Description
General Description
Natural human C5a is prepared from human C5 protein by cleavage of the peptide bond between C5a and C5b by the human C5 convertase. C5a is a naturally glycosylated
polypeptide containing 74 amino acids with a molecular weight of approx. 10,400 Daltons. It contains 25% carbohydrate attached to a single Assn residue at position 64. This
carbohydrate is of variable structure leading to a broad distribution of MW upon analysis by mass spectroscopy. C5a is the most potent anaphylatoxin (compared to C3a and C4a). Its
biological properties include being strongly chemotactic for neutrophils (PMN), causing smooth muscle contraction, increasing vascular permeability, causing histamine and
TNFalpha release, and causing lysosomal degranulation of immune cells. C5a acts through the C5a Receptor (C5aR, CD88, a G-protein coupled receptor) on PMN, monocytes, alveolar
macrophages, and mast cells. A second receptor of unknown function (C5L2, gpr77) has been identified. Due to the widespread expression of C5a receptors and the results from
C5aR KO mice it is believed that C5a and its receptors have many non-immunological functions in organ development, CNS development, neurodegeneration, tissue regeneration
and hematopoiesis (Monk, P.N. et al. (2007))
Native versus Recombinant C5a
Numerous recombinant forms of C5a are sold by many companies. In side-by-side biological testing, we have found that our native C5a is 10- to 100-fold more active per µg
than all but one of these recombinant proteins. Structurally not a single one of the recombinant proteins on the market has the correct amino acid sequence or structure. They
have extra amino acids at the N-terminal (such as 6 His tags), different amino acids in the sequence itself (some were produced from the original, but incorrect amino acid sequence),
and none possess the 25% carbohydrate at Assn 64. In fact, one recombinant C5a on the market has approximately 30 additional amino acids at the N-terminal end due to the cloning
vector used. This is a 40% addition of nonsense structure to the C5a molecule. Both our C5a and our C5adesArg are native proteins produced by the native human C5 convertase.
Physical Characteristics & Structure
Molecular weight: 10,400 (+ 1,000 due to variable glycosylation)
Deglycosylase MW: 8,271 (observed). Calculated monoisotopic mass 8268;
Calculated average mass 8273.
Isoelectric point: pI = 8.9
Carbohydrate content: ~25% carbohydrate (heterogeneous)
Amino acid sequence: TLQKKIEEIA AKYKHSVVKK CCYDGACVNN DETCEQRAAR ISLGPRCIKA FTECCVVASQ LRANISHKDM QLGR
CAS Number: 80295-54-1
MDL Number: MFCD00130842
NMR derived structure: FEBS Lett. 238:289-294, 1988; Biochemistry 28:172-185, 1989; Biochemistry 29:2895-2905, 1990; Proteins 28:261-267, 1997.
Function
C5a released from C5 by C5 convertases initiates a multitude of inflammatory reactions. C5a causes neutrophils to become adherent to endothelium and to migrate to the
site of complement activation by chemotaxis where it stimulates release of PMN granule contents and reactive oxygen species. The biological properties of C5a include being
strongly chemotactic for neutrophils (PMN), causing smooth muscle contraction, increasing vascular permeability, causing histamine release, and initiating lysosomal degranulation of a
variety of immune cells. C5a acts through the C5a Receptor (C5aR, a G-protein coupled receptor) on PMN, monocytes, alveolar macrophages, dendritic cells, mast cells, glial cells
and smooth muscle cells. Rapid release of C5a and other anaphylatoxins can cause systemic effects as well as local changes. Anaphylactic shock is a generalized circulatory collapse
similar to that caused by an allergic reaction and is caused by C3a and C5a which are generally released together.
Assays
The multitude of biological functions of C5a has resulted in the use of many different assay systems (Dodds, A.W. and Sim, R.B. (1997)). The most typical biological assays
being smooth muscle contraction assays using guinea pig ileum, chemotaxis assays using neutrophils or granule-release assays using human PMN or similar cell lines. Granule release
is generally followed by measuring the release of myeloperoxidase. In addition, assays have been described that measure ATP release from guinea pig platelets, serotonin release from
guinea pig platelets, N-acetyl-beta-D-glycosamides release from differentiated U937 cells and calcium release from differentiated U937 cells. These assays have been described in
detail (Dodds, A.W. and Sim, R.B. (1997)). Functional responses have been detected in the sub-picomolar concentration range for purified human C5a (Gerard, C. et al. (1981); Hugli,
T.E. et al. (1981)). ELISA kits for the assay of C5a levels (or more correctly C5a desArg levels) in blood and other fluids are sold by many companies. A radioimmunoassay for C5a/C5a desArg is
also available. These measurements are useful for detecting complement activation in vivo, but the interpretation of their meaning is complicated by the fact that clearance of the anaphylatoxins is rapid.
In vivo
The resting serum concentration has been reported to be approximately 4 nM although it is difficult to draw or store blood without 1 to 10 % C5 activation (Watkins, J.
(1987)). The presence of EDTA and Futhan in the collection tubes can minimize this
background. Full activation of all C5 in blood (75 µg/mL) would result in ~380 nM C5a
(~3.9 µg/mL). Due to the extreme sensitivity of many C5a responses, a response can
theoretically be initiated by activation of approximately one millionth of the C5 in a local
area.
Regulation
C5a levels are regulated by three processes: formation, inactivation and clearance. The enzymes that cleave C5 and release C5a (collectively called C5 convertases) do so at
very slow rates. Operating at Vmax the best enzymes only cleave one C5 every three minutes (Rawal, N. and Pang burn, M.K. (2001)). C5a is “inactivated” by removal
of its C-terminal arginine amino acid. The product C5a desArg (or C5a without the C-terminal arginine) is produced by the action of the plasma enzyme carboxypeptidase N (Mueller-Ortiz
S.L. et al. (2009)). This inactivation is rapid and most C5a is converted to C5a desArg within minutes of its formation. “Inactivated” C5a still possesses approx. 1% of its
anaphylatoxin and chemotactic activities, but its stimulatory activity is only reduced 10-fold. Thus, C5a desArg retains considerable biological activity even though it is frequently called
inactivated C5a. Because of the large number of cells bearing C5a receptors (endothelial, immune, smooth muscle, neuronal, etc.) the capture, internalization and digestion of C5a
results in its rapid removal from circulation.