproteins that bind O2
- Ferritin: Main store of iron in the body in non toxic form, because
free iron is toxic and oxidize cells (form reactive oxygen species).
Ferritin is present in liver, spleen and bone marrow. The amount of
ferritin stored reflects the amount of iron stored. Ferritin releases
iron to areas where it is required.
- Hemosidrin: another iron store. (report hemosidrosis)
-Transferrin: is the iron carrier protein in plasma.
Typically in the healthy individual, about 65% of the iron in the body
is in hemoglobin (in red blood cells) and about 4% in myoglobin
(protein in skeletal muscle). About 30% of the iron in the body is
stored (as ferritin or hemosiderin) in liver, bone marrow, and the
spleen. A small percentage of the body's iron is in transport between
various compartments of the body (in association with transferrin) or
is a component of enzymes in cells throughout the body.
Ferritin is a globular protein complex consisting of 24 subunits and is
the primary intracellular iron-storage protein keeping iron in a
soluble and non-toxic form. Ferritin that is not combined with iron is
called apoferritin.
Ferritin: The Iron-Storage Protein
How does the body regulate the amount of
iron? Fortunately, most of us are able to
maintain appropriate levels of available iron
in the body (enough available iron to ensure
an adequate supply of hemoglobin, but not
so much as to produce toxic effects),
Ferritin (Figure) is the key to this important
control of the amount of iron available to
the body. Ferritin is a protein that stores
iron and releases it in a controlled
fashion. Hence, the body has a "buffer"
against iron deficiency (if the blood has too
little iron, ferritin can release more) and, to
a lesser extent, iron overload (if the blood
and tissues of the body have too much iron,
ferritin can help to store the excess iron).
hemoglobin
Ferritin
b- Copper containing proteins:
e.g. - Ceruloplasmin which oxidizes ferrous ions into ferric ions.
c- Mg containing proteins:
e.g. Kinases and phosphatases.