Excitatory synapse

expansins are L-aspartate, NMDA and ibotenate; the agonists of the Q-type receptor are L-glutamate and quisqualate; agonists of the K-type are L-glutamate and kainate. All three receptor types are found widely in the CNS, and particularly the telencephalon. N- and Q-type receptors tend to occur together and may interact; their distribution is complementary to the K-type receptors. The ion fluxes through the Q and K receptors are relatively brief, whereas the flux through the N-type is longer, and carries a significant amount of calcium. Additionally, the N-type receptor is blockaded by magnesium near the resting potential, and thus shows voltage-gated ion channel properties, leading to a regenerative response; this is why N-type receptors have been linked to long-term potentiation. Invertebrate glutamate receptors may not have the same properties as those described above.

excitatory synapse A synapse (either chemical or electrical) in which an action potential in the presynaptic cell increases the probability of an action potential occurring in the postsynaptic cell. See inhibitory synapse.

excitotoxin Class of substances that damage neurons through paroxysmal overactivity. The best-known excito-toxins are the excitatory amino acids, which can produce lesions in the CNS similar to those of Huntington's chorea or Alzheimer's disease. Excitotoxicity is thought to contribute to neuronal cell death associated with stroke.

exendin Group of peptide hormones (39 residues), related to the glucagon family, found in the saliva of Gila monsters (Heloderma suspectum, H. horridum). Helospectin is exendin-1; helodermin is exendin-2.

exergonic A biochemical reaction in which there is a the release of energy - a negative change in free energy that can be used to produce work. Such reactions will proceed spontaneously.

exfoliatin Epidermolytic toxin produced by some strains of Staphylococcus aureus; causes detachment of outer layer of skin by disrupting desmosomes of the stratum granulosum.

exine External part of pollen wall that is often elaborately sculptured in a fashion characteristic of the plant species. Contains sporopollenin. The term is also used for the outer part of a spore wall.

exobiology The study of putative living systems that, statistically, are likely to exist elsewhere in the universe.

exocrine Exocrine glands release their secreted products into ducts that open onto epithelial surfaces. See endocrine.

exocytosis Release of material from the cell by fusion of a membrane-bounded vesicle with the plasma membrane.

exocytotic vesicle Vesicle, for example a secretory vesicle or zymogen granule, that can fuse with the plasma membrane to release its contents.

exoenzyme 1. An enzyme attached to the outer surface of a cell (an ectoenzyme) or released from the cell into the extracellular space. 2. An enzyme that only cleaves the terminal residue from a polymer (in contrast to an endoen-zyme).

exogen A dicotyledonous plant that grows by means of a peripheral cambial layer.

exoglycosidase Hydrolytic enzymes that cleave glyco-sidic bonds of terminal sugar moieties. Sequential exogly-cosidase cleavage is used to sequence carbohydrates. Can also be used to refer to glycosidases that act on the exterior of a cell although the prefix ecto- is less ambiguous.

exogonic Exothermic reaction that has negative AG; releases energy. Same as exergonic.

exon The sequences of the RNA primary transcript (or the DNA that encodes them) that exit the nucleus as part of a messenger RNA molecule. In the primary transcript, neighbouring exons are separated by introns.

exon shuffling Process by which the evolution of proteins with multifunctional domains could be accelerated. If exons each encoded individual functional domains, then introns would allow their recombination to form new functional proteins with minimal risk of damage to the sequences encoding the functional parts.

exon skipping Probably the commonest cause of alternative splicing in which, during RNA processing, one or more exons are omitted and the message is abbreviated accordingly. A recent estimate is that more than 1200 human genes exhibit exon skipping.

exon trapping Technique for identifying regions of a genomic DNA fragment that are part of an expressed gene. The genomic sequence is cloned into an intron, flanked by two exons, in a specialized exon trapping vector, and the construct expressed through a strong promoter. If the genomic fragment contains an exon, it will be spliced into the resulting mRNA, changing its size and allowing its detection.

exonuclease Enzyme that digests the ends of a piece of DNA (cf. endonuclease). The nature of the digestion is usually specified (e.g. 5' or 3' exonuclease).

exonuclease III Exo IIIEnzyme that degrades DNA from one end. Used to prepare deletions in cloned DNA, or for DNA footprinting.

exopeptidase Peptide hydrolases of the class EC 3.4 that cleave the N- or C-terminal amino acid from a peptide.

exosome 1. Antigen-presenting vesicle secreted by some professional antigen-presenting cells. Exosomes are membrane-bounded and enriched in MHC Class I and II proteins. 2. The exosome (nuclear exosome) complex is involved in multiple RNA processing and degradation pathways and contains 3' ^5' exoribonucleases. It is found in eukaryotes and archaea. 3. A DNA fragment taken up by a cell that does not become integrated with host DNA but nevertheless replicates.

exothermic Process or reaction in which heat is produced - the opposite of endothermic.

exotoxins Toxins released from Gram-positive and Gramnegative bacteria, as opposed to endotoxins that form part of the cell wall. Examples are cholera, pertussis and diphtheria toxins. Usually specific and highly toxic. See Table E2.

exp6 Exportin 6 A nuclear export receptor (exportin) that is specific for profilin-actin complexes and responsible for maintaining the nucleus actin-free.

expansins A superfamily of plant cell wall-loosening proteins that has been divided into four distinct families, designated alpha-expansin, beta-expansin, expansin-like A and expansin-like B. They facilitate cell expansion by selectively weakening the cell wall, although it appears that this may be achieved through a non-enzymatic mechanism.

Table E2



TABLE E2. Exotoxins



Target/mode of action


Aeromonas hydrophila

Pore forming


Clostridium perfringens

Phospholipase C


Staphylococcus aureus


Anthrax toxin

Bacillus anthracis

Three components, one a soluble adenyl cyclase


Plasmid in E. coli

Colicin E2 is a DNase, colicin E3 an RNase.

Botulinum toxins

Clostridium botulinum

Inhibits acetylcholine release


Clostridium botulinum

Cholesterol binding


Bacillus cereus.

Cholesterol binding

Cholera toxin

Vibrio cholerae

ADP-ribosylation of Gs

Diphtheria toxin

Corynebacterium diphtheriae

ADP-ribosylation of EF-2


Clostridium perfringens

Binds to cholesterol

ß, eand l toxins

Clostridium perfringens

Increase vascular permeability


Staphyloccus aureus



Pseudomonas aeruginosa

Causes diarrhoea

Erythrogenic toxin

Streptococcus pyogenes

Skin hypersensitivity


Staphyloccus aureus

Disrupts desmosomes

Haemolysins a, ß, x, 8

Staphyloccus aureus

3 is a sphingomyelinase C, y is haemolytic, 8 is a surfactant


Serratia marcescens

Pore forming; different method to RTX toxins


Pseudomonas aeruginosa

Toxic for macrophages

Heat-labile toxin

Bordetella pertussis


Heat-labile toxin

Escherichia coli

Similar to cholera toxin

Heat-stable enterotoxin

Escherichia coli

Analogue of guanylin

Kanagawa haemolysin

Vibrio haemolytica

Haemolytic, cardiotoxic


Clostridium perfringens



Staphyloccus aureus and Pseudomonas aeruginosa

Lyses neutrophils and macrophages

Listeriolysin O

Listeria monocytogenes


Perfringolysin (theta toxin)

Clostridium perfringens


Pertussis toxin

Bordetella pertussis

ADP-ribosylates Gi


Streptococcus pneumoniae

Binds to cholesterol

RTX family

Various Gram-negative

Calcium-dependent pore-forming



Shiga toxin/verotoxin

Shigella dysenteriae

Blocks eukaryotic protein synthesis

Stable toxin

Escherichia coli

Activates guanylate cyclase

Streptolysin D

Streptococcus pyogenes

Binds cholesterol

Streptolysin S

Streptococcus pyogenes



Bacillus subtilis

Haemolytic surfactant


Clostridium tetani

Binds cholesterol

Tetanus toxin

Clostridium tetani

Inhibits glycine release at synapse


Bacillus thuringiensis

Binds cholesterol

Toxin A

Pseudomonas aeruginosa

ADP-ribosylates EF-2

Gs, Gji see GTP-binding proteins EF-2, elongation factor 2.

Gs, Gji see GTP-binding proteins EF-2, elongation factor 2.

5 Common Skin Problems Answered

5 Common Skin Problems Answered

Our skin may just feel like a mere shield that protects us from the world outside. But, the fact is, its more than just the mask that keeps your insides in. It is a very unique and remarkable complex organ that reflects our general health.

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