Introduction to Proteins and Amino Acids, Lecture notes of Cell Biology

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Introduction to Proteins

and Amino Acids

Introduction

• Twenty percent of the human body is made up of proteins. Proteins are the large, complex molecules that are critical for normal functioning of cells.
• They are essential for the structure, function, and regulation of the body’s tissues and organs.
• Proteins are made up of smaller units called amino acids, which are building blocks of proteins. They are attached to one another by peptide bonds forming a long chain of proteins.

Classification of amino acids

• There are 20 amino acids. Based on the nature of

their ‘R’ group, they are classified based on their polarity as:

Classification based on essentiality: Essential amino acids are the amino acids which you need through your diet because your body cannot make them. Whereas non essential amino acids are the amino acids which are not an essential part of your diet because they can be synthesized by your body.

Essential Histidine Isoleucine Leucine Methionine Phenyl alanine Threonine Tryptophan Valine

Non essential Alanine Arginine Aspargine Aspartate Cystine Glutamic acid Glycine Ornithine Proline Serine Tyrosine

Structure of proteins

• The sequence of a protein is determined by the DNA of the gene that encodes the protein (or that encodes a portion of the protein, for multi- subunit proteins).
• A change in the gene's DNA sequence may lead to a change in the amino acid sequence of the protein. Even changing just one amino acid in a protein’s sequence can affect the protein’s overall structure and function.
• To understand how a protein gets its final shape or conformation, we need to understand the four levels of protein structure: primary, secondary, tertiary, and quaternary

Primary Structure

• The simplest level of protein structure, primary structure is simply the sequence of amino acids in a polypeptide chain.
• The hormone insulin has two polypeptide chains A, and B. The sequence of the A chain, and the sequence of the B chain can be considered as an example for primary structure.

Tertiary structure

• The overall three-dimensional structure of a polypeptide is called its tertiary structure. The tertiary structure is primarily due to interactions between the R groups of the amino acids that make up the protein.
• Important to tertiary structure are hydrophobic interactions , in which amino acids with nonpolar, hydrophobic R groups cluster together on the inside of the protein, leaving hydrophilic amino acids on the outside to interact with surrounding water molecules.
• Also, Disulfide bonds , covalent linkages between the sulfur- containing side chains of cysteines, are much stronger than the other types of bonds that contribute to tertiary structure

Quaternary structure

• When multiple polypeptide chain subunits come

together, then the protein attains its quaternary structure.

• An example for quaternary structure is

hemoglobin. The hemoglobin carries oxygen in the blood and is made up of four subunits, two each of the α and β types.

References

• https://www.khanacademy.org/science/biology/macro molecules/proteins-and-amino-acids/a/orders-of- protein-structure
• https://www.khanacademy.org/test- prep/mcat/biomolecules/amino-acids-and- proteins1/a/chemistry-of-amino-acids-and-protein- structure
• https://www.chem.wisc.edu/deptfiles/genchem/netori al/modules/biomolecules/modules/protein1/prot15.ht m
• http://cbc.arizona.edu/classes/bioc460/spring/460we b/lectures/LEC3_AminoAcids_08-ppt

Image references:

Peptide bond: https://www.chem.wisc.edu/deptfiles/genchem/netorial/modu les/biomolecules/modules/protein1/prot15.htm

Primary and tertiary structures: https://www.khanacademy.org/science/biology/macromolecul es/proteins-and-amino-acids/a/orders-of-protein-structure

Secondary structure: https://kimberlybiochemist.wordpress.com/category/amino- acids-and-proteins/

Quaternary structure: http://swift.cmbi.ru.nl/gv/students/mtom/QUA_1.html