lec16-17 Flashcards
What are prosthetic groups?
Tightly bound coenzymes, e.g., heme in hemoglobin.
What are coenzymes?
Small organic molecules that help enzymes catalyze reactions, bind to apoenzymes to form holoenzymes, often derived from vitamins.
What is the function of myoglobin and hemoglobin?
Oxygen transport and storage, necessary due to O2’s low solubility and poor diffusion in tissues.
How do myoglobin and hemoglobin illustrate protein structure?
They showcase secondary, tertiary, and quaternary structures, with myoglobin (tertiary) being monomeric and hemoglobin tetrameric.
How do myoglobin and hemoglobin differ in function and properties?
Myoglobin: Oxygen storage in tissues (e.g., muscles), monomeric, high O2 affinity, unaffected by pH, CO2, or BPG, binds 1 O2, doesn’t bind BPG.
Hemoglobin: Oxygen transport in blood, heterotetramer (2α, 2β), moderate O2 affinity, sensitive to pH, CO2, and BPG, binds 4 O2 molecules.
What is the structure and function of myoglobin?
A globular muscle protein for oxygen storage, with 153 amino acids, 77% α-helical, and 8 α-helices (A–H). inner residues are nonpolar except E7 and F8 where heme binds. with His F8 directly interacting with Fe²⁺.
How does myoglobin bind oxygen?
Through a permanently bound heme group (porphyrin + Fe²⁺), where Fe²⁺ binds O₂.
Structure of Heme (Cofactor in Hemoglobin & Myoglobin)
Protoporphyrin IX: Tetrapyrrole ring system that chelates Fe²⁺
Heme = Protoporphyrin IX + Fe²⁺
Iron-binding transport molecules must:
Bind O₂
Prevent oxidation
Release O₂ on demand
Heme synthesis requires multiple enzymes—defects can cause disorders like porphyrias.
Porphyria
Cause: Mutation in heme synthesis genes
Symptoms (Untreated):
Extreme sunlight sensitivity
Excessive hair growth
Skin sores, disfigurement
Finger/nose loss in severe cases
Tightened gums/lips → “fang-like” teeth
Garlic Effect: Worsens symptoms due to a chemical affecting heme metabolism
Historical Connection: Symptoms resemble vampire/werewolf myths
how does o2 stay in reduced form?
In addition to the 4 heme nitrogens, Fe2+ is
coordinated by His F8 (for 8th residue on the F helix). Oxygen provides a 6th ligand and is
stabilized by a hydrogen bond to His E7.
The hydrophobic environment of the protein in the
heme binding site keeps the iron in a reduced (Fe2+) form.
how does r state differ from t state in structure
the R state differs from the T state
by a rotation of about 15o of the
a1b1 dimer with respect to a2b2
together with a shift that brings the
b subunits closer together and
narrows central cavity
(as many at 50 non-covalent
interactions between the two ab
dimers are altered in the T to R
switch
o2 binding structural changes in hemoglobin
Before O₂ Binding: Fe²⁺ is outside the heme plane, heme is puckered
After O₂ Binding: Fe²⁺ moves into the heme plane, pulling His F8
Effect: Structural change is transmitted to other subunits, promoting R-state formation and cooperative binding
how is the communication between hemoglobin subunits possible?
due to quaternary structures.
thast why we only see either fully looaded (oxyhemoglobin ) or deoxyhemogflobin not rly an intermediate
Oxygen Binding: Myoglobin vs. Hemoglobin & Cooperativity
Flashcard Front: Oxygen Binding: Myoglobin vs. Hemoglobin & Cooperativity
Flashcard Back:
- Myoglobin: Binds O₂ where hemoglobin releases it; stores O₂ and releases it at very low P₀₂.
- Hemoglobin: Cooperative binding → High O₂ affinity in lungs (high P₀₂), low affinity in tissues (low P₀₂).
- Without Cooperativity:
- Strong binding at high P₀₂ but poor release (~10% at 20 torr).
- Good release at low P₀₂ but poor binding (~38% at high P₀₂).
how can o2 transport be inhibited by toxic materials and which ones?
Heme binds O₂ weakly & reversibly
CO, H₂S, CN⁻ bind strongly & irreversibly, blocking O₂ binding
These gases are toxic because they prevent O₂ transport
bpg!!!!
Hemoglobin can release O₂ on its own, but as soon as it starts, BPG swoops in and locks it into the T-state, making sure it keeps unloading O₂ efficiently. It’s like BPG saying, “No going back now!”
fetal hemoglobin
Fetal Hemoglobin (HbF): 2 α + 2 γ chains (vs. β in adults)
γ Chain vs. β Chain: 72% identical, but His143 → Ser in γ chain (his143 critical for binding bpg)
Effect: Weaker BPG binding → Higher O₂ affinity
Function: HbF efficiently takes O₂ from maternal hemoglobin
P₅₀ Difference: HbF reaches 50% saturation at lower P₀₂ than adult Hb
his146
get sprotonated whne theres low ph nad crseates one of two salt bridges that are improtant for stabilizing t state of hemoglobin
rmb his143 is the one in fetal its his to ser. 3 is youngest so baby and ser for sergio the baby
Three Factors That Help O₂ Release
- bpg-binds to the positively charged His143 residue on the β-chain of hemoglobin in the T-state. This interaction is crucial because it stabilizes the T-state, reducing hemoglobin’s affinity for O₂ and promoting O₂ release in tissues.
- ph-High [H⁺] (low pH) protonates His146 on hemoglobin.
Protonated His146 forms a salt bridge with Asp94, stabilizing the T-state, reducing hemoglobin’s ability to bind O₂, and promoting O₂ release.
This occurs in active tissues where CO₂ is high, and O₂ needs to be unloaded. - high co2-negative charge of HCO₃⁻ contributes to salt bridge formation in hemoglobin, which stabilizes the T-state, promoting O₂ release in tissues. Additionally, low pH (high CO₂ and H⁺) leads to the protonation of His146, further stabilizing the T-state
CO₂ and HCO₃⁻ in Hemoglobin’s O₂ Release
CO₂ is converted to HCO₃⁻ and H⁺ in tissues via carbonic anhydrase.
H⁺ protonates His146 on hemoglobin, stabilizing the T-state and promoting O₂ release.
The negative charge of HCO₃⁻ helps form salt bridges with positively charged residues (e.g., His146), further stabilizing the T-state.
CO₂ also binds to hemoglobin forming carbaminohemoglobin, contributing to O₂ unloading.
sickle cell
-mutation in the b chain of hb
-glu6->val
-val is np and casues hb to link up and form chains cuz of hydrophbic intetractions
-giving cells sickle shape
-
The sickling of red blood cells is caused by low [O2] because the interaction between the mutated beta chains occurs in the deoxy (T) form of hemoglobin
Would you expect the α and β subunits of hemoglobin to have more or fewer hydrophobic amino acids than myoglobin? Why?
Answer: The α and β subunits of hemoglobin would have more hydrophobic amino acids than myoglobin. This is because hemoglobin is a tetrameric protein with multiple subunits (two α and two β) that interact with each other. These interactions are stabilized by hydrophobic residues at the subunit interfaces. Myoglobin, being a monomer, does not have subunit interactions and thus has fewer hydrophobic residues involved in such interactions.
heterotropic vs homotropic allosteric effectors
Heterotropic effector: A molecule different from the substrate that binds allosterically and affects activity (e.g., 2,3-BPG for hemoglobin).
Homotropic effector: A molecule same as the substrate that binds allosterically and affects activity (e.g., O₂ for hemoglobin).
3 key fetaures of protein-ligand complex
(1) Ligand binding is a reversible process involving
weak noncovalent interactions. (2) Ligand binding
induces or stabilizes structural conformations in target
proteins. (3) The equilibrium between ligand-bound
protein and ligand-free protein can be altered by
effector molecules, which induce conformational
changes in the protein that increase or decrease ligand
affinity.
