Learning outcomes Describe the structure of an amino acid. Describe the formation and breakage of peptide bonds in the synthesis and hydrolysis of dipeptides.

Learning outcomes• Describe the structure of an amino acid.

• Describe the formation and breakage of peptide bonds in the synthesis and hydrolysis of dipeptides and polypeptides.

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Proteins

Basic amino acid structure - glycine

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Amino acid structure showing R-groups (side chains)

Proteins

Condensation and hydrolysis happens in the same way as in polysaccharide formation

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Proteins

Primary structure of insulin

In insulin there are 2 polypeptide chains, each with its own primary structure

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Explain the term primary structure.

The primary structure is just the sequence of amino acids in the polypeptide chain

Proteins

There are a huge number of possible combinations of amino acids that can make up proteins.

A four-amino-acid structure like this has 160,000 (20x20x20x20) possible forms, because each of the amino acids in the chain could be any one of the 20 that there are available.

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Learning Outcomes• Explain the term secondary structure with

reference to hydrogen bonding.

• Explain the term tertiary structure with reference to hydrophobic and hydrophilic interactions, disulphide bonds and ionic interactions.

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Proteins

The polypeptide chains are some times held in a spiral shape by hydrogen bonds between the amino and carboxylic acid groups. This is called an “Alpha Helix”

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Proteins

ProteinsThis ribbon diagram shows a number of alpha helices

linked together by less structured areas of the polypeptide chain to form the final protein structure

The secondary structures (alpha helices and beta pleated sheets) are held in place by different kinds of bonds , this is the tertiary (third) level of structure

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Proteins

ProteinsThere are a number of bonds holding this structure together at the tertiary level.

Proteins• The tertiary structure of proteins can be

DENATURED by heating.

• The heat increases the kinetic energy of the molecule and makes parts of it vibrate faster.

• This means that the bonds (not co-valent) that hold the protein in its globular shape are broken and its complex shape will unravel.

• This is particularly important in enzymes as it is the tertiary structure that defines the active site.

Learning Outcomes• Explain the term quaternary structure with

reference to the structure of haemoglobin.

• Describe the structure of a collagen molecule.

• Compare the structure and function of haemoglobin and collagen.

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Proteins

Haemoglobin• Haemoglobin is a globular

protein that has a quaternary structure

• This means it is made up of more than one polypeptide chain (it does not mean that all quaternary proteins have 4 polypeptide components!)

• The polypeptide chains are held together by similar bonds as the tertiary structure

• The haem area is called a “prosthetic” group. A molecule of oxygen can bind to each haem group

Proteins

Collagen molecules are made of repeating sequences of amino acids that form spiral structures. (Far left picture) Collagen protein is made of 3 polypeptide chains wound together rather like a rope. Hydrogen bonds hold the chains together(second picture from left)Each group of 3 chains is bonded to another group by co-valent bonds called cross links, thus fibrils are formedMany fibrils form a collagen fibreCollagen is a STRUCTURAL protein rather than a globular or metabolic protein