Grade 11 Protein Structure

Here is what you need to know about this topic:

Amino acids are linked together by condensation to form polypeptides
There are 20 different amino acids in polypeptides synthesised on ribosomes
Amino acids can be linked together in any sequence giving a huge range of possible polypeptides
The amino acid sequence of polypeptides is coded for by genes
A protein may consist of a single polypeptide or more than one polypeptide linked together
The amino acid sequence determines the three-dimensional conformation of a protein
Living organisms synthesise many different proteins with a wide range of functions

Amino Acids

Proteins are comprised of long chains of recurring monomers called amino acids

Amino acids all share a common basic structure, with a central carbon atom bound to:

An amine group (NH₂)
A carboxylic acid group (COOH)
A hydrogen atom (H)
A variable side chain (R)

Amino acids are joined together on the ribosome to form long chains called polypeptides, which make up proteins

Each type of amino acid differs in the composition of the variable side chain

These side chains will have distinct chemical properties (e.g. charged, non-polar, etc.) and hence cause the protein to fold and function differently according to its specific position within the polypeptide chain

Dipeptide Bonds are the bonds between two amino acids to create a dipeptide and eventually a longer chain of polypeptides

Amino acids can be covalently joined together in a condensation reaction to form a dipeptide and water

The covalent bond between the amino acids is called a peptide bond and, for this reason, long chains of covalently bonded amino acids are called polypeptides

Polypeptide chains can be broken down via hydrolysis reactions, which requires water to reverse the process

Peptide bonds are formed between the amine and carboxylic acid groups of adjacent amino acids

The amine group loses a hydrogen atom (H) and the carboxylic acid loses a hydroxyl (OH) – this forms water (H₂O)

Protein Structure

Amino acid sequences will commonly fold into two stable configurations, called secondary structures

Alpha helices occur when the amino acid sequence folds into a coil / spiral arrangement
Beta-pleated sheets occur when the amino acid sequence adopts a directionally-oriented staggered strand conformation

Amino acids are covalently joined via peptide bonds to form long chains called polypeptides

The order of the amino acid sequence is called the primary structure and determines the way the chain will fold

Different amino acid sequences will fold into different configurations due to the chemical properties of the variable side chains

Both α-helices and β-pleated sheets result from hydrogen bonds forming between non-adjacent amine and carboxyl groups

Where no secondary structure exists, the polypeptide chain will form a random coil

The overall three-dimensional configuration of the protein is referred to as the tertiary structure of the protein

The tertiary structure of a polypeptide chain will be determined by the interactions between the variable side chains

These interactions may include hydrogen bonds, disulphide bridges, ionic interactions, polar associations, etc.

The affinity or repulsion of side chains will affect the overall shape of the polypeptide chain and are determined by the position of specific amino acids within a sequence

The order of the amino acid sequence (primary structure) determines all subsequent levels of protein folding

Quaternary structures are found in proteins that consist of more than one polypeptide chain linked together

Alternatively, proteins may have a quaternary structure if they include inorganic prosthetic groups as part of their structure

An example of a protein with a quaternary structure is haemoglobin (O₂ carrying molecule in red blood cells)

Haemoglobin is composed of four polypeptide chains (two alpha chains and two beta chains)
It is also composed of iron-containing haeme groups (prosthetic groups responsible for binding oxygen)

Protein Functions

Fibrous vs. Globular

The following are specific examples of the different functions of proteins:

Structure

Collagen: A component of the connective tissue of animals (most abundant protein in mammals)
Spider silk: A fiber spun by spiders and used to make webs (by weight, is stronger than kevlar and steel)

Hormones

Insulin: Protein produced by the pancreas and triggers a reduction in blood glucose levels
Glucagon: Protein produced by the pancreas that triggers an increase in blood glucose levels

Immunity

Immunoglobulins: Antibodies produced by plasma cells that are capable of targeting specific antigens

Transport

Haemoglobin: A protein found in red blood cells that is responsible for the transport of oxygen
Cytochrome: A group of proteins located in the mitochondria and involved in the electron transport chain

Sensation

Rhodopsin: A pigment in the photoreceptor cells of the retina that is responsible for the detection of light

Movement

Actin: Thin filaments involved in the contraction of muscle fibres
Myosin: Thick filaments involved in the contraction of muscle fibres

Enzymes

Rubisco: An enzyme involved in the light independent stage of photosynthesis

Fibrous proteins are generally composed of long and narrow strands and have a structural role (they are something)
Globular proteins generally have a more compact and rounded shape and have functional roles (they do something)

Paper model of Insulin

https://pdb101.rcsb.org/learn/paper-models/insulin

Use this link to download the pdf and you can try it yourself

Haemoglobin vs. Collagen

Structure of Haemoglobin and how it functions to transport gasses.

Structure of collagen, focusing on the types of bonds

What is collagen?

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