What are Conjugated Molecules in Biology? Glycoprotein, Glycolipid, Lipoproteins and Nucleoproteins

Conjugated molecules

Conjugate means to join together, so conjugated molecules are molecules that are created when two distinct molecules from different categories are combined. 

What are conjugated molecules in biology

What are conjugated molecules in biology?

In biology, some conjugated molecules are very important for the proper functioning of the cells and any change or deficiency may lead to the dysfunctionality of the cell and eventually may lead to death.

Types of conjugated molecules in biology

Some important conjugated molecules in biology are:

  1. Glycoprotein
  2. Glycolipid
  3. Lipoproteins
  4. Nucleoproteins

Video Lesson


1. Glycoprotein

Glycoproteins are conjugated molecules formed when glycan (oligosaccharide chain) combine with protein covalently by the process of glycosylation during translation or posttranslational modification of the protein. They can be found in cells, tissues, and plasma. The most common sugars involved in the glycosylation process in the mammalian cell include glucose, galactose, mannose, xylose, and fucose.

The glycan attaches to the side chains of polypeptides in protein molecules. The presence of the OH group of carbohydrates makes the glycoproteins more hydrophilic compared to simple proteins that are more attracted to water. This hydrophilic property of glycoprotein helps the protein's tertiary structure to fold in a specific way.

Classification of glycoproteins

Glycoproteins are classified based on the attachment site of the carbohydrate to an amino acid in the protein. The two most common glycosylation types are:

1. O-Linked glycosylation

Sugars bind to oxygen (O) of the carboxyl group of threonine or serine amino acid to generate O-linked glycoproteins and this process is known as O-linked glycosylation or O-glycosylation. This type of glycosylation takes place in the Golgi complex.

2. N-Linked glycosylation

Sugars attach to nitrogen (N) of the amino group (-NH2) of asparagine amino acid to produce N-linked glycoprotein. This process is known as N-Linked glycosylation or N-glycosylation. The endoplasmic reticulum provides sugar to N-linked glycoproteins, which are then transported to the Golgi complex for processing. The N-linked sugar chains have more structural rules than O-linked sugar chains, according to an analysis of structural data from several glycoproteins.

Functions of glycoproteins

Glycoproteins perform many vital roles in many biological processes

  1. Glycoproteins are present on the surface of the lipid bilayer of the cell membrane. They function in the aqueous environment because of their hydrophilic nature and help in cell-to-cell recognition and molecule binding.
  2. Glycoproteins present on cell surfaces provide stability and strength to tissues by cross-linking cells and protein (for example collagen).
  3. They also aid in the communication of organ systems. Glycoproteins present in the grey matter of the brain work together with synaptosomes and axons.
  4. Blood clotting proteins including prothrombin, thrombin, and fibrinogen are glycoproteins in nature.
  5. Glycoproteins help plant cells to resist gravity and stand upright.
  6. Some hormones such as erythropoietin (EPO) and human chorionic gonadotropin (HCG) are glycoproteins in nature. Erythropoietin (EPO) is produced in the kidney and involved in red blood cell production stimulation. Human chorionic gonadotropin (HCG), on the other hand, is involved in embryogenesis and implantation of oocytes in the uterus upon fertilization.
  7. Immunoglobulins or antibodies are also glycoproteins and they play important role in the immune system of the body.

2. Glycolipid

Glycolipids are formed when glycan (oligosaccharide chain) combine with lipid through glycosidic bonding. In general, they include a wide variety of structurally heterogeneous biological compounds synthesized by microbes, plants, animals, and humans. They can be found in large quantities in the brain, tissue, and nerve cells.

The sugar in glycolipids alcohol sugars (e.g., erythritol, mannitol, or arabinol), sugar acids (e.g., glucuronic acids), mono, oligo, or polysaccharides (e.g. glucose, glycan, or cellobiose), and amino sugars (e.g., desosamine). While, the lipid part of glycolipid may contain fatty acids, fatty amino alcohols, fatty alcohols, sterols, polyketides, carotenoids, and hopanoids with various chain lengths, substitutions, branching, saturation levels, and polymerizations.

Classification of glycolipids

The glycolipids are classified into:

1. Simple glycolipid

In simple glycolipids, glycan and lipid are directly linked with each other. It is also known as saccharolipids. Examples include rhamnolipids, sophorolipids, Glucolipids, Glyco-carotenoids, and cellobiose lipids

2. Complex glycolipid

Complex glycolipids are more heterogeneous structurally and contain other residues such as peptide, glycerol, and acylated-sphingosine in addition to the glycan and lipid.

Functions of glycolipids

The main functions of glycolipids are:

  1. It is an important component of the cell membrane. They are present on the extracellular surface of the membrane and help in the maintenance of cell stability and facilitate in cell to cell interactions.
  2. In viruses, glycolipids help in the recognition of host cells.
  3. They are involved in immune responses and protect the body from pathogens.

3. Lipoproteins

Lipoproteins are conjugated molecules formed when lipid and protein are joined together by covalent bonding. They are more complex than glycolipids, generating enormous particles containing numerous lipid and protein types.

The central core of lipoprotein is made up of cholesterol ester and triacylglycerol molecules. An outer layer of phospholipids and cholesterol molecules protects them. Their hydrophobic regions face inward, toward the lipid core. Their outward hydrophilic charged regions are oriented toward the aquatic environment. Wrapped around the outer shell of the lipoprotein particle, which is also engaged in interactions with external water, are specialized apoproteins.

Classification of lipoprotein

Lipoproteins are generally classified based on their density. The main classifications are:

1. Chylomicrons

The main components of chylomicrons are dietary triacylglycerols (90%) and cholesterol. They are also known as ultra low-density lipoproteins (ULDL). They absorb lipids from the intestine and deliver them to cardiac, adipose, and skeletal muscle tissue.

2. Very-low-density lipoprotein (VLDL)

The primary components of very-low-density lipoprotein are endogenous triacylglycerols and cholesterol. The endogenous triglycerides are synthesized in the liver and taken to adipose tissues with the help of very-low-density lipoprotein.

3. Low-density lipoproteins (LDL)

Their primary component is cholesterol and transports it to the cells all around the body. LDL causes atherosclerosis in which the walls of arteries develop lesions due to oxidation of LDL.

4. High-density lipoproteins (HDL)

They are made up of Phospholipid and multiple proteins. Proteins help in the transport of fat around the body through the aqueous environment of the blood.

Functions of lipoprotein

The primary functions of lipoproteins include:

  1. The main function of lipoproteins is to transport and deliver triacylglycerol, fatty acids, and cholesterol to the cells all around the body.
  2. They are the basic framework of all types of the membrane in a cell.
  3. They allow fats and cholesterol to travel through the water-based environment of blood.
  4. Plasma lipoproteins reduce the toxicity of toxins produced by bacteria.
  5. Many enzymes, antigens, structural proteins, toxins, and adhesions are also lipoproteins in nature.

4. Nucleoproteins

A nucleoprotein is a conjugated protein made up of a protein and a nucleic acid. Nucleic acid can either be DNA (deoxyribonucleic acid) or RNA (ribonucleic acid). The most common protein attached with nucleic acid is histone and protamine. The nucleoproteins can be located in chromosomes.

Classification of nucleoproteins

Typically they can be classified into two types:

1. Deoxyribonucleoproteins

This type of nucleoprotein is formed when protein combines with DNA. The typical example of Deoxyribonucleoproteins is nucleosomes.

The nucleosome is formed with a cluster of eight histone proteins are wrapped around genomic DNA in a compact structure. This complex helps in the packing of the long DNA molecule in a small nucleus structure of a cell. Protamine protein replaces histones during spermatogenesis.

2. Ribonucleoproteins

The attachment of RNA binding protein with RNA makes ribonucleoproteins. The most common example is the ribosome.

Ribosomes are the site of protein synthesis in the cell. They can be found in the cytoplasm freely or attached to the rough endoplasmic reticulum.

Functions of nucleoproteins

The main functions of nucleoproteins are:

  1. They help in the regulation of replication and transcription of DNA.
  2. They are involved in the repairing of damaged DNA.
  3. Nucleoproteins are also play important role in the translation and regulation of genes.
  4. They are normally the main antigens for viruses.

Summary of four conjugated molecules in biology

Characteristics

Glycoprotein

 

Glycolipid

 

Lipoproteins

 

Nucleoproteins

 

Components

Glycan and protein

Glycan and lipid

Lipid and protein

Nucleic acid and protein

 

Types

O-Linked Glycoprotein and

N-Linked Glycoprotein

 

Simple glycolipid and complex glycolipid

 

Chylomicrons, very-low-density lipoprotein, low-density lipoproteins, and high-density lipoproteins

 

Deoxyribonucleop-roteins and ribonucleoproteins

 

Functions

Cellular secretion

Cell-surface receptors

Cell-adhesion molecules

Immunoglobulins

Cell membrane component

 

Cell membrane component

Maintain cell stability

Facilitate cell to cell interactions

Adhesion to neighboring cells

 

The basic framework of all types of membrane in a cell.

Many enzymes

Transporters

Structural proteins

Antigens

Adhesins

Toxins

Structurally and functionally important.

Regulation of gene expression

 


Some Questions and Answers

1. What are conjugated molecules?

A. Conjugated molecules are molecules that are created when two distinct molecules from different categories are combined.

2. Give examples of conjugated molecules found in living organisms.

A. Four important conjugated molecules found in living organisms are glycoprotein, glycolipid, lipoproteins, and nucleoproteins.

3. Which conjugate molecules are involved in blood clotting?

A. Prothrombin, thrombin, and fibrinogen belonging to glycoproteins are involved in blood clotting.

4. What is the common example of nucleoprotein?

A. Nucleosomes and ribosomes are common examples of nucleoprotein.

5. Which conjugate molecules are part of the cell membrane?

A. Glycolipids, glycoproteins, and lipoproteins are part of the cell membrane.





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