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Proteins and amino acids

Proteins are a diverse group of large and complex polymer molecules, made up of long chains of amino acids.

The R group, also attached to the same carbon atom, can vary. There are 22 amino acids that are used to make proteins (proteinogenic). The simplest is glycine, where R = H.

Amino acids contain both amine (NH2) and carboxyl (COOH) functional groups.

In alpha amino acids, these groupsare attached to the same carbon atom.

glycine (gly)

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Proteinogenic amino acids

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Zwitterions

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Acid–base properties of amino acids

The presence of a carboxyl group and an amine group mean that amino acids have both acidic and basic properties.

NH2 group acts as a base

COOH acid group acts as an acid

When acting as an acid, the COOH group loses a H+ ion:

When acting as a base, the NH2 group gains a H+ ion:

H2NCHRCOOH + OH- H2NCHRCOO- + H2O

H2NCHRCOOH + H+ H3N+CHRCOOH

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Effect of pH on amino acids

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Peptide formation

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Hydrolysis of peptide bonds

A peptide bond can be split by refluxing with hydrochloric acid. During hydrolysis, the water molecule adds across the peptide bond, forming a mixture of the two amino acids.

Peptide links can also be broken using a solution of alkali, such as aqueous sodium hydroxide at above 100°C.

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Protein structure

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Bonds in proteins

The 3D shape of a protein is maintained by several types of bond, including:

hydrogen bonds:involved in all levels of structure.

hydrophobic interactions:between non-polar sections of the protein.

disulfide bonds: one of the strongest and most important type of bond in proteins. Occur between two cysteine amino acids.

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Types of protein

There are two broad classes of protein: fibrous and globular.

Fibrous proteins are formed from parallel polypeptide chains held together by cross-links. These form long, rope-like fibres, with high tensile strength and are generally insoluble in water.

Globular proteins usually have a spherical shape causedby tightly folded polypeptide chains with hydrophobicgroups on the inside, and hydrophilic groups on the outside.

Examples of fibrous proteins include collagen, keratin and silk.

Examples of globular proteins include enzymes and some hormones.

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Structure of enzymes

All enzymes are globular proteins. They are soluble in water due to the presence of many hydrophilic side groups on their constituent amino acids.

Most enzymes are very large molecules but only a small part of them is involved in catalysis. This is called the active site and it may consist of just a few amino acids.

The remainder of the amino acids maintain the precise shape of the enzyme and the active site.

active site

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Amino acids and proteins: summary