3.6
Cosa sono le proteine?
Panoramica
Le proteine sono catene di aminoacidi che sono collegati da legami di peptidi e impilati in una struttura tridimensionale. Le catene latera…
- [Narratore] La proteina è una catena di amminoacidi,
composti che contengono gruppi amminici e carbossilici.
Insieme a diversi gruppi laterali.
Denotato come R, collegato a un nucleo di carbonio.
Gli amminoacidi sono collegati attraverso legami peptidici
covalenti. E la serie ripetitiva di atomi lungo il nucleo,
è chiamato la spina dorsale polipeptidica.
La sequenza inizia al N-terminale,
con un gruppo amminico libero NH3 plus.
E finisce con il C-terminale,
un COO gruppo carbossilico libero negativo.
Ogni proteina è unica,
a seconda del numero di
amminoacidi e delle loro catene laterali. Ad esempio,
l'ormone che rilascia la tiroide ha 234 amminoacidi.
Considerando che, la proteina muscolare elastica
di connectina ne ha oltre trentaquattromila.
Inoltre, ci sono venti diverse catene laterali
che rientrano in uno dei quattro gruppi.
Gli amminoacidi polari negativi,
come acidi aspartico e glutammico,
hanno catene laterali con un gruppo carbossile.
Per questo agiscono come un acido a pH neutro.
Gli amminoacidi polari positivi agiscono come basi
e hanno un gruppo amminico nella loro catena laterale.
Diversi amminoacidi sono polari ma non caricati.
Tendono ad essere idrofilici e compongono
la parte esterna di una proteina piegata,
dove sono liberi di interagire con l'ambiente neutro.
Infine, un grande gruppo di aminoacidi
contengono catene laterali non polari.
Possono essere semplici, come nella glicina,
o complesso come la prolina e il triptofano.
Catene laterali non polari generalmente
rendeno l'amminoacido idrofobico,
formando il nucleo di molte proteine.
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Q1: What is the basic structure of an amino acid?
An amino acid contains a carboxyl group and an amino group attached to the same carbon atom, called the alpha-carbon. A side chain, or R-group, extends from this central carbon and determines the amino acid's chemical identity and properties. The simplest amino acid is glycine, which has only a hydrogen atom as its side chain.
Q2: How are amino acids linked together in proteins?
Amino acids are linked through covalent peptide bonds formed between the amino group of one amino acid and the carboxyl group of an adjacent amino acid. This process occurs through dehydration synthesis, where one water molecule is released during each linkage. The resulting chain forms the polypeptide backbone, with a free amino group at the N-terminus and a free carboxyl group at the C-terminus.
Q3: What are the four main groups of amino acid side chains?
The 20 amino acids used in proteins have side chains classified into four groups: negative polar amino acids with carboxyl groups that act as acids; positive polar amino acids with amino groups that act as bases; polar uncharged amino acids that are hydrophilic; and nonpolar amino acids with hydrophobic side chains. These properties determine how amino acids interact and fold within proteins.
Q4: How does pH affect the chemical properties of amino acids?
At low pH, both carboxyl and amino groups are protonated, making the amino acid act as a base. At high pH, both groups are deprotonated, causing it to act as an acid. At neutral pH, typical in physiological environments, the amino group is protonated and the carboxyl group is deprotonated, creating a zwitterion with both positive and negative charges.
Q5: What determines the uniqueness of each protein?
Each protein's uniqueness depends on the number of amino acids in its chain and the specific sequence of their side chains. For example, thyroid-releasing hormone contains 234 amino acids, while the elastic muscle protein connectin has over 34,000. The side chain composition and arrangement directly influence how the protein folds and functions.
Q6: What is the difference between a polypeptide and a protein?
A polypeptide is a chain of amino acids linked by peptide bonds, with chains containing fewer than 20 amino acids called oligopeptides. A protein is a polypeptide that has folded into a three-dimensional structure and is ready to perform its specific cellular function. Length, structural complexity, and functionality distinguish proteins from simpler polypeptide chains.
Q7: What are the major functions of proteins in cells?
Proteins serve diverse cellular functions including providing structural support like collagen, enabling movement through actin and myosin in muscles, catalyzing reactions as enzymes, transporting molecules across cell membranes, and defending organisms through antibodies. Together with carbohydrates, nucleic acids, and lipids, proteins are fundamental building blocks essential to all life.