Biology - Globular and Fibrous Proteins

These are complete notes copied off Wikipedia and other sites, completed for homework, on globular and fibrous proteins, and haemoglobin and collagen. Diagrams included. It is based on the OCR specification for AS Biology. Happy revising! =P

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Preview of Biology - Globular and Fibrous Proteins

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Reianna Shakil L6EFBiology4/12/12
Proteins
Globular proteins
Molecule folded into spherical (globular) shapes, forms a coiled shape (globule).
Have complex tertiary and sometimes quaternary structures.
Hydrophobic groups point into centre of molecule away from water.
Only hydrophilic groups are exposed outside the molecule so globular proteins are soluble.
Globular proteins have roles in metabolic reactions:
Enzymes - catalyse metabolic reactions.
Haemoglobin - binds to oxygen to transport it around body.
Haemoglobin (Hb)
Haemoglobin has a quaternary structure made up of 4 separate polypeptide chains:
2 identical -chains and 2 identical -chains.
Each polypeptide chain is folded/coiled into a compact
shape due to hydrophobic interactions between the
(hydrophobic) R-groups.
Nearly spherical - hydrophobic side chains point inwards.
All 4 polypeptide chains are linked to form a roughly
spherical Hb molecule.
The Hydrophilic R-groups are arranged around the outside
of the molecule which allows Hb to mix with the watery
medium inside red blood cells.
Outward pointing hydrophilic side chains maintain solubility.
Attached to each polypeptide chain is a prosthetic HAEM group with a Fe2+ ion.
Haem group is a prosthetic group (i.e. an important permanent part of a protein molecule
which is not made from amino acids) - when combined with 4 polypeptide chains it forms a
conjugated protein.
Haem group has an iron ion (Fe2+) at its centre, and each Fe2+ ion can combine with one
O2 molecule.
The iron combines with oxygen at high oxygen concentrations and releases oxygen at low
oxygen concentrations.
Human Hb has 4 polypeptide chains with 1 haem group each and can therefore carry 4 x O2
molecules.
When Hb is bound to O2, it is called oxyhaemoglobin and the colour changes from purplish
red to bright red.
The precise 3D-shape of globular proteins is critical to their oxygen-carrying function ­ slight
changes can have radical effects ­ e.g. in sickle cell anaemia, one amino acid change causes a
shape change in the molecule; that in turns reduces the ability of Hb to bind to O2, and
changes the shape of the whole red blood cell from a biconcave disk to a sickle shape.
Severe sickle cell anaemia can be fatal.

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Reianna Shakil L6EFBiology4/12/12
Fibrous proteins
Fibrous proteins are long filamentous protein molecules only found in animals.
Little or no tertiary structure.
Cross linkages at intervals forming long fibres or sheets.
Fibrous proteins form 'rod' or 'wire' -like shapes and are usually inert structural or storage
proteins.
They are generally water-insoluble.…read more

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Reianna Shakil L6EFBiology4/12/12
The ends of parallel molecules are staggered, preventing weak areas from running across
the collagen fibre.…read more

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