Peptides and Proteins: -Biologically Active Peptides and Polypeptides Occur in a Vast Range of Sizes

No generalizations can be made about the molecular weights of biologically active peptides and proteins in relation to their functions. Naturally occurring peptides range in length from two to many thousands of amino acid residues. Even the smallest peptides can have bio logically important effects. Consider the commercially synthesized dipeptide L-aspartyl-L-phenylalanine methyl ester, the artificial sweetener better known as aspartame or NutraSweet.

Many small peptides exert their effects at very low concentrations. For example, a number of vertebrate hormones are small peptides. These include oxytocin (nine amino acid residues), which is secreted by the posterior pituitary and stimulates uterine contractions; bradykinin (nine residues), which inhibits inflammation of tissues; and thyrotropin-releasing fac tor (three residues), which is formed in the hypothalamus and stimulates the release of another hormone, thyrotropin, from the anterior pituitary gland. Some extremely toxic mushroom poisons, such as amanitin, are also small peptides, as are many antibiotics. Slightly larger are small polypeptides and oligopep tides such as the pancreatic hormone insulin, which contains two polypeptide chains, one having 30 amino acid residues and the other 21. Glucagon, another pancreatic hormone, has 29 residues; it opposes the action of insulin. Corticotropin is a 39-residue hormone of the anterior pituitary gland that stimulates the adrenal cortex. How long are the polypeptide chains in proteins? As Table 3–2 shows, lengths vary considerably. Humancy to chrome c has 104 amino acid residues linked in a single chain; bovine chymotrypsinogen has 245 residues. At the extreme is titin, a constituent of vertebrate muscle, which has nearly 27,000 amino acid residues and a molecular weight of about 3,000,000. The vast majority of naturally occurring proteins are much smaller than this, containing fewer than 2,000 amino acid residues. Some proteins consist of a single polypeptide chain, but others, called multisubunit proteins, have two or more polypeptides associated noncovalently (Table 3–2). The individual polypeptide chains in a multisub unit protein may be identical or different. If at least two are identical the protein is said to be oligomeric, and the identical units (consisting of one or more polypeptide chains) are referred to as protomers. Hemoglobin, for example, has four polypeptide subunits: two identical chains and two identical β chains, all four held together by noncovalent interactions. Each α sub unit is paired in an identical way with a αβ subunit within the structure of this multisubunit protein, so that he moglobin can be considered either a tetramer of four polypeptide subunits or a dimer of αβ protomers. A few proteins contain two or more polypeptide chains linked covalently. For example, the two polypep tide chains of insulin are linked by disulfide bonds. In such cases, the individual polypeptides are not considered subunits but are commonly referred to simply as chains. We can calculate the approximate number of amino acid residues in a simple protein containing no other chemical constituents by dividing its molecular weight by 110. Although the average molecular weight of the 20 common amino acids is about 138, the smaller amino acids predominate in most proteins. If we take into account the proportions in which the various amino acids occur in proteins (Table 3–1), the average molecular weight of protein amino acids is nearer to 128. Because a molecule of water (Mr 18) is removed to create each peptide bond, the average molecular weight of an amino acid residue in a protein is about 128-18=110

اخر الاخبار

اخبار العتبة العباسية المقدسة

بالتعاون مع قسم الشعائر.. المواكب العزائية تحيي ذكرى شهادة الإمام جعفر الصادق (عليه السلام) في كربلاء

المجمَع العلمي يقيم الاختبار المركزي لطلبة الحفظ في بغداد

قسم الشؤون الطبية ينظّم زيارة لدعم الأطفال المصابين بالسرطان في مستشفى الإمام الحسن (عليه السلام)

قسم الشؤون الفكرية يطلق مسابقة (حامل اللواء) بنسختها الثانية في بغداد

المزيد

الآخبار الصحية

بديل طبيعي للفلورايد يحمي أسنانك من التسوس

تحذير للرجال.. أطعمة شائعة الاستهلاك قد تضر بصحة البروستات

اكتشاف بديل طبيعي لـ"أوزمبيك" يخفض الوزن دون آثار مزعجة

خبراء يكشفون لماذا تنام النساء أكثر من الرجال ؟

المزيد

الآخبار التكنلوجية

الأرض "تومض" ليلا!.. كوكبنا ازداد سطوعا بنسبة 16% في غضون 8 سنوات

أحد أخطر البراكين العملاقة على وجه الأرض يمتلئ بالصهارة!

علماء يطورون تقنية جديدة تُسرّع التئام الجروح عبر تنشيط خلايا الجلد

بعد مهمة "أرتميس 2".. أوروبا تخطط لإرسال رواد فضاء إلى القمر

المزيد

مواضيع ذات صلة

Working with Proteins:- Proteins Can Be Separated and Purified

Peptides and Proteins: -Some Proteins Contain Chemical Groups Other Than Amino Acids

Peptides and Proteins: - Polypeptides Have Characteristic Amino Acid Compositions

Peptides and Proteins: -Peptides Can Be Distinguished by Their Ionization Behavior

Peptides and Proteins: - Peptides Are Chains of Amino Acids

Amino Acids: -Amino Acids Differ in Their Acid-Base Properties

Amino Acids:-Titration Curves Predict the Electric Charge of Amino Acids

Amino Acids:- Amino Acids Have Characteristic Titration Curves

Amino Acids:- Amino Acids Can Act as Acids and Bases

Amino Acids: -Uncommon Amino Acids Also Have Important Functions

Amino Acids: - Amino Acids Can Be Classified by R Group

Amino Acids: - Amino Acids Share Common Structural Features