3.6
Proteinler Nelerdir?
Genel Bakış
Proteinler, peptit bağlarıyla bağlanan ve 3 boyutlu bir yapıda katlanan amino asit zincirleridir. Ayrı ayrı amino asit rezidülerinin yan z…
- [Anlatıcı] Protein bir amino asit zinciridir, amin ve karboksil grupları içerir ve farklı yan gruplarla birlikte birleşir. Bir çekirdek karbona bağlı bu gruplar R olarak adlandırılır. Amino asitler kovalent peptid bağlarıyla bağlanır.
Ve çekirdek boyunca yinelenen atom dizisine polipeptit omurgası denir. Dizi, N-terminalinde, NH3 artı serbest bir amino grubu ile başlar. Ve C-terminalinde COO eksi serbest karboksil grubu ile biter.
Her protein, amino asitlerin sayısına ve yan zincirlerine bağlı olarak benzersizdir. Örneğin, tiroid salgılayan hormon 234 amino aside sahiptir. Elastik kas proteini konnektinde ise otuz dört binin üzerindedir.
Ayrıca, dört gruptan birine giren yirmi farklı yan zincir var. Aspartik ve glutamik asitler gibi negatif polar amino asitler, bir karboksil grubuna sahip yan zincirlere sahiptir. Bu nedenle nötr pH'da bir asit gibi davranırlar.
Pozitif polar amino asitler bazlar gibi davranır ve yan zincirlerinde bir amino grubuna sahiptir. Birçok amino asit polar fakat yüksüzdür. Bunlar hidrofilik olma eğilimindedir ve katlanmış bir proteinin su çevresiyle etkileşime girme konusunda serbest oldukları dış kısmını oluştururlar.
Son olarak, amino asitlerin büyük bir kısmı nonpolar yan zincirlere sahiptir. Glisinde olduğu gibi basit veya prolin ve triptofan gibi kompleks olabilirler. Pek çok proteinin çekirdeğini oluşturan nonpolar yan zincirler genellikle amino asitlerin hidrofobik olmasına sebep olur.
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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.