Name: how protein shape determines function
Uploaded: 2023-04-30T18:28:47.000Z
Duration: 2 min 15 s
Channel: JoVE

K12 Standards

Next Generation Science Standards

High School

Life Science

from-molecules-to-organisms-structures-and-processes

From Molecules to Organisms: Structures and Processes

HS-LS1-1

Protein and Protein Structures

Le proteine sono polimeri di amminoacidi legati insieme tramite legami peptidici. La maggior parte delle proteine si piega in specifiche strutture tridimensionali per svolgere la loro funzione. La struttura delle proteine è classificata in quattro categorie:primaria, secondaria, terziaria e quaternaria.

La sequenza lineare degli amminoacidi nella proteina è la sua struttura primaria. Queste sequenze vengono lette nell'ordine di sintesi del polimero, dall'estremità N-amminica libera, al terminale C, l'estremità carbossilica libera. Le strutture secondarie si formano grazie al legame idrogeno fra l'ossigeno del gruppo carbonile di un amminoacido e l'idrogeno del gruppo amminico di un altro.

Le strutture secondarie comuni includono strutture ad α-elica e a foglio β-pieghettato. Queste e altre strutture di collegamento, come le β-turns, interagiscono ulteriormente per formare la struttura tridimensionale della proteina, detta struttura terziaria. La formazione della struttura terziaria avviene grazie alle interazioni fra i gruppi R degli amminoacidi.

I gruppi R con cariche opposte possono formare legami ionici, e lo zolfo in due cisteine può formare un ponte disolfuro covalente. Altri contributori includono interazioni idrofobiche fra catene laterali idrofobiche e legami idrogeno fra catene laterali polari. Inoltre, alcune proteine formano strutture quaternarie, assemblaggi di due o più catene polipeptidiche.

Per esempio, l'emoglobina è una proteina composta da quattro subunità, due α e due β. Gli scienziati possono utilizzare la struttura di una proteina per prevedere la sua funzione e posizione in una cellula. Le proteine nel citoplasma cellulare hanno aminoacidi idrofili sulla loro superficie che interagiscono con l'acqua nel citoplasma e hanno nuclei idrofobici.

Al contrario, le proteine presenti nelle membrane cellulari hanno spesso aminoacidi idrofobici sulla superficie, i quali interagiscono con i glicerofosfolipidi nella membrana, e hanno nuclei idrofili.

<ol>
	<li>Nelson, DL e Cox, M. M. (2008). Principi di biochimica di Lehninger (5a ed.). W.H. Freeman. Pagine 113-140</li>
	<li>Struttura cristallina dell'emoglobina dello stato T con ossigeno legato a tutti e quattro gli Haems.Paoli, M., Liddington, R., Tame, J., Wilkinson, A., Dodson, G. (1996) J Mol Biol 256: 775</li>
</ol>

proteine e strutture proteiche

Le proteine sono una delle molecole organiche più abbondanti nei sistemi viventi e ha la gamma di funzioni più diversificata di tutte le macromolecole. Le proteine possono essere strutturali, regolatrici, contrattili o protettive. Possono servire nel trasporto, nello stoccaggio o nelle membrane; oppure possono essere tossine o enzimi. Le loro strutture, come le loro funzioni, variano notevolmente. Sono tutti, però, polimeri di amminoacidi disposti in sequenza lineare.

La forma di una proteina è fondamentale per la sua funzione. Ad esempio, un enzima può legare uno specifico substrato nel suo sito attivo. Se questo sito attivo viene alterato a causa di cambiamenti locali o cambiamenti nella struttura complessiva della proteina, l'enzima potrebbe non essere in grado di legarsi al substrato. Per capire come la proteina ottiene la sua forma o conformazione finale, dobbiamo comprendere i quattro livelli della struttura proteica: primario, secondario, terziario e quaternario.

Struttura primaria

La sequenza unica degli amminoacidi in una catena polipeptidica è la sua struttura primaria. Ad esempio, l’ormone pancreatico insulina ha due catene polipeptidiche, A e B, e sono collegate insieme da legami disolfuro. L'amminoacido N terminale della catena A è la glicina; mentre l'amminoacido C terminale è l'asparagina. Le sequenze di aminoacidi nelle catene A e B sono esclusive dell'insulina.

Il gene che codifica per la proteina determina in definitiva la sequenza unica per ogni proteina. Un cambiamento nella sequenza nucleotidica della regione codificante del gene può portare all’aggiunta di un amminoacido diverso alla catena polipeptidica in crescita, causando un cambiamento nella struttura e nella funzione della proteina. Nell’anemia falciforme, la catena β dell’emoglobina ha una singola sostituzione di aminoacidi, causando un cambiamento nella struttura e nella funzione delle proteine. Nello specifico, la valina nella catena β sostituisce l'amminoacido acido glutammico. A causa di questo cambiamento di un amminoacido nella catena, le molecole di emoglobina formano lunghe fibre che distorcono i globuli rossi biconcavi, o a forma di disco, e gli fanno assumere una forma a mezzaluna o “falce”, che ostruisce i vasi sanguigni. Per le persone affette da questa malattia questo può portare a una miriade di gravi problemi di salute come mancanza di respiro, vertigini, mal di testa e dolori addominali.

Struttura secondaria

Il ripiegamento locale del polipeptide in alcune regioni dà origine alla struttura secondaria della proteina. Le più comuni sono le strutture a fogli α-elica e β-pieghettate. Entrambe le strutture sono mantenute in forma da legami idrogeno. I legami idrogeno si formano tra l'atomo di ossigeno nel gruppo carbonilico in un amminoacido e un altro amminoacido che si trova quattro amminoacidi più avanti lungo la catena.

Ogni giro elicoidale in un'alfa elica ha 3,6 residui di amminoacidi. I gruppi R del polipeptide (i gruppi varianti) sporgono dalla catena α-elica. Nel foglio β-pieghettato, il legame idrogeno tra gli atomi sulla struttura portante della catena polipeptidica forma le "pieghe". I gruppi R sono attaccati ai carboni e si estendono sopra e sotto le pieghe della piega. I segmenti pieghettati si allineano paralleli o antiparalleli tra loro e si formano legami idrogeno tra l'atomo di idrogeno parzialmente positivo nel gruppo amminico e l'atomo di ossigeno parzialmente negativo nel gruppo carbonilico della struttura peptidica. Le strutture a fogli α-elica e β-pieghettate si trovano nella maggior parte delle proteine globulari e fibrose e svolgono un importante ruolo strutturale.

Struttura terziaria

La struttura tridimensionale unica del polipeptide è la sua struttura terziaria. Questa struttura è in parte dovuta alle interazioni chimiche che agiscono sulla catena polipeptidica. Principalmente, le interazioni tra i gruppi R creano la complessa struttura terziaria tridimensionale della proteina. La natura dei gruppi R negli amminoacidi coinvolti può contrastare la formazione dei legami idrogeno che abbiamo descritto per le strutture secondarie standard. Ad esempio, i gruppi R con cariche simili si respingono a vicenda, mentre quelli con cariche diverse si attraggono tra loro (legami ionici). Quando avviene il ripiegamento delle proteine, i gruppi R idrofobici degli amminoacidi non polari si trovano all'interno della proteina; mentre i gruppi R idrofili si trovano all'esterno. Le interazioni tra le catene laterali della cisteina formano legami disolfuro in presenza di ossigeno, l'unico legame covalente che si forma durante il ripiegamento delle proteine.

Tutte queste interazioni, deboli e forti, determinano la forma tridimensionale finale della proteina. Quando una proteina perde la sua forma tridimensionale, potrebbe non essere più funzionale.

Struttura quaternaria

In natura, alcune proteine si formano da diversi polipeptidi, o subunità, e l'interazione di queste subunità forma la struttura quaternaria. Le deboli interazioni tra le subunità aiutano a stabilizzare la struttura complessiva. Ad esempio, l’insulina (una proteina globulare) ha una combinazione di legami idrogeno e disolfuro che la fanno raggruppare principalmente a forma di palla. L'insulina inizia come un singolo polipeptide e perde alcune sequenze interne in presenza di modifiche post-traduzionali dopo aver formato i collegamenti disolfuro che tengono insieme le restanti catene. La seta (una proteina fibrosa), tuttavia, ha una struttura a fogli β-pieghettati che è il risultato del legame idrogeno tra diverse catene.

Questo testo è adattato da <a href="https://openstax.org/books/biology-2e/pages/3-4-proteins">Openstax, Biology 2e, Chapter 3.4: Proteins</a>.

Proteine e strutture proteiche

Le proteine sono una delle molecole organiche più abbondanti nei sistemi viventi e hanno la gamma di funzioni più diversificata di tutte le macromolecole. Le proteine possono...

Education

Didattica

JoVE Core

Cell Biology

Biologia cellulare

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Biochemistry of the Cell

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KEY TERMS AND CONCEPTS

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how protein shape determines function

Proteins are essential molecules in living things, and their shape affects how they work. Protein structure is described at several levels, including primary, secondary, tertiary, and quaternary structure. These levels help explain how a protein is built and how its final form supports its job.

Primary structure is the sequence of amino acids in a protein. Secondary structure forms when parts of the chain fold into regular patterns. Tertiary structure is the overall three-dimensional shape of one protein chain. Quaternary structure appears when more than one protein chain joins together to make a working protein.

The video also shows how protein structure can be represented in different ways. A simple line drawing can show the amino acid sequence. Other models can show folding and the full 3D shape. These visual forms help students compare levels of structure and see why protein shape matters.

How Protein Shape Determines Function

the periodic table and organismal elements

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Elements in the Human Body and Periodic Table

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Learn how proteins are built and folded through primary, secondary, tertiary, and quaternary structure, and how protein shape affects function.

How Protein Shape Determines Function | JoVE K12

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SC.912.L.18

SC.912.L.18.1

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Structure and Function

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Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms. [Clarification Statement: Emphasis is on functions at the organism system level such as nutrient uptake, water delivery, and organism movement in response to neural stimuli. An example of an interacting system could be an artery depending on the proper function of elastic tissue and smooth muscle to regulate and deliver the proper amount of blood within the circulatory system.] [Assessment Boundary: Assessment does not include interactions and functions at the molecular or chemical reaction level.]

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How Protein Shape Determines Function

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