Proteins, The Structure and Function, An Introduction

Hi everyone, this is a shorter post about a glance at proteins – most of the information came from my notes and Timberlake’s book which is referenced down the bottom.

Structure and Function Of Proteins

Proteins are made up for 20 different amino acids, all arranged in different sequences. This determines the characteristics and functions. Proteins function as Enzymes, Hormones and are important for structure, transport, protection, storage and muscle contraction.

Types of Proteins

Structural Level Characteristics
Primary Peptide bonds join amino acids in a specific sequence in a polypeptide
Secondary The alpha helix, and beta pleated sheet, or triple helix forms by hydrogen bonding between the atoms in the peptide bond along the chain
Tertiary A polypeptide folds into a 3D shape stabilized by interactions between R groups to form a biologically active protein
Quaternary 2 or more protein subunits combine to form a biologically active protein

This table is from Pearson Education in the reference.

Formation of a Peptide Bond

As we discussed in Function and Structure of Amino Acids, a peptide bond is an Amide Bond between the carboxylic group of one amino acid and the next amino acid group

Primary, Secondary, Tertiary and Quaternary Structures of Proteins

The Primary Structure of a Protein is:

  • the particular sequence of amino acids
  • the backbone of a peptide chain or protein

The nonapeptides (9 amino acids) oxytocin (induces labour) and vasopressin (antidiuretic hormone):

  • Have similar primary structures
  • Differ only in the amino acids at positions 3 and 8

Insulin:

  • First protein to have its structure determined
  • Has a primary structure of two polypeptide chains linked by disulphide bonds.

Has a chain A with 21 amino acids and a chain B with 30 amino acids

The Secondary Structure

The secondary structure of a protein indicates the 3D spatial arrangements of the polypeptide.

An Alpha Helix:

  • has a coiled shape held in place by the hydrogen bonds between the amide groups and the carbonyl groups of the amino acids along the chain.
  • The hydrogen bonds between the H of a –N-H group and the O of C=O of the fourth amino acid down the chain.

A Beta Pleated Sheet:

  • Consists of polypeptide chains arranged side by side
  • Has hydrogen bonds between chains
  • Has R groups above and bellow the sheet
  • Is a typical of fibrous proteins such as silk

A Triple Helix:

  • Consists  of three alpha helix chains woven together
  • Contains large amounts of glycine, proline, hydroxy proline, and hydroxyzine that contain –OH groups for hydrogen bonding
  • Is found in collagen, connective tissue, skin, tendons and cartilage

Tertiary Structure

The interactions of the R side group give a protein it’s specific 3D tertiary structure. The side groups only interact in Tertiary Structures.

Globular Proteins:

  • Have compact, spherical shapes (Globe – globular like the world)
  • Carry out Synthesis, Transport and metabolism in the cells
  • Such as myoglobin store and transport oxygen in muscle

Fibrous Proteins:

  • Consists of long, fiber-like shapes
  • Such as alpha keratins make up hair, wool, nails and skin

Quaternary Structure

This is the combination of two or more tertiary units. It is stabilized by the same interactions found in tertiary structures. The Quaternary Structure of Hemoglobin consists of 2 alpha chains and 2 Beta chains. The heme group in each subunit picks up oxygen (Oxygen binds at Heme group) for transport in the blood to the tissues. This occurs in Proteins with more than one chain.

 

Duration of Hydrolysis of Proteins

Protein Hydrolysis is essentially  the chemical reaction where you break the peptide bond – but it needs enzymes. It “splits the peptide bond to give smaller peptides and amino acids” (Timberlake ). Hydrolysis happens in digestion of proteins (by digestive enzymes). Hydrolysis happens when amino acids are needed to synthesise new proteins or repair tissues.

Types of Agents that Denature Proteins

Denaturing is when a disruption of bonds in secondary, tertiary and quaternary protein structure occurs…. A bit like “death”

  • Heat– like when you cook eggs – the egg white solidifies and you cant reverse > this is what happens when organic compounds break apart hydrogen bonds and disrupt hydrophobic interactions.
  • Acids and Bases –like when you poach an egg in vinegar. Egg + Vinegar = Acid
  • Reducing agents – like a hair perm
  • Detergent – disrupt hydrophobic interaction causing protein chains to unfold.

Heat –this is why kids cant/shouldn’t have a temperature higher than 39degrees Celsius or death will follow as vital enzymes denature. Adults cant/shouldn’t go past 40 degrees Celsius or death will follow.

Reference:

Timberlake, KC 2007, General, Organic and Biological Chemistry, 4th Edition, Pearson, USA

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