PROTEINS AND THEIR FUNCTIONS

 

What Are Proteins?

Proteins are very important molecules in cells. By weight, proteins are collectively the major component of the dry weight of cells. They can be used for a variety of functions from cellular support to cell signaling and cellular locomotion. While proteins have many diverse functions, all are typically constructed from one set of 20 amino acids. Examples of proteins include antibodies, enzymes, and some types of hormones (insulin).

Most amino acids have the following structural properties:
A carbon (the alpha carbon) bonded to four different groups:

• A hydrogen atom (H)
• A Carboxyl group (-COOH)
• An Amino group (-NH₂)
• A "variable" group

Of the 20 amino acids that typically make up proteins, the "variable" group determines the differences among the amino acids. All amino acids have the hydrogen atom, carboxyl group and amino group bonds.

Polypeptide Chains

Amino acids are joined together through dehydration synthesis to form a peptide bond. When a number of amino acids are linked together by peptide bonds, a polypeptide chain is formed. One or more polypeptide chains twisted into a 3-D shape forms a protein.

Protein Structure

There are two general classes of protein molecules: globular proteins and fibrous proteins. Globular proteins are generally compact, soluble, and spherical in shape. Fibrous proteins are typically elongated and insoluble. Globular and fibrous proteins may exhibit one or more of four types of protein structure. The four structure types are primary, secondary, tertiary, and qua ternary structure. A protein's structure determines its function. For instance, structural proteins such as collagen and keratin are fibrous and stringy. Globular proteins like hemoglobin, on the other hand, are folded and compact. Hemoglobin, found in red blood cells, is an iron-containing protein that binds oxygen molecules. Its compact structure is ideal for traveling through narrow blood vessels.

Protein Synthesis

 

Proteins are synthesized in the body through a process called translation. Translation occurs in the cytoplasm and involves the rendering of genetic codes that are assembled during DNA transcription into proteins. Cell structures called ribosomes help translate these genetic codes into polypeptide chains. The polypeptide chains undergo several modifications before becoming fully functioning proteins.

Organic Polymers

• Polymers
• Carbohydrates
• Lipids
• Nucleic Acids
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  Functions

• Antibodies - are specialized proteins involved in defending the body from antigens (foreign invaders). They can travel through the bloodstream and are utilized by the immune system to identify and defend against bacteria, viruses, and other foreign intruders. One way antibodies counteract antigens is by immobilizing them so that they can be destroyed by white blood cells. 
• • Contractile Proteins - are responsible for movement. Examples include actin and myosin. These proteins are involved in muscle contraction and movement. 
  • Enzymes - are proteins that facilitate biochemical reactions. They are often referred to as catalysts because they speed up chemical reactions. Examples include the enzymes lactase and pepsin. Lactase breaks down the sugar lactose found in milk. Pepsin is a digestive enzyme that works in the stomach to break down proteins in food.
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    • Hormonal Proteins - are messenger proteins which help to coordinate certain bodily activities. Examples include insulin, oxytocin, and somatotropin. Insulin regulates glucose metabolism by controlling the blood-sugar concentration. Oxytocin stimulates contractions in females during childbirth. Somatotropin is a growth hormone that stimulates protein production in muscle cells. 
    • Structural Proteins - are fibrous and stringy and provide support. Examples include keratin, collagen, and elastin. Keratins strengthen protective coverings such as skin, hair, quills, feathers, horns, and beaks. Collagens and elastin provide support for connective tissues such as tendons and ligaments. 
  • Storage Proteins - store amino acids. Examples include ovalbumin, casein, ferritin. Ovalbumin is found in egg whites and casein is a milk-based protein. Ferritin stores iron in hemoglobin. 
  • Transport Proteins - are carrier proteins which move molecules from one place to another around the body. Examples include hemoglobin and cytochromes. Hemoglobin transports oxygen through the blood via red blood cells. Cytochromes operate in the electron transport chain as electron carrier proteins.

  • Synthesis

     
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   Proteins are synthesized in the body through a process called translation. Translation occurs in the cytoplasm and involves the translation of genetic codes into proteins. These gene codes are assembled during DNA transcription, where DNA is transcribed into an RNA transcript. Cell structures called ribosomes help translate the gene codes in RNA into polypeptide chains that undergo several modifications before becoming fully functioning proteins.
• PROTEINS 
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