Chapter 5 Chem 341 Suroviec Fall 2013. I. Introduction Every protein has a unique 3-D structure

Chapter 5

Chem 341

Suroviec Fall 2013

I. Introduction

• Every protein has a unique 3-D structure

II. Myoglobin

• Small intercellular protein

A. Heme group

• Heme contains 4 pyrrole groups• Fe(II) atom at the center is coordinated

by the 4 porphyrin N atoms and one N from a His side chain

B. Equilibrium of O2 binding

• Myoglobin binding of O2 is simple equilibrium

C. Binding Curve

Steepness of hyperbola increases as K decreases

III. Hemoglobin Structure & Mechanism

• 4 polypeptide chains– 2 subunits– 2 subunits

III. Hemoglobin Structure & Mechanism

• Oxygenation causes extensive quaternary structural changes

• Oxy- and Deoxy- Hb have different forms

A. Binding of O2

• T-state (deoxy)

• R-state (oxy)

In T state (blue) Fe(II) located 0.6 Å out of heme plane

When O2 binds Fe-N porphyrin bonds contract and Fe(II) moves in plane (red)

B. 2 Stable Positions

• Difference between T and R occur at 1-2 and 2-1 interface

C. Role of Globin in Binding of O2

• Protect Fe(II)

• His attached to backside of porphyrin

D. Relative Stability of T and R

• With no O2 present: T more stable

• With O2 present: R more stable

V. Hemoglobin binding and pH

• Effect of pH on Hb transport

• Lung pH = 7.6

• Blood pH = 7.2

• pO2 in tissues = 30 torr

• pO2 in lungs = 95 torr

Bohr Effect

VI. 2 – 3 Bis-phosphoglycerate

• Red blood cells use BPG to fine tune hemoglobin function

VII. Abnormal Hemoglobins• Sickle Cell Anemia

– Deoxyhemoglobin S forms insoluble filaments that deform red blood cells

– Rigid sickle shaped cells cannot pass through the capillaries

– Results in tissue death: lack of oxygen

– Mutant hemoglobin where hemoglobin S contains Val instead of Glu at the 6th position of the chain

– Causes polymerization of hemoglobins

VIII. Structural Proteins

Typical eukaryotic cells have 3 types of cytoskeletal proteins that form fibers

A. Microfilaments

• Made of actin

• Network of microfilaments support plasma membrane

B. Microfilaments extend/retract

• Polymerization of actin monomers is reversible process so the polymer undergoes constant shrinking and growing as subunits add to and dissociate from one or both ends of the microfilaments

C. Microtubules

• Microtubules are cytoskeletal fibers built from globular protein subunits

• Microtubules can assemble and disassemble on a time scale that allow the cell to rapidly change shape in response to external or internal stimuli

D. -Keratin

• Intermediate filaments are structural proteins• Chemically un-reactive• Component of hair, horns, nails and feathers -helix shape, but exhibits smaller than

expected spacing - due to coiled coil structure

E. Collagen

• Most abundant animal protein• Major stress-bearing components of connective tissues (bone, teeth, tendons)• Has distinct amino acid composition

– Every 3rd amino acid = glycine

E. Collagen

• Cross-linking between fibrils also increases insolubility• Can’t be S-S bonds• Cross-link between Lys and His chains using Lysyl oxidase• Tends to occur near termini