Chapter 5 - Protein Function

Transport

Structural

Motor function

Other functions:

Catalysis

Immunity

Regulation of gene expression

Myoglobin transports O2 throughout muscles

Hemoglobin transports O2 in blood

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

Myosin interacts with actin to facilitate muscular movement

Kinesin moves along microtubules to support a variety of cellular functions

In muscle cells

Promote diffusion of O2 into and through muscle cells

In red blood cell

Promote diffusion of O2 through the body ( binds O2 at lungs releases at tissues)

1. O2 transport is critical for sustaining life

2. Hemoglobin mutation can possibly lead to disease

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

A prosthetic group

A porphyrin that chelates iron for oxygen transport

Classical globular protei

Via the Fe in hem

Histidine residues play a key role in anchoring both O2 and iron

With iron supplements or an iron-rich diet

MbO2

K = {[Mb][O2] / [MbO2]}

Hyperbolic trend

Such that binding is half-maximal when the oxygen concentration is equal to the dissociation constant

Organic molecule bound to protein that is essential for the protein's function

Subunits are structurally similar

Only approximately 18% identical in primary sequence

Similar in their secondary and tertiary structures (they almost overlap perfectly)

Hemoglobin

A common evolutionary background

Essential for function

Somewhat important for function

Not important for functio

Hb . H+ + O2 <-- --> Hb . O2 + H+

pH increases = O2 affinity increases

BPG

BPG binds only to the tense (deoxy) conformation of Hb

BPG stabilizes the deoxy state

Cells need BPG in order for Hb to release enough O2, otherwise O2 binds too tightly

Glu --> Val mutatio

Fatigue

Shortness of breath

Joint pain

Splenic sequestration

Lung and other tissue damage

Results in polymerization of hemoglobin (fiber formation)

People with sickle cell anemia have two copies of the mutated gene

Carriers of the sickle cell trait only have one copy of the mutated gene

A recessive disease

Three

Cytoskeleton

Actin forms microfilaments in cells

Polymers of actin

Support plasma membrane and determines shape

Associates with motor proteins

Formed by globular actin subunits associate in a double chain

Monomeric actin is a globular protein that binds to ATP

Actin polymerization is driven by the hydrolysis of ATP

Do not associate with motor proteins

Basic structural unit is a dimer of alpha-helices that wind around each other --> coiled coil

The dimers then associate to form higher-order fibrous structures that consist of 16-32 polypeptides

Intermediate filament

Intermediate in thickness to microfilaments and microtubules

Tubulin dimers constitute microtubules

Hollow fibers built from tubulin dimers

Associates with motor proteins

Thicker and more rigid than microfilaments

Constructed as a hollow tube built from tubulin dimers

Made from dimers of the proteins alpha- and beta tubulin

Keratin filament

Collagen

Actin is added and removed on both the (-) and (+) ends

Actin is added more rapidly on the (+) end

The ability for microfilaments to change and adjust in lengt

Able to assemble and disassemble to allow for cells to change shape

Can be observed as dividing cells

The disassembly of microtubules, along with action of motor proteins, draws the chromosomes apart

Keratin forms a coiled-coil structure

Keratin fibers are cross-linked through disulfide bonds

Keratin is used to make hair, nails, and ski

Structural protein that is not part of the cytoskeleton

Helps hold cells together --> in connective tissue and bone

Most abundant protein in animal

Triple helix

Covalently cross-linked

Every 3rd amino acid = Gly

Approximately 30% or remaining amino acids are proline or hydroxyproline

Cross-linking

Ascorbate or Vitamin C

Molecular motors that use ATP hydrolysis to move along cytoskeletal filaments within the cel

Acts on actin filaments to carry out muscle contraction

Two heads and a long tail

Binds to ATP

ATP hydrolysis drive the physical movement of myosin along an actin filament

1. The reaction sequence begins with a myosin head bound to an actin subunit of the thin filament. ATP binding alters the configuration of the myosin head so that it releases actin

2. The rapid hydrolysis of ATP to ADP + Pi triggers conformational change that rotates the myosin lever and increases the affinity of myosin for actin

3. Myosin binds to an actin subunit farther along the thin filament

4. Binding to actin causes Pi and then ADP to be released. As these reaction products exit, the myosin lever returns to its original position. This causes the thin filament to move relative to the thick filament (the power stroke). ATP replaces the lost ADP to repeat the reaction cycle

Moves along microtubules for intracellular transport

A microtubule-association protein

By moving processively along a microtubule trac

Similar to myosin, except the two heads work in a coordinated fashion, not independently