biology

Streptococcal pyrogenic exotoxin

Streptococcal pyrogenic exotoxin is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Streptococcal pyrogenic exotoxin rather than just read about it. In short: Streptococcal pyrogenic exotoxins also known as erythrogenic toxins, are exotoxins secreted by strains of the bacterial species Streptococcus pyogenes. SpeA and SpeC are superantigens, which induce inflammation by nonspecifically activating T cells and stimulating the production of inflammatory cytokines.

Key takeaways

Streptococcal pyrogenic exotoxin belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Streptococcal pyrogenic exotoxin to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Streptococcal pyrogenic exotoxin from memory before moving on to harder problems.

Reference excerpt

Streptococcal pyrogenic exotoxins also known as erythrogenic toxins, are exotoxins secreted by strains of the bacterial species Streptococcus pyogenes. SpeA and SpeC are superantigens, which induce inflammation by nonspecifically activating T cells and stimulating the production of inflammatory cytokines. SpeB, the most abundant streptococcal extracellular protein, is a cysteine protease. Pyrogenic exotoxins are implicated as the causative agent of scarlet fever and streptococcal toxic shock syndrome. There is no consensus on the exact number of pyrogenic exotoxins. Serotypes A, B, and C are the most extensively studied and recognized by all sources, but others note up to thirteen distinct types, categorizing SpeF through SpeM as additional superantigens. Erythrogenic toxins are known to damage the plasma membranes of blood capillaries under the skin and produce a red skin rash (characteristic of scarlet fever). Past studies have shown that multiple variants of erythrogenic toxins may be produced, depending on the strain of S. pyogenes in question. Some strains may not produce a detectable toxin at all. Bacteriophage T12 infection of S. pyogenes enables the production of SpeA, and increases virulence.

History

Discovery and nomenclature SpeB was identified in 1919 as an ectoenzyme secreted by certain strains of streptococci. It was originally studied as two separate toxins, streptococcal pyrogenic exotoxin B and streptococcal cysteine proteinase, until it was shown that both proteins were encoded by the speB gene and that the attributed pyrogenic activities were due to contamination by SpeA and SpeC. Pyrogenic, in streptococcal pyrogenic exotoxin, means "causes fever." Erythrogenic refers to the typical red rash of scarlet fever. In older literature, these toxins are also referred to as scarlatina toxins or scarlet fever toxins due to their role as the causative agents of the disease. SpeB is known as streptococcal pyrogenic exotoxin B, streptopain and streptococcal cysteine proteinase as a result of its original misidentification as two separate toxins, and is neither an exotoxin nor pyrogenic.

Structure

Location of genes The speB and speJ genes are located in the core bacterial chromosome of all strains of S. pyogenes. However, despite its presence and high levels of conservation in the nucleotide sequence, 25–40% of these strains do not express the SpeB toxin in significant amounts. In contrast, speA, speC and speH-M are encoded within prophages, bacteriophage genomes that have integrated into the chromosome of the host bacterium genome. There is a lack of consensus over the location of the speG gene, which has been attributed to both the core chromosome and lysogenic phages.

Protein structure

SpeB is a 28 kDa protein with three major forms, mSpeB1, mSpeB2 and mSpeB3, which are categorized by variations the primary amino acid sequence. Three amino acids, C192, H340, and W357, are vital for enzymatic activity in all variants. The toxin contains a canonical papain-like domain, and mSpeB2 has an additional human integrin binding domain.

All superantigenic streptococcal pyrogenic exotoxins contain two major conserved protein domains that are linked by an α-helix, which consist of an amino-terminal oligosaccharide/oligonucleotide binding fold and a carboxy-terminal β-grasp domain, as well as a dodecapeptide binding region. SpeA also has a cystine loop, a low-affinity α-chain MHC II binding site, and the Vβ-TCR binding site. SpeC, SpeG, SpeH and SpeJ contains a Zn2+-dependent high β-chain MHC II binding site in addition to the low affinity site present in SpeA, and lacks the cystine loop. SpeH also has an additional α3-β8 loop that mediates the specificity of the toxin's Vβ-TCR binding site.

Processing and regulation

The speB gene encodes for an amino acid sequence that becomes the 40 kDa zymogen, known as SpeBz, after cleavage of the signal sequence. SpeBz undergoes autocatalysis through at least eight intermediates to create the 28 kDa SpeBm. Finally cystine-192 and histidine-340 form a catalytic dyad. Each step is tightly regulated by multiple factors, allowing sophisticated temporal expression of the mature proteinase.

Mechanisms of action

SpeA and SpeC

SpeA and SpeC bind to MHC Class II molecules, are presented to T cells, and bind to the variable region of the beta chain of T-cell receptors (TCRs). Once activated, the T cells release pro-inflammatory cytokines and chemokines. The interactions with TCRs are characterized by low affinities and fast dissociation, allowing the toxin to activate multiple T cells in succession. The lack of specificity allow the activation of up to 50% of the T cells in the body.

SpeB SpeB cleaves degrades multiple proteins through hydrolysis, including cytokines, extracellular matrix proteins and immunoglobulin. It requires three amino acids before the cleavage site, known as P1, P2 and P3. Of these, SpeB has a preference for hydrophobic P2 and positively charged P1 residues, with greater importance of the P2 amino acid.

Roles in virulence, pathogenesis and infection

SpeB Streptococcal cysteine proteinase has roles in immune evasion and apoptosis, as well as potential influence on bacterial internalization. There is contradictory evidence regarding the effect of SpeB on virulence. Some studies have reported increased protease levels in strains that cause scarlet fever in comparison to those associated with streptococcal toxic shock syndrome, while others show decreased expression in more virulent strains. SpeB degrades immunoglobulins and cytokines, as well as through cleavage of C3b, inhibiting recruitment of phagocytic cells and the complement activation pathway. This results in decreased inflammation and neutrophil levels around the site of infection, preventing clearance and through phagocytosis and promoting the survival of S. pyogenes. The toxin also induces apoptosis in host cells after GAS internalization. Evidence suggests that this may take place through extrinsic and intrinsic caspase pathways. The receptor-binding pathway and Fas-mediated apoptotic signaling pathway have been implicated in this process. The induction of apoptosis results in necrotizing fasciitis.

References

External links Media related to Erythrogenic toxin at Wikimedia Commons Todar's Online Textbook of Bacteriology Streptococcal Pyrogenic Exotoxin A1

Illustrations

Worked examples

Example 1 — a first encounter with Streptococcal pyrogenic exotoxin

Start with the simplest possible case. Write down what Streptococcal pyrogenic exotoxin claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Streptococcal pyrogenic exotoxin before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Streptococcal pyrogenic exotoxin ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Streptococcal pyrogenic exotoxin

In research: Streptococcal pyrogenic exotoxin appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Streptococcal pyrogenic exotoxin in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Streptococcal pyrogenic exotoxin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bacterial toxins, Proteins, Scarlet fever, so understanding it makes those chapters shorter.
In everyday life: Look for Streptococcal pyrogenic exotoxin outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Streptococcal pyrogenic exotoxin in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Streptococcal pyrogenic exotoxin means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Streptococcal pyrogenic exotoxin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Streptococcal pyrogenic exotoxin in simple terms?

Why does Streptococcal pyrogenic exotoxin matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Streptococcal pyrogenic exotoxin?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Streptococcal pyrogenic exotoxin.