3.6
O que são Proteínas?
Visão Geral
As proteínas são cadeias de aminoácidos que são conectados por ligações peptídicas e dobrados em uma estrutura tridimensional. As cadeias…
A proteína é uma cadeia de aminoácidos compostos que contêm grupos amina e carboxilo, junto com diferentes grupos paralelos denotado como R ligado a um carbono central. Os aminoácidos estão ligados por meio de ligações peptídicas covalentes, e uma série repetitiva de átomos ao longo do núcleo é chamada de estrutura polipeptídica. A sequência começa no terminal N, com um grupo amino livre NH3 mais, e termina com o terminal C, um grupo carboxilo livre COO menos.
Cada proteína é única dependendo do número de aminoácidos e suas cadeias laterais. Por exemplo, o hormônio estimulador da tireoide tem 234 aminoácidos, enquanto a proteína muscular elástica conectina tem mais de 34.000. Além disso, existem 20 cadeias laterais diferentes que se enquadram em um dos quatro grupos;aminoácidos polares negativos, como os ácidos aspártico e glutâmico têm cadeias laterais com um grupo carboxilo.
Por causa disso, eles agem como um ácido com pH neutro. Aminoácidos polares positivos agem como bases e tem um grupo amino em sua cadeia lateral. Vários aminoácidos são polares, mas sem carga.
Eles tendem a ser hidrofílicos e compõem a porção externa da proteína dobrada onde eles estão livres para interagir com o meio aquoso. Por último, um grande grupo de aminoácidos contêm cadeias laterais não polares. Eles podem ser simples, como a glicina, ou complexos, como a prolina e o triptofano.
As cadeias laterais não polares geralmente formam aminoácidos hidrofóbicos, formando o núcleo de muitas proteínas.
View the full transcript and gain access to JoVE Core videos
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.