BIOLOGY FORM 4

Chapter 4: Proteins

CONTENTS INTODUCTION PROTEIN STRUCTURES TYPES OF AMINO ACIDS SOURCES OF REFERENCE

INTRODUCTION

A protein is a linear polymer built from about 20 different amino acids. The type and the sequence of amino acids in a protein are specified by the DNA in the cell that produces them. This sequence of amino acids is essential since it determines the overall structure and function of a protein.

Protein Functions

Antibodies

• are specialized proteins involved in defending the body from antigens (foreign invaders).

Contractile Proteins

• are responsible for movement. Examples include actin and myosin.

Enzymes

• are proteins that facilitate biochemical reactions. They are often referred to as catalysts because they speed up chemical reactions.

Hormonal Proteins

• - are messenger proteins which help to coordinate certain bodily activities. Examples include insulin, oxytocin, and somatotropin.

Structural Proteins

• are fibrous and stringy and provide support. Examples include keratin, collagen, and elastin.

Storage Proteins

• - store amino acids. Examples include ovalbumin and casein. Ovalbumin is found in egg whites and casein is a milk-based protein.

Transport Proteins - are carrier proteins which move molecules from one

Transport Proteins

• are carrier proteins which move molecules from one place to another around the body. Examples include hemoglobin and cytochromes.

PROTEIN STRUCTURES Primary Structure of Proteins The primary structure of peptides and proteins refers to the linear

number and order of the amino acids present. The convention for the designation of the order of amino acids is that the N-terminal end (i.e. the end bearing the residue with the free α-amino group) is to the left (and the number 1 amino acid) and the C-terminal end (i.e. the end with the residue containing a free α-carboxyl group) is to the right.

Secondary Structure in Proteins The ordered array of amino acids in a protein confer regular

conformational forms upon that protein. These conformations constitute the secondary structures of a protein. In general proteins fold into two broad classes of structure termed, globular proteins or fibrous proteins. Globular proteins are compactly folded and coiled, whereas, fibrous proteins are more filamentous or elongated. It is the partial double-bond character of the peptide bond that defines the conformations a polypeptide chain may assume. Within a single protein different regions of the polypeptide chain may assume different conformations determined by the primary sequence of the amino acids.

Tertiary Structure of Proteins Tertiary structure refers to the complete three-dimensional

structure of the polypeptide units of a given protein. Included in this description is the spatial relationship of different secondary structures to one another within a polypeptide chain and how these secondary structures themselves fold into the three-dimensional form of the protein. Secondary structures of proteins often constitute distinct domains. Therefore, tertiary structure also describes the relationship of different domains to one another within a protein. The interactions of different domains is governed by several forces: These include hydrogen bonding, hydrophobic interactions, electrostatic interactions and van der Waals forces.

Quaternary Structure Many proteins contain 2 or more different polypeptide chains that are

held in association by the same non-covalent forces that stabilize the tertiary structures of proteins. Proteins with multiple polypetide chains are oligomeric proteins. The structure formed by monomer-monomer interaction in an oligomeric protein is known as quaternary structure.

Oligomeric proteins can be composed of multiple identical polypeptide chains or multiple distinct polypeptide chains. Proteins with identical subunits are termed homo-oligomers. Proteins containing several distinct polypeptide chains are termed hetero-oligomers.

Hemoglobin, the oxygen carrying protein of the blood, contains two α and two β subunits arranged with a quaternary structure in the form, α2β2. Hemoglobin is, therefore, a hetero-oligomeric protein.

TYPES OF AMINO ACIDS

Amino acids is an organic compound containing an amino group (NH2), a carboxylic acid group (COOH), and any of various side groups, especially any of the 20 compounds that have the basic formula NH2CHRCOOH, and that link together by peptide bonds to form proteins or that function as chemical messengers and as intermediates in metabolism. There are twenty kinds of amino acids that support the body, each having their own functions.

20 Types Of Amino Acids

SOURCES OF REFERENCE http://biology.about.com/od/molecularbiology/a/aa101904a.htm

http://www.biology-online.org/dictionary/Protein

http://themedicalbiochemistrypage.org/protein-structure.html

http://www.hornetjuice.com/amino-acids-types.html

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