Download Polymers and Amino Acids

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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.
Amino acids contain both amine
(NH2) and carboxyl (COOH)
functional groups.
In alpha amino acids, these groups
are attached to the same carbon atom.
glycine (gly)
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.
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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:
H2NCHRCOOH + OH-  H2NCHRCOO- + H2O
When acting as a base, the NH2 group gains a H+ ion:
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.
Examples of fibrous proteins include collagen, keratin and
silk.

Globular proteins usually have a spherical shape caused
by tightly folded polypeptide chains with hydrophobic
groups on the inside, and hydrophilic groups on the outside.
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.
active site
The remainder of the amino acids maintain the precise
shape of the enzyme and the active site.
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Amino acids and proteins: summary
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