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Questions and Answers
Globular hemeproteins have heme as a ______ group.
Globular hemeproteins have heme as a ______ group.
prosthetic
Myoglobin functions as a reservoir for ______.
Myoglobin functions as a reservoir for ______.
oxygen
The structure of myoglobin is primarily composed of ______ helices.
The structure of myoglobin is primarily composed of ______ helices.
alpha
The interior of myoglobin is composed of ______ amino acids.
The interior of myoglobin is composed of ______ amino acids.
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Charged amino acids on the surface of myoglobin form ______ bonds with each other.
Charged amino acids on the surface of myoglobin form ______ bonds with each other.
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One molecule of 2,3-BPG binds to a pocket in the center of the deoxyhemoglobin ______
One molecule of 2,3-BPG binds to a pocket in the center of the deoxyhemoglobin ______
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In response to chronic hypoxia, the concentration of 2,3-BPG in red blood cells ______
In response to chronic hypoxia, the concentration of 2,3-BPG in red blood cells ______
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Increasing levels of 2,3-BPG decreases the ______ affinity of hemoglobin
Increasing levels of 2,3-BPG decreases the ______ affinity of hemoglobin
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Chemicals used to preserve blood can cause a decrease in 2,3-BPG, making blood an 'oxygen ______'
Chemicals used to preserve blood can cause a decrease in 2,3-BPG, making blood an 'oxygen ______'
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To counteract the decrease in 2,3-BPG in preserved blood, ______ can be added.
To counteract the decrease in 2,3-BPG in preserved blood, ______ can be added.
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The proximal histidine, known as F8, binds the iron of ______.
The proximal histidine, known as F8, binds the iron of ______.
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Hemoglobin A consists of two α-chains and two ______-chains.
Hemoglobin A consists of two α-chains and two ______-chains.
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Myoglobin can only carry ______ molecule of oxygen.
Myoglobin can only carry ______ molecule of oxygen.
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Hemoglobin has ______ binding sites, one in each subunit.
Hemoglobin has ______ binding sites, one in each subunit.
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The T-form of hemoglobin is also known as the taut or ______ form.
The T-form of hemoglobin is also known as the taut or ______ form.
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Polypeptide chains within each dimer of hemoglobin are held together primarily by ______ interactions.
Polypeptide chains within each dimer of hemoglobin are held together primarily by ______ interactions.
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Hemoglobin can transport both H+ and ______ from tissues to the lungs.
Hemoglobin can transport both H+ and ______ from tissues to the lungs.
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The two dimers in hemoglobin are held together by ______ bonds.
The two dimers in hemoglobin are held together by ______ bonds.
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CO2 binding stabilizes the T-form (deoxy form) of hemoglobin, causing a decrease in ______ affinity.
CO2 binding stabilizes the T-form (deoxy form) of hemoglobin, causing a decrease in ______ affinity.
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Collagen has a repeating sequence of -Gly – ______ – ______ – Gly-
Collagen has a repeating sequence of -Gly – ______ – ______ – Gly-
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CO is very toxic because its affinity for hemoglobin is ______ times greater than that for oxygen.
CO is very toxic because its affinity for hemoglobin is ______ times greater than that for oxygen.
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Hyp stabilizes the triple-helical structure of collagen by maximizing interchain ______ bond formation.
Hyp stabilizes the triple-helical structure of collagen by maximizing interchain ______ bond formation.
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Fetal hemoglobin (HbF) has a higher affinity for ______ than adult hemoglobin (HbA).
Fetal hemoglobin (HbF) has a higher affinity for ______ than adult hemoglobin (HbA).
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HbA1c is formed when hemoglobin is slowly ______ under physiological conditions.
HbA1c is formed when hemoglobin is slowly ______ under physiological conditions.
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Vitamin C deficiency can lead to ______, which results in fragile capillaries and excessive bruising.
Vitamin C deficiency can lead to ______, which results in fragile capillaries and excessive bruising.
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In collagen, proline at position X is essential for ______ conformation of each α-chain.
In collagen, proline at position X is essential for ______ conformation of each α-chain.
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Collagen is the most abundant ______ in the human body.
Collagen is the most abundant ______ in the human body.
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The hydroxyl group of hydroxylysine can be glycosylated by adding glucose and ______ to the polypeptide chain.
The hydroxyl group of hydroxylysine can be glycosylated by adding glucose and ______ to the polypeptide chain.
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Fibril-forming collagen type I is found in skin, bone, tendon, blood vessels, and ______.
Fibril-forming collagen type I is found in skin, bone, tendon, blood vessels, and ______.
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Fibrous proteins, including collagen and elastin, are primarily responsible for ______ functions.
Fibrous proteins, including collagen and elastin, are primarily responsible for ______ functions.
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Type IV collagen forms the ______ membrane.
Type IV collagen forms the ______ membrane.
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Study Notes
Globular Hemeproteins
- Hemeproteins are proteins that have heme as a prosthetic group
- Prosthetic groups are non-protein molecules associated with proteins and are essential for their activity
- Heme is a complex of protoporphyrin IX and ferrous iron (Fe2+)
- Heme in proteins can have different functions:
- Hemoglobin/myoglobin – reversible binding of oxygen
- Catalase – involved in the breakdown of hydrogen peroxide
- Cytochrome – electron carrier
Structure of Heme
- Iron can form 6 bonds:
- 4 with nitrogens of the porphyrin ring
- 1 with a histidine residue in the globin protein
- 1 binds oxygen
Myoglobin
- Present in heart and skeletal muscle
- Acts as:
- A reservoir for oxygen
- An oxygen carrier, increasing the rate of oxygen transport in muscle cells
- Composed of a single polypeptide chain, therefore there is no quaternary structure
Structure of Myoglobin
- Myoglobin is composed of 8 α-helices labelled A to H
- α-helices are terminated by proline and connected by β-bends and loops
- Myoglobin is stabilized by hydrogen and ionic bonds
- The interior of myoglobin is composed of nonpolar amino acids, stabilized by hydrophobic interactions
- The surface of myoglobin is composed of charged amino acids, forming hydrogen bonds with each other and surrounding water
- Heme sits in the center of the molecule, lined with nonpolar amino acids
- Two histidine residues:
- F8 proximal histidine binds iron of heme
- Distal histidine (E7) stabilizes binding of oxygen to ferrous ion
- The protein of myoglobin facilitates the reversible binding of oxygen
Hemoglobin
- Present only in red blood cells
- Transports oxygen from the lungs to capillaries of all tissues
- Hemoglobin A is composed of 4 polypeptide chains (2 α-chains, 2 β-chains)
- Polypeptide chains are held together by noncovalent interactions
- Deoxy form of hemoglobin is called T-form (tense or taut)
- Oxy form of hemoglobin is called R-form (relaxed)
Structure of Hemoglobin
- The 4 polypeptide chains form 4 subunits, making hemoglobin a tetrameric protein
- Each subunit has stretches of α-helices
- Heme binding site is similar to myoglobin with two histidine molecules (4 heme binding sites, one in each subunit)
Properties of Hemoglobin
- Hemoglobin can transport H+ and CO2 from tissues to lungs (myoglobin transports only O2)
- Hemoglobin can carry 4 molecules of oxygen from the lungs to tissues in the body (myoglobin only carries one oxygen molecule)
- Oxygen-binding properties of hemoglobin are regulated by interaction with allosteric effectors
Quaternary Structure of Hemoglobin
- Hemoglobin is often viewed as having two identical dimers, (αβ)1 and (αβ)2
- Each dimer contains 2 polypeptide chains
- Polypeptide chains within dimers are held together primarily by hydrophobic interactions
- Two dimers are held together by polar bonds
- Interactions holding the two dimers together are weaker than the interactions within each dimer
- Both dimers are able to move with respect to each other
- Both dimers occupy different relative positions in deoxyhemoglobin compared with oxyhemoglobin
Factors Affecting Binding of Oxygen by Hemoglobin
- 2,3-bisphosphoglycerate (BPG) binds to a pocket in the center of the deoxyhemoglobin tetramer and is expelled from the tetramer on oxygenation
- BPG stabilizes the T-form of hemoglobin, which has a lower affinity for oxygen
- The concentration of BPG in red blood cells increases in response to chronic hypoxia such as:
- Obstructive pulmonary emphysema
- High altitudes
- Chronic anemia
- During hypoxia, hemoglobin does not receive enough oxygen, leading to an increase in the level of BPG. This decrease in oxygen affinity allows for greater unloading of oxygen to the tissues.
Role of BPG in Transfused Blood
- BPG is essential for normal oxygen transport by hemoglobin
- Chemicals used to preserve blood e.g. acid-citrate-dextrose, cause a decrease in BPG in blood
- This results in blood having a very high affinity for oxygen, leading to it not releasing oxygen to tissues (“oxygen trap”)
- Solution: add inosine (hypoxanthine-ribose) to release ribose, which enters the hexose-monophosphate pathway to produce 2,3-BPG
Binding of CO2 to Hemoglobin
- Most metabolic CO2 is hydrated and transported as bicarbonate
- Some CO2 is carried as carbamate bound to uncharged α-amino groups of hemoglobin, forming carbamino-hemoglobin Hb-NH2+CO2↔ Hb-NH-COO-+H+
- CO2 binding stabilizes the T-form of hemoglobin, causing a decrease in oxygen affinity
- Allows dissociation of CO2 from hemoglobin in the lungs
Binding of Carbon Monoxide (CO) to Hemoglobin
- CO binds to iron of hemoglobin (reversible), forms carbon monoxyhemoglobin
- Binding of CO to a site on heme increases the affinity of other oxygen binding sites, moving the oxygen dissociation curve to the left
- Affected hemoglobin is not able to unload oxygen to tissues
- Affinity of hemoglobin for CO is 220 times greater than affinity for oxygen, making CO highly toxic
- CO poisoning is treated with 100% oxygen, facilitating dissociation of CO from hemoglobin
Minor Hemoglobins
Fetal Hemoglobin (HbF)
- Major hemoglobin in the fetus and newborn
- Higher affinity for oxygen than HbA
- Binds weakly to 2,3-BPG
- Facilitates transfer of oxygen from maternal circulation across the placenta to red blood cells of the fetus
Hemoglobin HbA1c
- HbA is slowly glycosylated under physiological conditions
- Extent of glycosylation depends on plasma concentration of hexose
- HbA1c has glucose residues attached to amino groups of N-terminal valines
- Increased amounts of HbA1c are found in the blood of diabetic patients
Fibrous Proteins
- Fibrous proteins have structural functions such as:
- Skin
- Connective tissue
- Blood vessel walls
- Cornea
- Examples of fibrous proteins include collagen and elastin
Collagen
- Most abundant protein in the human body
- Three polypeptide chains wound around each other (α-chains)
- Collagen has a structural role which include:
- Eye: supports vitreous humor
- Tendons: tight fibers provide strength
Types of Collagen
- Collagen has different types with varying tissue distributions:
- I – Skin, bone, tendon, blood vessels, cornea
- II – Cartilage, intervertebral disk, vitreous body
- III – Blood vessels, fetal skin
- IV – Basement membrane
- VII – Beneath stratified squamous epithelia
- IX – Cartilage
- XII – Tendon, ligaments
Structure of Collagen
- Collagen has a repeating sequence -Gly – X – Y – Gly – X – Y – Gly-
- Glycine is every third amino acid
- Proline is at position X and is essential for the helical conformation of each α-chain
- Y is either hydroxyproline or hydroxylysine
- Collagen is composed of 3 polypeptide chains intertwined to form a triple-stranded helix
- Most amino acids are exposed on the surface, allowing bond formation between α-chains and collagen monomers
- Hyp and Hyl are not present in most proteins
- Pro and lys residues are hydroxylated after incorporation into polypeptide chains
- Hyp stabilizes the triple-helical structure of collagen by maximizing interchain hydrogen bond formation
- Hydroxylation requires vitamin C (ascorbic acid)
- Vitamin C deficiency causes scurvy, decreasing the strength of collagen fibrils and causing capillary fragility and excessive bruising
- The hydroxyl group of Hyl can be glycosylated enzymatically, adding glucose and galactose sequentially to the polypeptide chain prior to triple-helix formation
Collagen Diseases
-
Ehlers-Danlos syndrome:
- Occurs due to mutations in the gene needed for the production of type III collagen
- Serious vascular problems occur due to vascular weakness
- Patients also have defects in collagen type I, resulting in stretchy, fragile skin
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Description
Explore the fascinating world of globular hemeproteins, focusing on their structures and functions. This quiz covers essential functions of heme in proteins like hemoglobin, myoglobin, and catalase, as well as the specific structure of myoglobin. Test your knowledge on how these proteins contribute to biological processes!
