Transcript Binding of oxygen to myoglobin and hemoglobin

‫المحاضره كامله‬
GLOBULAR
HEMOPROTEINS
• Hemeproteins are a group of ‫مكون من‬1- specialized proteins that
contain 2-heme as a tightly bound prosthetic group ( non protein
part attach fraimly to protein ).
• Heme is a complex of protoporphyrin IX ( cyclic tetra (4) birol
ring ) ) ‫ كربونات ونتروجين واحده ( انظر الرسمه الشريحه التاليه‬4 ‫حلقه خماسيه بها‬
‫اللون االصفر نوت اضافي‬
and ferrous iron (Fe2+)‫ مهمه‬.
• The iron is held ‫ موجود‬in the center of the heme molecule by
bonds to the four nitrogens of the porphyrin ring ‫كالحديد مرتبط باربع‬
‫ ذرات نتروجين‬.
• The heme Fe2+ can form two additional bonds, one on each
side of the planar porphyrin ring ‫واحده ليرتبط بها االكسجين واالخرى‬
‫ روابط‬6 ‫مكتوبه باالسفل – اذا مجموع روابط الحديد‬.
• In myoglobin and hemoglobin, one of these positions is
coordinated to the side chain of a histidine residue of the
globin molecule, whereas the other position is available to bind
oxygen
A .Hemeprotein (cytochrome c(
B .Structure of heme ‫هذه الصوره توضح ما‬
Myoglobin : ‫اول نوع من هيموبروتين‬
:‫الوظيفه يقول الدكتور سؤال استغفر هللا‬Structure and function
• Myoglobin, a hemeprotein present in heart and skeletal
muscle, functions both as a reservoir for oxygen, and as an
oxygen carrier that increases the rate of transport of oxygen
within the muscle cell. )‫مهمه جدا ( سؤااال خصوصا االخضر‬
• Myoglobin consists of a single polypeptide chain that is
structurally similar to the individual subunit polypeptide
chains of the hemoglobin molecule.
• Myoglobin is a compact ‫ مرصوص‬molecule, with approximately
80 % ) ‫ مهمه جدا ( سؤال‬of its polypeptide chain folded into 8
stretches of α-helix ) ‫هذي مهمه جدا ( سؤال‬. These α-helical
regions, labeled A to H Myoglobin has more affinity to o2 than hemoglobin
mussels and get the o2 from
• A-B-C-D-E-F-G-H ‫ يعني‬Myoglobin located in
hemoglobin
• The interior ‫ الداخلي‬of the myoglobin molecule is composed
almost entirely of nonpolar amino acids.
• In contrast, polar amino acids are located almost
exclusively on the surface of the molecule.
• The heme ( non polar ) group of myoglobin is located in a
crevice ‫ المسافه بين‬in the molecule between helix E and helix
F, ‫ مهمه جدا ( سؤال ) – تتوضح من الرسمه القادمه‬which is lined with
nonpolar amino acids. Notable exceptions are two histidine
residues ‫اسم احماض امينيه قطبيه في المركز غير القطبي تتوضح بالرسمه‬
‫وسوف تشرح الحقا‬.
‫الحظ مكان الهيمي بين‬
F and E
A. Model of myoglobin showing helices A to
H.
B. Schematic diagram of the oxygen-binding
site of myoglobin
‫هذه االحماض االمينه عباره عن‬
‫اجزاء قطبيه في مركز غير قطبي‬
Distal :
‫بعيد عن جزيء الحديد‬
Proximal :
‫قريب من جزيء الحديد‬
• One, the proximal histidine (F8), binds directly to the iron
of heme.
• The second, or distal histidine (E7), does not directly
interact with the heme group, ‫ وظيفة الديستال‬but helps
stabilize the binding of oxygen to the ferrous iron.
• ‫ فائده البروتين‬: The protein, or globin, portion of myoglobin
prevents the oxidation of iron of heme. ‫مهمه‬
‫نوع الحديد دائما‬
FE++ ferrous
‫سواء كان االكسجين مرتبط به او ال بسبب ارتباط‬
Globin
‫ ثانيا‬: Hemoglobin
Structure and function ‫الوظيفه سؤااال‬:
• Hemoglobin is found exclusively in red blood cells, where
its main function is to transport oxygen from the
lungs to the capillaries of the tissues.
• Hemoglobin A, the major hemoglobin in adults, is
composed of four ‫ عكس السابق المكون من واحده فقط‬polypeptide
chains - two α chains and two β chains - held together by
noncovalent interactions.
• Each subunit has stretches of α-helical structure, and a
heme-binding pocket similar to that described for
myoglobin.
• However, the tetrameric hemoglobin molecule is
structurally and functionally more complex than
myoglobin.
• For example, hemoglobin can transport H+ and CO2 from
the tissues to the lungs, and can carry four molecules of
O2 from the lungs to the cells of the body.
• Furthermore, the oxygen-binding properties of
hemoglobin are regulated by interaction with allosteric
effectors .
DIFFREENSES BETWEEN MYOGLOBIN AND HEMOGLOBIN
1- hemoglobin carry 4 o2 atoms while myoglobin carry 1 o2 atom only
2- the affenity of Hb to o2 affected by 5 factors ( which will be expliened later )
while myoglobin not affected by any factor
3- myoglobin carry o2 only while hemoglobin can carry o2 , co2 and H+
‫السؤال االول على التركيب‬
‫شريحه‬
‫مهمه جدا‬
‫وعليها‬
‫سؤالين‬
‫السؤال‬
‫االول‬
HbA1c :
‫اصله‬
HbA
‫ولكن ارتبط به‬
‫جلكوز وهذا‬
‫االرتباط غير‬
‫عكسي ( يعني غير‬
) ‫قابل لالنفصال‬
‫السؤال‬
‫الثاني‬
3–6 %
Normal adult human hemoglobins.
[Note: The α-chains in these hemo-globins are identical]
HbA1c could be used as a monitor for the control of the blood
glucose level during the last 2 months for diabetic patients
Schematic diagram showing structural changes resulting
from oxygenation and deoxygenation of hemoglobin .
‫شريحه مهمه وتشرح في الشرائح االقادمه‬
Quaternary
‫رباعي‬
structure of hemoglobin:
• The hemoglobin tetramer ‫ البروتين المكون من اربع وحدات‬can
be envisioned ‫ ينقسم لصورتين متطابقتين‬as being composed of
two identical dimers, )αβ(1 and )αβ(2, in which the
numbers refer to dimers one and two. ‫كما في الصوره‬
• The two polypeptide chains within each dimer are held
tightly together, primarily by hydrophobic interactions
• In contrast, the two dimers are able to move with
respect to each other, being held together primarily by
polar bonds.
• The weaker interactions between these mobile dimers
result in the two dimers occupying different relative
positions in deoxyhemoglobin as compared with
oxyhemoglobin
• T form ‫ ) تكون‬2 – 1 ‫اذا اقتربوا من بعض ( الديامير‬: ‫مهمه وتشرح‬
‫ الصوره‬The deoxy ‫ التحرر من االكسجين‬form of hemoglobin is
called the “T,” or taut (tense ‫ )توتر‬form.
•In the T form, the two αβ dimers interact through a network
of ionic bonds that constrain the movement of the
polypeptide chains. The T form is the low-oxygen-affinity
form of hemoglobin.
• R form : ‫اذا ابتعدوا تكون‬: ‫ سيم سيم‬The binding of oxygen to
hemoglobin causes the rupture of some of the ionic bonds
between the αβ dimers.
•This leads to a structure called the “R,” or relaxed form, in
which the polypeptide chains have more freedom of
movement . The R form is the high- oxygen-affinity form of
hemoglobin.
Binding of oxygen to myoglobin and hemoglobin
• Myoglobin can bind only one molecule of oxygen (O2),
because it contains only one heme group.
• In contrast, hemoglobin can bind four oxygen
molecules—one at each of its four heme groups.
• The degree of saturation (Y) of these oxygen-binding
sites on all myoglobin or hemoglobin molecules can vary
between zero (all sites are empty) and 100% (all sites
are full
‫نفس مخطط‬
‫الفيسيو‬
‫الحظ ان‬
‫‪P50‬‬
‫لل‪Myoglobin‬‬
‫اكبر بكثير من‬
‫الهيموجلوبين الن‬
‫جداذبيه االول اكبر‬
‫تجاه االكسجين‬
‫‪Myoglobin :‬‬
‫له شكل‬
‫‪Hyperpolic‬‬
‫‪curve‬‬
‫‪Hemoglobin‬‬
‫له شكل‬
‫‪S shape‬‬
‫بسبب وجود اربع‬
‫مركبات هيمي ( تفصل‬
‫الحقا‬
‫‪Oxygen dissociation curves for myoglobin and hemoglobin‬‬
Oxygen dissociation curve:
• A plot of Y measured at different partial pressures of
oxygen (pO2) is called the oxygen dissociation curve.
• The curves for myoglobin and hemoglobin show
important differences.
• This graph illustrates that myoglobin has a higher
oxygen affinity at all pO2 values than does hemoglobin
Allosteric effects: ‫التاثير الجانبي على‬
‫الهيموجلوبين‬
• The ability of hemoglobin to reversibly bind oxygen is
affected by the pO2 ‫ تتاثر بالعوامل التاليه‬: (1-through hemeheme interactions , 2-the pH of the environment, 3-the
pCO2,4- the availability of 2,3-bisphosphoglycerate and
5- CO.
• These are collectively called allosteric (“other site”)
effectors, because their interaction at one site on the
hemoglobin molecule affects the binding of oxygen to
heme groups at other locations on the molecule.
• [Note: The binding of oxygen to myoglobin is not
influenced by allosteric effectors.]
 Effect of O2 binding (Heme-heme interactions):
 The sigmoidal ‫ له شكل اس‬oxygen-binding curve reflects
specific structural changes that are initiated at one heme
group and transmitted to other heme groups in the
hemoglobin tetramer.
 The net effect is that the affinity of hemoglobin for the last
oxygen bound is approximately 300 times greater than its
affinity for the first oxygen bound. ‫اذا ارتبط االكسجين بهيمي ( واحده‬
‫من االربع ) فان هذا االرتباط يحفز الهيمي الباقيه لالرتباط باالكسجين ويزيد جاذبيتها‬
) ‫ ضعف الهيمي ( االول‬300 ‫ ( االخير ) الى‬4 ‫تجاهه حتى يصل عند الهيمي رقم‬
 O2 favors the R- form ‫ الشكل االكثر جاذبيه لالكسجين‬of Hb. ( ‫سؤااال‬
) ‫مهمه‬
 Deoxygenation favors ‫ تؤدي الى‬the T- form of Hb.
Effect of pH and CO2 (Bohr effect):
• The release of oxygen from hemoglobin is enhanced when the pH is
lowered ( H+) or when the hemoglobin is in the presence of an
increased partial pressure of CO2.
• Both result in a decreased oxygen affinity of hemoglobin and
both, stabilize the T state.
• In the tissues, CO2 is converted by carbonic anhydrase (CA) to
carbonic acid:
CO2 + H2O
H2CO3
which spontaneously loses a proton, becoming bicarbonate
(the major blood buffer):
H2CO3
HCO3¯ + H+
• The H+ produced by this pair of reactions   pH
• H+ increases the ionic bonds on Hb T-form  O2
release to the tissues.
• Lactic acid produced during muscular exercise   pH
Effect of 2,3-bisphosphoglycerate ( ‫ ) سالب الشحنه‬:‫سؤال‬on
oxygen affinity:
• 2,3- Bisphosphoglycerate (2,3-BPG) is an important regulator of the
binding of oxygen to hemoglobin.
• It is the most abundant organic phosphate in the red blood cell, where
its concentration is approximately that of hemoglobin.
• 2,3-BPG is synthesized from an intermediate of the glycolysis.
Binding of 2,3-BPG to deoxyhemoglobin ( ‫)موحب الشحنه‬:
• 2,3-BPG decreases the oxygen affinity of hemoglobin by binding to
deoxyhemoglobin but not to oxyhemoglobin.
• This preferential binding stabilizes the T conformation of
deoxyhemoglobin.( ‫ ) يجعله اكثر سالبيه‬: ‫سؤال‬
Response of 2,3-BPG levels to chronic ‫ مهمه‬hypoxia or anemia:

The concentration of 2,3-BPG in the red blood cell increases in response to
chronic hypoxia or anemia.
• Elevated 2,3-BPG levels lower the oxygen affinity of hemoglobin, permitting
greater unloading of oxygen in the capillaries of the tissues.
Binding of CO:
• Carbon monoxide (CO) binds tightly (but reversibly) to
the hemoglobin iron, forming carboxyhemoglobin.
• When carbon monoxide binds to one or more of the four
heme sites, hemoglobin shifts to the relaxed
conformation (R-form), ‫مهمه‬causing the remaining heme
sites to bind oxygen with high affinity.
• As a result, the affected hemoglobin is unable to release
oxygen to the tissues.
• [Note: The affinity of hemoglobin for CO is 220 times
greater than for oxygen.
Co compute with O2 on the binding
site ( hemoglobin )
Factors favoring the T-form of Hb. are:
• Deoxygenation
• Low pH (  H+)
• CO2
• Lactic acid
• 2,3 bisphosphoglycerate
Factors favoring the R-form of Hb. are:
• O2
• CO
Hemoglobinopathies:
• Hemoglobinopathies have traditionally ‫ تقليديا‬been defined
as a family of genetic disorders caused by ‫ سببين‬:
1- production of a structurally abnormal hemoglobin
molecule,2- synthesis of insufficient quantities ‫كميات غير‬
‫ كافيه‬of normal hemoglobin, or, rarely, both.
• ‫ االمراض‬1-Sickle cell anemia (Hb S),2- hemoglobin C
disease (Hb C), and 3-the thalassemia ‫قلت كميه الهيموجلوبين‬
syndromes are representative hemoglobinopathies that
can have severe clinical consequences.
• The first two conditions result from production of
hemoglobin with an altered amino acid sequence
(qualitative ‫ تغيير بالجوده‬hemoglobinopathy), whereas the
thalassemias are caused by decreased production of
normal hemoglobin (quantitative ‫تغير الكميه‬
hemoglobinopathy).
Sickle cell disease (hemoglobin S disease)
(SCD)
• Sickle cell disease (also called sickle cell anemia) is a
genetic disorder of the blood caused by a single
nucleotide alteration )a point mutation( in the β-globin
gene.
• Sickle cell disease is an autosomal recessive disorder.
• It occurs in individuals who have inherited two mutant
genes (one from each parent) that code for synthesis of
the β chains of the globin molecules.
 A molecule of Hb S contains two normal α-globin chains
and two mutant β-globin chains )βS(, ) ‫مهمه جدا جدا ( سؤاال‬in
which glutamate ‫ ذو الشحنه السالبه‬at position six has
been replaced with valine ‫ ال يحمل شحنه‬.
 Therefore, during electrophoresis ‫ جهاز االقطاب الكهربائي‬at
alkaline pH, Hb S migrates more slowly toward the anode
(positive electrode) than does Hb A ) ‫ مهمه ( سؤال‬.
 Electrophoresis of hemoglobin obtained from lysed red
blood cells is routinely used in the diagnosis of sickle cell
trait and sickle cell disease.
 ‫ طريقه الكشف عن المرض‬: HbS is slower in electrophoretic
motility than HbA at pH 8.6
 The lifetime of an erythrocyte in SCD is less than 20 days
compared to 120 days in normal RBC, hence causing
anemia.
 HbS is less soluble than HbA and precipitates ‫تترسب‬
specially when in T-form giving the RBCs the sickle shape.
) ‫الن الذوبانيه تعتمد على زياده الشحنه ( كلما زادت الشحنه زادت الذوبانيه‬