05-Biochemistry of protein -Protein Classification -3.pdf

Protein Classification Simple – composed only of amino acid residues Conjugated – contain prosthetic groups (metal ions, co-factors, lipids, carbohydrates) Example: Hemoglobin – Heme  Protein Classification One polypeptide chain - monomeric protein More than one - mu...

Protein Classification Simple – composed only of amino acid residues Conjugated – contain prosthetic groups (metal ions, co-factors, lipids, carbohydrates) Example: Hemoglobin – Heme  Protein Classification One polypeptide chain - monomeric protein More than one - multimeric protein Homomultimer - one kind of chain Heteromultimer - two or more different chains (e.g. Hemoglobin is a heterotetramer. It has two alpha chains and two beta chains.)  Protein Classification Fibrous – 1) polypeptides arranged in long strands or sheets 2) water insoluble (lots of hydrophobic AA’s) 3) strong but flexible 4) Structural (keratin, collagen) Globular – 1) polypeptide chains folded into spherical or globular form 2) water soluble 3) contain several types of secondary structure 4) diverse functions (enzymes, regulatory proteins)  Protein Function Catalysis – enzymes Structural – keratin Transport – hemoglobin Trans-membrane transport – Na+/K+ ATPases Toxins – rattle snake venom, ricin Contractile function – actin, myosin Hormones – insulin Storage Proteins – seeds and eggs Defensive proteins – antibodies  Globular Proteins Myoglobin/Hemoglobin Hemeproteins: group of specialized proteins that contain heme group as a tightly bound prosthetic group. prosthetic group: is a non-protein compound that is permanently associated with protein The role of heme group is dependent on the environment created by the three-dimensional structure of the protein. e.g. heme in cytochrome  electron carrier, enzyme catalase  active site Myoglobin and hemoglobin  Oxygen carrier  Overview  Hemeproteins are a group of specialized proteins that contain heme as a tightly bound prosthetic group. The role of the heme group is dictated by the environment created by the three-dimensional structure of the protein.  Hemoglobin and myoglobin, the two most abundant hemeproteins in humans, the heme group serves to reversibly bind oxygen.  Hemoglobin is the main constituent of RBCs and it is responsible for the red color of t he blood (Each RBC has 640 m illion m olecules of hemoglobin)  Its normal value 12 – 16 gm% (female) and 13.5 – 17.5 gm% (male).  It consists of 4 heme groups and surrounded by 4 polypeptide chains (Globin) which protect the iron from oxidation.  Myoglobin/Hemoglobin  First protein structures determined  Oxygen carriers  Hemoglobin transport O2 from lungs to tissues  Myoglobin O2 storage protein  Structure and function of Hemoglobin Mb and Hb subunits structurally similar 8 alpha-helices Contain heme group Mb monomeric protein Hb heterotetramer (α22) - Hb found exclusively in red blood cells -transport O2 from lungs to capillaries of tissues and transfer CO2 from tissues to lungs -composed of 4 polypeptides held together by non-covalent interaction -each subunit is similar to myoglobin hemoglobin myoglobin and contains a heme group  -Myoglobin is O2 binding protein found in almost all mammals mainly in muscles and heart  -its main function is to store O2 for periods where energy demands is high, it also increases the rate of transport of oxygen within the muscle cells.  - Compact structure, 80% of its polypeptide chain is α-helix that labeled A to h that terminated by Proline or by -bends The interior of the myoglobin is composed of NON-polar a.a. they packed together stabilized by hydrophobic interaction. Charged a.a are located at the surface. Structure of Heme Heme is a complex of protoporphyrine IX (4 pyrrole rings linked by methene bridges) and ferrous iron (Fe+2) Ferrous ion has 6 coordination bonds: 4 with the N of pyrrole rings and 2 are perpendicular one with N of histidine and the other is with O2 protoporphyrine IX porphyrine  Heme = Fe++ bound to tertapyrrole ring (protoporphyrin IX complex)  Heme non-covalently bound to globin proteins through His residue  O2 binds non-covalently to heme Fe++, stabilized through H-bonding with another His residue  Heme group in hydrophobic crevice of globin protein Oxygen binding to Heme group Distal histidine: stabilizes the binding ofO2 to heme Heme Oxygen Ferrous ion Proximal histidine Oxygen binding to Myoglobin Myglobin has one heme group bind only one oxygen molecule. Hemoglobin has 4 heme groups  bind to 4 oxygen molecules O2 dissociation curve has hyperbolic shape Myglobin has higher O2 affinity than of Hemoglobin P50 of Mb is about 1 Degree of saturation mmHg and for Hb is 26 Oxygen dissociation curve P50 is O2 Partial pressure needed to half saturation of the Mb of Hb Concentration of Oxygen (Partial pressure) Oxygen transport proteins Efficient O2 transport protein should bind to O2 at high partial pressure (loading in lung) and release it (low affinity) at low Partial pressure of (unloading in the tissue)  Oxygen Binding Curves tissues Mb has hyperbolic O2 binding curve lungs Mb binds O2 tightly. Releases at very low pO2 Strong-binding Hb has sigmoidal O2 binding curve Transition from weak to Hb high affinity for O2 at strong binding high pO2 (lungs) Hb low affinity for O2 at low pO2 (tissues) Weak-binding  O2 Binding to Hb shows positive cooperativity O2 Binding to Hb shows sigmoidal shape,  low binding affinity at low con of Oxygen and high affinity at Hb higher con cooperative binding by the four subunit of Hb  The binding of one O2 molecule at one heme group increases the oxygen affinity of the remaining heme a groups in the same hemoglobin molecule. The affinity of hemoglobin for the last O2 bound is 300 times greater than its affinity for the first O2 O2 affinity increases as each O2 molecule binds Increased affinity due to conformation change Deoxygenated form = T (tense) form = low affinity Increasing Oxygenated form = R (relaxed) form = high affinity affinity for O2 Hemoglobin is efficient in delivering the O2 to the tissues from lung, myglobin which has hyperbolic O2-dissociation curve is unable to do that  How is CO2 Exported? CO2 + H2O  H2CO3  HCO3 + H+ Most of the CO2 produced in metabolism is hydrated and transported as bicarbonate ion. the hydration of CO2 by the zinc-dependent enzyme carbonic anhydrase.  Binding of Hemoglobin to CO2 Some CO2 is carried as carbamate bound to the uncharged α-amino group Carbon dioxide is transported in the form of a carbamate on the amino terminal residues of each of the polypeptide subunits. Direct binding of CO2 to Hb stabilizes the T- form (deoxy) of Hb  resulting in a decrease in its affinity for oxygen The formation of a carbamate also results in release of a proton into solution  indirectly induces the Bohr effect O H+ C O H H O H H2N C C P rotein C N C C P rotein O R O R O A mi no Terminus Carbamate on A mi no Termi nus The End