Proteins are compounds made of large number of amino acids joined together by peptide linkages to form long polypeptide chains.
|Learning Objective: Demonstrate monomeric units of proteins, identify various protein structure, hemoglobin, and several major functions of proteins|
Amino acids are the building blocks of peptides and proteins. They act as intermediates in cellular metabolism.
This is what you will learn:
General Structure of Amino Acids
These are molecules containing amine group, carboxylic acid group and side chain alkyl group (R).
The alkyl groups vary between different amino acids (Figure 1).
Types of Amino Acids
There are 20 different types of amino acids. Biological activity of a protein is determined by the chemical properties of the amino acids forming it.
Amino acids are usually classified as acidic, basic, or neutral-based type of side chain (alkyl group, commonly denoted as ‘R’) present in their structures (Figure 2, 3 and 4 ).
Characteristics of Amino Acids
For every protein, there is a specific pH value at which it exhibits no net charge. It is called isoelectric point (pI).
Amino acids can exist as zwitterions in solids and in polar solutions such as, water, but not in gas phase. Zwitterions contain both amine and carboxylic acid groups being present in their structure (Figure 5).
Peptides are short polymers of amino acids linked together by peptide bond. They have the same chemical structure as protein, but they are shorter in length.
A peptide bond (-CO-NH-) is covalent bond formed between carboxyl group of one amino acid molecule and amine group of another amino acid molecule, thereby releasing a water molecule (Figure 6).
The end of a polypeptide chain where the —COOH group of the amino acid is not involved in peptide linkage is called as C-terminal end. Whereas, the other end of the polypeptide chain with amino acid having free —NH2 group is called as N-terminal end.
Proteins are class of organic compounds that forms structural and functional unit of the cell .
Examples are, skin, hair, callus, cartilage, muscles, tendons, ligaments, enzymes, hormones, antibodies, albumin, globulins, hemoglobin, myoglobin, and lipoproteins.
Primary Structure of a Protein
Primary structure of protein is a simple linear arrangement of amino acids in a protein (Figure 7).
The secondary structure of a segment of polypeptide chain is the local spatial arrangement of its main-chain atoms without regard to the conformation of its side chains or to its relationship with other segments (IUPAC-IUB, 1970).
Alpha helices (Figure 8), beta sheets (Figure 9), and turns are three common secondary structures of proteins.
Secondary Structure of a Protein
Rao and Rossmann (1973) coined the term supersecondary structures (present in parvalbumin, calmodulin, or troponin-C) (Figure 10).
They found structural components comprising of few alpha helices or beta-strands were frequently repeated within a structure.
These structures play a key role in protein-folding process. Examples of supersecondary structures are β hairpin, α-helix hairpins, and β-α-β motifs.
Tertiary Structure of a Protein
Tertiary structure refers to three-dimensional structure of a single protein molecule formed by hydrophobic interaction, salt bridges, hydrogen bonds and disulfide bonds.
Examples of tertiary proteins are lysozyme, and myoglobin.
Myoglobin(Figure 11) is a single-chain, iron-containing protein found in muscle fibers.
It is structurally similar to a single subunit of hemoglobin, although its affinity for oxygen is even higher than hemoglobin of the blood.
Quaternary Structure of a Protein
Quaternary structure of proteins refers to association of two or more polypeptide chains to form a complex. It can composed of two or more identical polypeptides, or it may include different polypeptides.
Quaternary structure is maintained by various forces like di-sulphide – linkages, H-bonds etc., between the different polypeptide chains of the protein.
Non-covalent interactions and disulfide bonds stabilize the quaternary structures.
For example, hemoglobin (Figure 12 ), oxygen carrying protein of blood, contains two α and two β subunits arranged with a quaternary structure in the form, α2β2.
Hemoglobin (Hb) is the globular protein contained in red blood cells that is responsible for delivery of oxygen to the tissues (Figure 12).
To ensure adequate tissue oxygenation, a sufficient hemoglobin level must be maintained.
The amount of hemoglobin in whole blood is expressed in grams per deciliter (g/dl). The normal Hb level for males is 14 to 18 g/dl; that for females is 12 to 16 g/dl.
The hematocrit (Figure 13), measures the volume of red blood cells compared to the total blood volume (red blood cells and plasma).
Things to Remember
- Proteins are compounds made of large number of amino acids joined together by peptide linkages to form long polypeptide chains.
- The building blocks of proteins monomers are amino acids.
- Each amino acid has a central carbon that is linked to an amino group, a carboxyl group, a hydrogen atom, and an R group or side chain.
- Protein structures are of four types: primary, secondary, tertiary, and quaternary.
- Hemoglobin (Hb) is the globular protein contained in red blood cells that is responsible for delivery of oxygen to the tissues
- The hematocrit, measures the volume of red blood cells compared to the total blood volume (red blood cells and plasma)