Proteins serve a variety of functions. Transport –Myoglobin transports O 2 throughout muscles. –Hemoglobin transports O 2 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 O 2 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. © 2014 John Wiley & Sons, Inc. All rights reserved.

Why focus on myoglobin and hemoglobin? Sickled red blood cellHealthy red blood cell O 2 transport is critical for sustaining life. Hemoglobin mutation can possibly lead to disease. Characteristics about O 2 binding to myoglobin and hemoglobin are observed in many areas of biochemistry. © 2014 John Wiley & Sons, Inc. All rights reserved.

Myoglobin is a classical globular protein. Space-filling representation Ribbon diagram with heme in purple What is heme? © 2014 John Wiley & Sons, Inc. All rights reserved.

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

Myoglobin transports O 2 via the Fe in heme. O2O2 His residues play a key role in anchoring both O 2 and iron. Anemia is often treated with iron supplements or an iron-rich diet. © 2014 John Wiley & Sons, Inc. All rights reserved.

How can binding of O 2 to myoglobin be described? Mb + O 2 MbO 2 K = [MbO 2 ][Mb] [MbO 2 ] © 2014 John Wiley & Sons, Inc. All rights reserved.

Myoglobin binds to O 2 in a hyperbolic trend. Fractional Saturation (Y): the proportion of myoglobin molecules that have bound O 2 Y = Bound Mb Total Mb [MbO 2 ][Mb] + [MbO 2 ] Y = pO 2 K + pO 2 Y = Equation of a hyperbolic curve © 2014 John Wiley & Sons, Inc. All rights reserved.

O 2 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 © 2014 John Wiley & Sons, Inc. All rights reserved.

Mb and Hb are only ~18% identical in primary sequence. Invariant Identical in all Identical in Hb © 2014 John Wiley & Sons, Inc. All rights reserved.

Mb and Hb are similar in their secondary and tertiary structures. Myoglobin α -Subunit of Hemoglobin β -Subunit of Hemoglobin Heme Even though myoglobin and hemoglobin have only ~18% identical residues, their secondary and tertiary structures overlap almost perfectly when superimposed! Hb has quaternary structure, but Mb does not. © 2014 John Wiley & Sons, Inc. All rights reserved.

The similarities in structure and sequence between myoglobin and hemoglobin indicate a common evolutionary origin. © 2014 John Wiley & Sons, Inc. All rights reserved.

How to Express Affinity in Biochemistry? K d = dissociation constant Mb + O 2 reaction Fractional saturation Plot of fractional saturation vs. pO 2 © 2014 John Wiley & Sons, Inc. All rights reserved.

KEY CONCEPTS: Section 5-1 O 2 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. © 2014 John Wiley & Sons, Inc. All rights reserved.

Oxygen binds cooperatively to Hb. Dotted line represents O 2 binding to myoglobin (hyperbola). Solid line represents O 2 binding to hemoglobin (sigmoid). Note: Sigmoidal data are indicative of cooperativity. Cooperativity: Binding of O 2 to one subunit induces easier binding to other subunits. © 2014 John Wiley & Sons, Inc. All rights reserved.

Bohr Effect and O 2 Transport What is happening biochemically when you breathe? From Metabolism + H 2 O © 2014 John Wiley & Sons, Inc. All rights reserved.

As pH , O 2 affinity  © 2014 John Wiley & Sons, Inc. All rights reserved. From Metabolism + H 2 O

BPG decreases Hb’s O 2 affinity. Lower O 2 affinity Fractional Saturation of O 2 BPG binds only to the tense (deoxy) conformation of Hb. © 2014 John Wiley & Sons, Inc. All rights reserved.

KEY CONCEPTS: Section 5-2 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. © 2014 John Wiley & Sons, Inc. All rights reserved.

Microfilaments are polymers of actin. Actin monomer © 2014 John Wiley & Sons, Inc. All rights reserved.

Globular actin subunits associate in a double chain to form a microfilament. Actin monomer Polymerization © 2014 John Wiley & Sons, Inc. All rights reserved.

α and β-Tubulin form dimers. © 2014 John Wiley & Sons, Inc. All rights reserved.

Microtubules are hollow fibers built from tubulin dimers. © 2014 John Wiley & Sons, Inc. All rights reserved.

Cryoelectron microscopy reveals tubular structure of a microtubule. © 2014 John Wiley & Sons, Inc. All rights reserved.

Microtubules can be observed in dividing cells. Microtubules are shown in green fluorescence. Chromosomes are detected in blue fluorescence. © 2014 John Wiley & Sons, Inc. All rights reserved.

KEY CONCEPTS: Section 5-2 Intermediate filaments are long-lasting fibrous proteins consisting of coiled α helices. Three left-handed Gly-rich helical polypeptides form the collagen triple helix. © 2014 John Wiley & Sons, Inc. All rights reserved.

Keratin is an intermediate filament. Keratin forms a coiled-coil structure shown in the three representations here. BackboneStick Space- filling © 2014 John Wiley & Sons, Inc. All rights reserved.

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

Collagen is covalently cross-linked. Cross-linking stabilizes collagen’s structure. Oxidation © 2014 John Wiley & Sons, Inc. All rights reserved.

Collagen has a noteworthy sequence. Every 3 rd amino acid = Gly ~30% of remaining amino acids are proline or hydroxyproline. © 2014 John Wiley & Sons, Inc. All rights reserved.

The Arrangement of Collagen Fibrils in Various Tissues.

KEY CONCEPTS: Section 5-3 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. © 2014 John Wiley & Sons, Inc. All rights reserved.

Myosin has two heads and a long tail. © 2014 John Wiley & Sons, Inc. All rights reserved.

Myosin binds to ATP. © 2014 John Wiley & Sons, Inc. All rights reserved.

ATP hydrolysis drives the physical movement of myosin along an actin filament. © 2014 John Wiley & Sons, Inc. All rights reserved.

Kinesin is a microtubule-associated protein. Vesicle (cargo) binding region © 2014 John Wiley & Sons, Inc. All rights reserved.

Kinesin transports cargo by moving processively along a microtubule track. © 2014 John Wiley & Sons, Inc. All rights reserved.

Kinesin transports cargo by moving processively along a microtubule track. © 2014 John Wiley & Sons, Inc. All rights reserved.

Kinesin transports cargo by moving processively along a microtubule track. © 2014 John Wiley & Sons, Inc. All rights reserved.

Kinesin transports cargo by moving processively along a microtubule track. © 2014 John Wiley & Sons, Inc. All rights reserved.