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High-Yield Terms
Heme: is formed when iron is inserted into the chemical compound protoporphyrin
Hemin: normal heme contains iron in the ferrous oxidation state (Fe2+), whereas hemin contains iron in the ferric oxidation state (Fe3+)
Methemoglobin: the form of the hemoglobin protein that contains ferric iron (Fe3+) in the heme prosthetic groups due to oxidation
Hemoglobinopathy: any disease resulting from either (or both) quantitative or qualitative defects in α-globin or β-globin proteins
Thalassemia: specifically refers to quantitative hemoglobinopathies due to either α-globin or β-globin protein defects
Sickle cell anemia: most commonly occurring qualitative hemoglobinopathy, results from a single amino acid substitution in the adult β-globin gene
Cooley anemia: is thalassemia major, which is either β0− and β+-thalassemia
Myoglobin and hemoglobin are hemeproteins whose physiological importance is principally related to their ability to bind molecular oxygen. Hemoglobin is a heterotetrameric oxygen transport protein found in red blood cells (erythrocytes), whereas myoglobin is a monomeric protein found mainly in muscle tissue where it serves as an intracellular storage site for oxygen. The oxygen carried by hemeproteins such as hemoglobin and myoglobin is bound directly to the ferrous iron (Fe2+) atom of the heme prosthetic group. Oxidation of the iron to the ferric (Fe3+) state renders the molecule incapable of normal oxygen binding. When the iron in heme is in the ferric state, the molecule is referred to as hemin.
The tertiary structure of myoglobin is that of a typical water-soluble globular protein. Its secondary structure is unusual in which it contains a very high proportion (75%) of α-helical secondary structure. Each myoglobin molecule contains a single heme group inserted into a hydrophobic cleft in the protein. Hydrophobic interactions between the tetrapyrrole ring and hydrophobic amino acid R groups on the interior of the cleft in the protein strongly stabilize the heme–protein conjugate. In addition, a nitrogen atom from a histidine R group located above the plane of the heme ring is coordinated with the iron atom further stabilizing the interaction between the heme and the protein. In oxymyoglobin the remaining bonding site on the iron atom (the 6th coordinate position) is occupied by the oxygen, whose binding is stabilized by a second histidine residue.
Adult hemoglobin is a heterotetrameric [α(2):β(2)] hemeprotein (Figure 6-1) found in erythrocytes where it is responsible for binding oxygen in the lung and transporting the bound oxygen throughout the body, where it is used in aerobic metabolic pathways. Each subunit of a hemoglobin tetramer has a heme prosthetic group identical to that described for myoglobin. The quaternary structure of hemoglobin leads to physiologically important allosteric interactions between the subunits, a property lacking in monomeric myoglobin, which is otherwise very similar to the α-subunit of hemoglobin.
Hemoglobin. Shown is ...
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Classification of protein; on the basis of structure, composition and function
I. Classification of protein on the basis of Structure and composition:
• This Classification of protein is based on shape or structure and composition. They are classified into three types; fibrous, globular and derived protein.
1. Fibrous protein:
They are elongated or fiber like protein.
Axial ratio (length: breadth ratio) is more than 10
They are static in nature with simple structure.
They have less biological functions
They are mostly present in animals
Fibrous proteins are further classified as- simple and conjugated
i. Simple fibrous protein:
Examples; Scleroprotein (Keratine, elastin, collagen, fibroin etc)
Scleroprotein or Albuminoids: they make animal skeleton and they are water insoluble.
ii. Conjugated fibrous proteins:
Examples; pigments present in chicken feather.
2. Globular protein:
They are spherical or globular in shape.
Axial ratio is always less than 10
They are dynamic in nature (can flow or move) with higher degree of complexity in structure.
They have variety of biological functions
Examples; enzymes, hormones etc
Globular protein is further classified on the basis of composition or solubility.
i. Simple or homo globular protein:
They are composed of amino acids only.
Some examples are;
a. Protamine:
They are positively charged (basic) proteins mostly present in animals and fishes (sperm)
Protamines binds with DNA in embryonic stage and later replaced by histone
It is soluble in water and ammonium hydroxide solution
It is not coagulated by heat
It precipitate out in aqueous solution of alcohol
Protamine are rich in arginine and lysine whereas devoid of sulfur containing and aromatic amino acids.
b. histone:
They are basic protein but weak base in comparison to protamine.
Histone is low molecular weight protein and are water soluble.
It is not coagulated by heat.
Histone is present in nucleic acids as nucleohistone binding with DNA.
c. Albumin:
It is the most abundant protein in nature
It is most commonly found in seeds in plants and in blood and muscles in animals.
Molecular weight of albumin is 65000 KD
It is water soluble and can be coagulated by heat
Plant albumins; Leucosine, Legumelins etc
Animal albumins; serum albumin, myosin, lactalbumin, ova-albumin etc
d. Globulin:
Pseudoglobulin (water soluble) and Euglobulin (water insoluble)
e. Glutelins:
Water insoluble. Eg. Glttenin (wheat), glutelin (corn), oryzenin (rice)
f. Prolamine:
They are storage protein found in seeds.
They are water insoluble. But soluble in dilute acid or detergents and 60-80% alcohol.
They are coagulated by heat
Prolamine is rich in proline and glutamine
Examples; Gliadin (wheat), zein (corn), Hordein (barley), Avenin (oats)
ii. Complex or conjugate or hetero globular protein:
These proteins in which protein are always linked by non-protein moiety to become functional. So, they are composed of both protein and non- protein components. The non-protein component is known as prosthetic group.
On the basis of prosthetic group, they are classified as follows;
a. Metalloprotein:
They have metal prosthetic group.
Some metals such as Hg, Ag, CU, Zn etc, strongly binds with proteins such as collagen, albumin, casein by –SH group of side chain of amino acids.
Eg. Ceruloplasmin; contains copper as prosthetic group
Some other metals such as Calcium weakly binds with protein. Eg. Calsequestrin, calmodulin
Some metals such as Na, K etc do not binds with protein but associate with nucleic acids protein.
b. Chromoprotein:
They have colored prosthetic group.
Some examples are;
Haemoprotein: Haemoglobin, myoglobin, chlorophyll, cytochrome, peroxidase, haemocyanin
Flavoprotein: Riboflavin (Vit B2) give yellow/orange color to FAD requiring enzymes
c. Glycoprotein/Mucoprotein:
They have carbohydrate as prosthetic group
Eg. Antibody, complement proteins, Heparin, Hyaluronic acid
d. Phosphoprotein:
They have phosphate group as prosthetic group.
Eg. Caesein (milk protein binds with calcium ion to form calcium salt of caseinate)
Ovovitellin; present in egg yolk
e. Lipoprotein:
They have lipid as prosthetic group.
Eg. Lipovitelline, chylomicrons
3. Derived protein:
These protein are the derivatives of either simple or complex protein resulting from the action of heat, enzymes and chemicals.
Some artificially produced protein are included in this group.
They are classified as primary derived protein and secondary derived protein.
i. Primary derived protein:
The derived protein in which the size of protein molecules are not altered materially but only the arrangement is changed.
Some examples are;
a. Proteans:
Obtained as a first product after the action of acid or enzymes or water on protein.
They are insoluble in water.
Eg. Edestan, myosin
b. Metaprotein:
They are produced by further action of acid or alkali on protein at 30-60°C.
They are water insoluble but soluble in dil acid or alkali.
Also known as Infraprotein.E.g. Curd
c. Coagulated protein:
They are produced by the action of heat or alcohol on protein.
They are insoluble in water.
Eg. Coagulated egg
ii. Secondary derived protein:
The derived protein in which size of original protein are altered.
Hydrolysis has occurred due to which size of protein molecule are smaller than original one.
Examples; a) Proteoses:
They are produced by the action of dilute acid or digestive enzymes when the hydrolysis proceeds beyond the level of metaprotein.
They are soluble in water
They are not coagulated by heat. • Eg. Albumose, Globulose etc.
II. Classification of protein on the basis of biological functions:
Catalytic protein:
They catalyze biochemical reaction in cells. Eg. Enzymes and co-enzymes
2. Structural protein;
They make various structural component of living beings.
Eg. Collagen make bone, Elastin make ligamnets and keratin make hair and nails
3. Nutrient protein:
They have nutritional value and provide nutrition when consumed.
Eg. Casein in milk
4. Regulatory protein:
They regulate metabolic and cellular activities in cell and tissue.
Eg. Hormones
5. Defense protein:
They provide defensive mechanism against pathogens.
Eg. Antibodies, complement proteins
6. Transport protein:
They transport nutrients and other molecules from one organ to other.
Eg. Haemoglobin
7. Storage protein:
They stores various molecules and ions in cells.
Eg. Ferritin store Iron
8. Contractile or mobile protein:
They help in movement and locomotion of various body parts.
Eg. Actin, myosin, tubulin etc
9. Toxic protein:
They are toxic and can damage tissues.
Eg. Snake venom, bacterial exotoxins.
Levels of Protein Organization
Levels of Protein Organization
A protein's primary structure is defined as the amino acid sequence of its polypeptide chain; secondary structure is the local spatial arrangement of a polypeptide's backbone (main chain) atoms; tertiary structure refers to the three-dimensional structure of an entire polypeptide chain; and quaternary structure is the three-dimensional arrangement of the subunits in a multisubunit protein. In this series of pages we examine the different levels of protein organization. We also view structures in lots of ways -- Cα backbone, ball-and-stick, CPK, ribbon, spacefilling -- as well color is used to highlight different aspects of the amino acids, structure, etc. As you traverse though this module please note these aspects.
Primary Structure (1˚)
The primary structure of a peptide or protein is the linear sequence of its amino acids (AAs). By convention, the primary structure of a protein is read and written from the amino-terminal (N) to the carboxyl-terminal (C) end. Each amino acid is connected to the next by a peptide bond.
Secondary Structure (2˚) -- Alpha Helices
While primary structure describes the sequence of amino acids forming a peptide chain, secondary structure refers to the local arrangement of the chain in space. Several common secondary structures have been identified in proteins. These will be described in the following sections and visualized using the KiNG software mentioned previously.
To load the KiNG Java Applet, just click here. Upon loading this page, the KiNG Java Applet should automatically spawn. If you need information on using King, please hover here.
The Alpha Helix
helix.gifAn alpha helix is an element of s...

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