proteins serve a variety of functions. transport –myoglobin transports o 2 throughout muscles....

Proteins serve a variety of functions.

• Transport– Myoglobin transports O2 throughout muscles.

– Hemoglobin transports O2 in blood.

• Structural– Actin forms microfilaments in cells.– Tubulin dimers constitute microtubules.– Keratin filaments constitute the bulk of animal

hair.– Collagen is a major protein in connective

tissue.© 2014 John Wiley & Sons, Inc. All rights reserved.

Proteins serve a variety of functions.

• Motor function– Myosin interacts with actin to facilitate

muscular movement.– Kinesin moves along microtubules to support

a variety of cellular functions.

• Other functions of proteins– Catalysis– Immunity– Regulation of gene expression

© 2014 John Wiley & Sons, Inc. All rights reserved.

KEY CONCEPTS: Section 5-1

• O2 binds to the heme group of myoglobin such that binding is half-maximal when the oxygen concentration is equal to the dissociation constant.

• The similarities in structure and sequence between myoglobin and hemoglobin indicate a common evolutionary origin.


Why focus on myoglobin and hemoglobin?

Sickled red blood cellHealthy red blood cell

• O2 transport is critical for sustaining life.

• Hemoglobin mutation can possibly lead to disease.

• Characteristics about O2 binding to myoglobin and hemoglobin are observed in many areas of biochemistry.


Myoglobin is a classical globular protein.

Space-filling representation Ribbon diagram with heme in purple

What is heme?


Heme is a prosthetic group.• Prosthetic group =

organic molecule bound to protein that aids protein function

• Heme is a porphyrin that chelates iron for oxygen transport.


Myoglobin transports O2 via the Fe in heme.

O2

• His residues play a key role in anchoring both O2 and iron.

• Anemia is often treated with iron supplements or an iron-rich diet.


How can binding of O2 to myoglobin be described?

Mb + O2 MbO2

K = [MbO2][Mb]

[MbO2]


Myoglobin binds to O2 in a hyperbolic trend.

Fractional Saturation (Y):the proportion of myoglobin

molecules that have bound O2

Y = Bound Mb

Total Mb

[MbO2][Mb] +

[MbO2]Y =

pO2K +

pO2Y =

Equation of a hyperbolic curve


O2 binds to the heme group of myoglobin such that binding is

half-maximal when the oxygen concentration

is equal to the dissociation constant.

Hyperbolic data is common in biochemistry!© 2014 John Wiley & Sons, Inc. All rights reserved.

Remember!

• Proteins have four possible levels of structure.– Primary sequence

– Secondary: alpha helices and beta sheets

– Tertiary: 3D fold

– Quaternary: interaction of multiple subunits


Mb and Hb are only ~18% identical in primary sequence.

Invariant Identical in all Identical in Hb


Mb and Hb are similar in their secondary and tertiary structures.

Myoglobin α-Subunit of Hemoglobinβ-Subunit of Hemoglobin

Heme

Even though myoglobin and hemoglobin haveonly ~18% identical residues, their secondary and tertiary structures overlap almost perfectly when superimposed!

Hb has quaternary structure, but Mb

does not.


The similarities in structure and sequence between myoglobin and

hemoglobin indicate a common evolutionary origin.


How to Express Affinity in Biochemistry?

• Kd = dissociation constant

• Mb + O2 reaction

• Fractional saturation

• Plot of fractional saturation vs. pO2



• O2 can bind cooperatively to hemoglobin as the protein shifts from the deoxy to the oxy conformation.

• The Bohr effect and BPG modulate hemoglobin function in vivo.


Oxygen binds cooperatively to Hb.

Dotted line represents O2

binding to myoglobin (hyperbola).

Solid line represents O2

binding to hemoglobin (sigmoid).

Note: Sigmoidal data are

indicative of cooperativity.

Cooperativity: Binding of O2 to one subunit induces easier binding to other subunits.


Bohr Effect and O2 TransportWhat is happening biochemically when you breathe?

From Metabolism

+ H2O


As pH , O2 affinity


From Metabolism

+ H2O

BPG decreases Hb’s O2 affinity.

Lower O2 affinity

Fra

ctio

nal

Sat

ura

tio

n o

f O

2

BPG binds only to the tense (deoxy)

conformation of Hb.



• Globular actin subunits associate in a double chain to form a microfilament.

• The growth and regression of actin filaments can change a cell’s shape.

• Microtubules are hollow tubes built from tubulin dimers.


Microfilaments are polymers of actin.

Actin monomer


Globular actin subunits associate in a double chain to form a

microfilament.

Actin monomer

Polymerization


α and β-Tubulin form dimers.


Microtubules are hollow fibers built from tubulin dimers.


Cryoelectron microscopy reveals tubular structure of a microtubule.


Microtubules can be observed in dividing cells.

Microtubules are shown in greenfluorescence.

Chromosomes are detected inblue fluorescence.



• Intermediate filaments are long-lasting fibrous proteins consisting of coiled α helices.

• Three left-handed Gly-rich helical polypeptides form the collagen triple helix.


Keratin is an intermediate filament.

Keratin forms acoiled-coil structureshown in the three

representationshere.

Backbone Stick Space-filling


Collagen

• Gly-Pro-xxx repeat discourages a-helices or b-sheets

• Triple helix packs Gly in center• Pro is modified for H-bonding• Triple helices bound together to

make strong fibrils for hair/skin

Collagen is a triple helix.


Collagen is covalently

cross-linked.

Cross-linking

stabilizes collagen’s structure.

Oxidation Oxidation


Collagen has a noteworthy sequence.

• Every 3rd amino acid = Gly

• ~30% of remaining amino acids are proline or hydroxyproline.


The Arrangement of Collagen

Fibrils in Various Tissues.


• The motor protein myosin couples the steps of ATP hydrolysis to conformational changes, resulting in muscle contraction.

• Kinesin transports cargo by moving processively along a microtubule track.


Myosin has two heads and a long tail.


Myosin binds to ATP.


ATP hydrolysis drives the physical movement of myosin along an actin

filament.


Kinesin is a microtubule-associated protein.

Vesicle (cargo) binding region


Kinesin transports cargo by moving processively along a microtubule

track.


proteins serve a variety of functions. transport –myoglobin transports o 2 throughout muscles....

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