Types of proteins

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Proteins types are categorized as per their structure, chemical composition and biological function.

Proteins types in relation to the structure

Proteins are made up of long chains of amino acids the sequence of which is decided by gene expression. A simple polypeptide chain of amino acid is called monomeric chain and those with several polypeptide chains and bonds are called oligomeric types. With relation to the structure and number of amino acids present and the nature of bonding, they are broadly categorized into four classes. Primary structure:
This is a basic structure made up of amino acids chains and the sequence is defined by the gene expression.
Secondary structure:
In these proteins covalent bonding within and outside the chains is made and a structure, usually of helix shape, is formed.
Tertiary structure:
In these types of proteins the structure is much more complex.
These structures are three dimensional in nature with complex formations of folds and loops due to repulsion of like charged molecules and attraction of oppositely charged molecules.
Quaternary structure:
These types of proteins are very large complex molecules with many many polypeptide chains. There are two main classes of quaternary structure and they are scleroproteins and globular proteins.

Scleroproteins:

These are also called fibrous proteins. They form long rod or wire-like shaped filaments. these are not soluble in aqueous solutions.

They are usually inert and are storage or structural types of protein having role in supportive and protective function.

They are found in bone matrix, connective tissues, muscle fiber and tendons. Some of the examples of this class are collagen, elastin, keratin and fibroin (in silk).

Enzyme hexokinase ball and stick model (enlarge) Globularproteins:

These are also called spheroproteins having globe like structure and are soluble in aqueous solutions. Many types of enzymes, catalysts, hormones, transporters, signal transduction proteins, plasma, coagulation factors, growth factors and hemoproteins come under this category. Nucleosome, spliceosome, signal recognition particle and ribonucleoprotein are complexes with many components.

Types of proteins based on chemical composition

Depending upon the chemical composition, they are classified as simple or conjugated proteins.
Simple proteins:
Simple types contain only amino acids and do not contain any other chemical or chemical groups.
Conjugated proteins:
Many of these contain apart from amino acids other metal and chemical components and are named conjugated proteins.
The non amino acid parts are called prosthetic groups.
Some of the conjugated types and their prosthetic groups are given below.
Lipoproteins:

These conjugated protein types contain lipids in their biochemical assembly.
Lipid groups may be bound non-covalently or covalently.
Many protein like structural proteins, enzymes, transporters, antigens, toxins, adhesins, HDL (high density lipoproteins), LDL (low density lipoproteins), mitochondria and pathogen lipoprotein are lipoproteins.
Glycoproteins:

These conjugated proteins contain glycans (oligosaccharide chains) covalently attached to the side-chains of polypeptides.
These types are very important in integral membrane proteins.
There are many important glycoproteins like mucins (present in mucus), antibodies (immunoglobulins), glycoprotein llb/llla ( found on blood platelets and required for their aggregation), components of zona pellucida surrounding ovum, structural protein in the connective tissue, egg white, blood plasma, hormones (Follicle-stimulating hormone, Alpha-fetoprotein and Thyroid-stimulating hormone, Luteinizing hormone and Human chorionic gonadotropin and Erythropoietin).
Phosphoproteins:
These are chemically bonded to compounds containing phosphoric acid. Calcineurins are one of the examples.
Hemoproteins:
These conjugated types contain heme prosthetic group bound non covalently or covalently to the proteins.

Heme consists of iron atom contained in the center of a large heterocyclic organic ring called a porphyrin.

Many hemoproteins like hemoglobin, myoglobin, cytoglobin, hemocyanin, neuroglobin and leghemoglobin are required for oxygen transport.
Hemeproteins like peroxidases, ligninases and cytochrome c oxidase have the function of catalysis.
Hemeproteins, cytochrome c and catalase have a role in electron transfer whereas for active membrane transport cytochromes are needed.
Flavoproteins: 
These contain nucleic acid derivative of riboflavin like flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD).
These have many functions in DNA repair, photosynthesis, apoptosis and cellular respiration.
Metalloproteins:
These types have a metal ion as cofactor.
Many hemeproteins, enzymes (metalloenzymes), signal transduction factors come under these types.
Some of the metals in metalloproteins are iron (hemoglobin), zinc (carbonic anhydrase), cobalt (Vitamin B12 or cobalamin), copper (superoxide dismutase), magnesium (chlorophyll, Hexokinase, Glucose 6-phosphatase, DNA polymerase), calcium (calmodulin), vanadium (vanabins), nickel (Hydrogenase, Urease), manganese (arginase), selenium (Glutathione peroxidase) and molybdenum (nitrate reductase).
Opsins:
Opsins are conjugated types present in photoreceptor cells of the retina.
Opsins bind covalently to vitamin A based retinaldehyde chromophore and function the vision perception.

Other conjugated proteins like cytochrome have function in electron transport and phytochromes function in plants for photoreception regulating photoperiodism, circadian rhythms and germination of seeds.

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Different types of proteins.