Protein Structure: Myoglobin, Hemoglobin, Structural Proteins, and Motor Proteins

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Noncovalent forces (hydrogen bonds, ionic interactions, van der Waals forces) are crucial in biological molecules.
Proteins have four levels of structure: primary, secondary, tertiary, and quaternary.
Some proteins can adopt more than one conformation.
Proteins with multiple polypeptide chains have quaternary structure.

Myoglobin is a classical globular protein found in muscle tissue, responsible for oxygen storage.
Hemoglobin is a tetrameric protein in red blood cells, responsible for oxygen transport from lungs to tissues.
Both proteins contain a heme prosthetic group, a porphyrin ring that chelates an Fe(II) ion, enabling reversible oxygen binding.
Myoglobin binds O2 via the Fe in heme; oxygen binding depends on O2 concentration.
Myoglobin's O2 binding curve is hyperbolic:

Where is fractional saturation, is oxygen partial pressure, and is the dissociation constant.

Where is the Hill coefficient (degree of cooperativity).

Myoglobin and hemoglobin have similar secondary and tertiary structures but only ~18% identity in primary sequence.
Structural and sequence similarities suggest a common evolutionary origin.

Hemoglobin undergoes conformational changes upon O2 binding, shifting from the T (tense, low affinity) to R (relaxed, high affinity) state.
The Bohr effect: As pH increases, hemoglobin's O2 affinity increases. In tissues, CO2 production lowers pH, promoting O2 release.
2,3-Bisphosphoglycerate (BPG) binds to deoxyhemoglobin, stabilizing the T state and decreasing O2 affinity.

Over 1200 hemoglobin variants exist; ~7% of the world's population carries a variant.
Sickle cell hemoglobin (Hb S): E6V mutation (Glu → Val) in β chain causes sickling of red blood cells, leading to anemia and other complications.
Carriers (heterozygotes) of Hb S are protected against malaria (Plasmodium falciparum).
Hemoglobin C: E6K mutation in β chain, also confers malaria resistance, associated with mild anemia.
Thalassemias: Genetic defects reducing synthesis of α or β globin chains, common in Mediterranean and South Asia, symptoms range from mild to severe anemia.

Variant	Chain	Position	Amino Acid Change	Effect
Sickle cell (Hb S)	β	6	Glu → Val	Sickling, malaria resistance
Hemoglobin C	β	6	Glu → Lys	Mild anemia, malaria resistance
Thalassemia	α or β	Various	Deletions/mutations	Reduced chain synthesis
Additional info: …				Other variants affect stability, O2 affinity, or cooperativity

Actin filaments (microfilaments): Polymers of globular actin, form double chains, most abundant in cells, involved in cell shape and movement.
Microtubules: Hollow fibers built from α- and β-tubulin dimers, form 13-protofilament tubes, essential for cell division, intracellular transport, and structure.
Intermediate filaments: Rope-like fibers (e.g., keratin), provide mechanical strength, less dynamic than actin or microtubules.
Collagen: Triple helix structure, every third residue is glycine, rich in proline and hydroxyproline, provides tensile strength to connective tissues.

Actin filaments and microtubules are dynamic, capable of rapid assembly/disassembly (treadmilling in actin, dynamic instability in microtubules).
Intermediate filaments and collagen are more stable, providing long-term structural support.

Protein	Subunit Structure	Size	Dynamics	Function
Actin filament	Globular actin	7 nm	Highly dynamic	Cell shape, movement
Microtubule	α/β-tubulin dimer	25 nm	Highly dynamic	Cell division, transport
Intermediate filament	Fibrous proteins	10 nm	Stable	Mechanical strength
Collagen	Triple helix	Varies	Stable	Connective tissue

Myosin: Moves along actin filaments, has two heads (actin and ATP binding sites) and a long tail. ATP hydrolysis drives conformational changes for movement.
Kinesin: Moves along microtubules, transports cargo toward the (+) end, operates processively (takes many steps without detaching).
Both use ATP hydrolysis to convert chemical energy into mechanical work.

Property	Myosin	Kinesin
Track	Actin filament	Microtubule
Direction	Toward (+) end	Toward (+) end
Processivity	Non-processive (single stroke)	Processive (multiple steps)
Function	Muscle contraction, vesicle transport	Organelle/vesicle transport

ATP binding, hydrolysis, and product release drive conformational changes in both myosin and kinesin, enabling movement along their respective tracks.