Last modified 6th April '95 © Birkbeck College 1995
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A Rough Guide to Protein Types
This is meant to provide a quick and dirty overview, and is not
exhaustive, although it is hoped that it helps in appreciating the wide
variety of roles that various different forms of protein fulfill. You
will appreciate the links on this resource best if you have RasMol
installed, and configured for
Chemical MIME.
Firstly three classes that are a little separate from the bulk of
proteins :-
eg photosynthetic reaction centre, bacteriorhodopsin, 7-trans-membrane helix
G-protein-coupled receptors, nicotinic acetylcholine receptor, various proton
and ion pumps, voltage-gated ion channels, gap junction proteins, etc.
Intracellular
muscle proteins, actin, myosin, dystrophin, troponins
cellular cytoskeleton, F- and G-actin, dynein, microtubules,
ankyrin, vilin, etc
Extracellular
Extra-cellular Matrix, collagen,
fibronectin, laminin, vitronectin, fibrinogen, fibrin
titin, twitchin, adhesion proteins, glycoproteins
keratin, silks,
Coat proteins, integrases,
proteases, RNaseH/Reverse Transcriptase
INTRACELLULAR
- Includes cytosolic and nuclear
- Mostly all-alpha or alpha-beta (alternating)
- This is a mildly reducing environment
- Most cysteines are free thiols -SH
- Many are globular and soluble
- Often oligomeric
Enzymes
Hundreds or thousands of them -
catalysing all the biochemical pathways, mostly in the cytosol -
transferases, hydrolases, lyases, isomerases, etc.
Many are variations on the basic Rossman-fold (as in
lactate dehydrogenase)
Many use ATP
and/or other co-factors, such as metal ions, mono- or dinucleotides,
flavins, etc.
Often multimers, with allosteric cooperativity
Nucleic Acid Manipulation and Regulation Proteins
Polymerases,
nucleases,
ligases, gyrases, topo-isomerases,
DNA-binding proteins,
transcription factors,
zinc-fingers, repressors, histones, steroid
receptors
Response Elements
Elements of 2nd.messenger pathways - kinases, phosphatases,
G-proteins,
calmodulin,
lipases, adaptors (
SH2,
SH3, PH domains), etc.
Calcium ions and phosphate groups play a large role here.
Some of these are membrane associated.
Redox and Electron Transport Proteins
e.g.
dismutases,
cytochromes,
thioredoxin,
ferredoxin, etc.
Haem groups and metal ions often employed as co-factors
EXTRACELLULAR
- Includes interior of lysosomes and endoplasmic reticulum
- Mostly all-beta or mixed alpha + beta (BUT 4-helix bundles in cytokines)
- Many are globular and soluble
- The environment is oxidizing
- Most cysteines are in disulphide bridges
Hydrolases
Nucleases cut nucleic acids (eg
pancreatic RNase) and
Lipases cut and process lipid chains, and
Glycosidases cut sugar moieties, (eg
lysozyme, and
Proteinases `cut' other peptide chains - there are FOUR main classes