Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: - Oxygen Can Be Bound to a Heme Prosthetic Group

Oxygen is poorly soluble in aqueous solutions (see Table 2–3) and cannot be carried to tissues in sufficient quantity if it is simply dissolved in blood serum. Diffusion of oxygen through tissues is also ineffective over distances greater than a few millimeters. The evolution of larger, multicellular animals depended on the evolution of proteins that could transport and store oxygen. However, none of the amino acid side chains in proteins is suited for the reversible binding of oxygen molecules. This role is filled by certain transition metals, among them iron and copper, that have a strong tendency to bind oxy gen. Multicellular organisms exploit the properties of metals, most commonly iron, for oxygen transport. How ever, free iron promotes the formation of highly reactive oxygen species such as hydroxyl radicals that can damage DNA and other macromolecules. Iron used in cells is therefore bound in forms that sequester it and/or make it less reactive. In multicellular organisms—especially those in which iron, in its oxygen-carrying capacity, must be transported over large distances—iron is often incorporated into a protein-bound prosthetic group called heme. Heme (or haem) consists of a complex organic ring structure, protoporphyrin, to which is bound a single iron atom in its ferrous (Fe²⁺) state (Fig. 5–1). The iron atom has six coordination bonds, four to nitrogen atoms that are part of the flat porphyrin ring system and two perpendicular to the porphyrin. The coordinated nitrogen atoms (which have an electron-donating char acter) help prevent conversion of the heme iron to the ferric (Fe³⁺) state. Iron in the Fe²⁺ state binds oxygen reversibly; in the Fe³⁺ state it does not bind oxygen. Heme is found in a number of oxygen-transporting proteins, as well as in some proteins, such as the cytochromes, that participate in oxidation-reduction (electron-transfer) reactions. In free heme molecules (heme not bound to protein), reaction of oxygen at one of the two “open” coordination bonds of iron (perpendicular to the plane of the porphyrin molecule, above and below) can result in irreversible conversion of Fe²⁺ to Fe³⁺. In heme containing proteins, this reaction is prevented by sequestering of the heme deep within the protein structure where access to the two open coordination bonds is restricted. One of these two coordination bonds is occupied by a side-chain nitrogen of a His residue. The other is the binding site for molecular oxygen (O₂) (Fig. 5–2). When oxygen binds, the electronic properties of heme iron change; this accounts for the change in color from the dark purple of oxygen-depleted venous blood to the bright red of oxygen-rich arterial blood. Some small molecules, such as carbon monoxide (CO) and nitric oxide (NO), coordinate to heme iron with greater affinity than does O₂. When a molecule of CO is bound to heme, O₂ is excluded, which is why CO is highly toxic to aerobic organisms (a topic explored later, in Box 5–1). By surrounding and sequestering heme, oxygen binding proteins regulate the access of CO and other small molecules to the heme iron.

FIGURE 5–1 Heme. The heme group is present in myoglobin, hemoglobin, and many other proteins, designated heme proteins. Heme consists of a complex organic ring structure, protoporphyrin IX, to which is bound an iron atom in its ferrous (Fe²⁺) state. (a) Porphyrins, of which protoporphyrin IX is only one example, consist of four pyrrole rings linked by methene bridges, with substitutions at one or more of the positions denoted X. (b, c) Two representations of heme. (De rived from PDB ID 1CCR.) The iron atom of heme has six coordination bonds: four in the plane of, and bonded to, the flat porphyrin ring system, and (d) two perpendicular to it.

FIGURE 5–2 The heme group viewed from the side. This view shows the two coordination bonds to Fe²⁺ perpendicular to the porphyrin ring system. One of these two bonds is occupied by a His residue, sometimes called the proximal His. The other bond is the binding site for oxygen. The remaining four coordination bonds are in the plane of, and bonded to, the flat porphyrin ring system.

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شعبة التوجيه الديني النسوي تنظم مسابقة ثقافية بذكرى ولادة السيدة المعصومة (عليها السلام)

المجمع العلمي ينظّم محاضرةً تربوية لطلبة مشروع حفظ القرآن الكريم في كربلاء

قسم الشؤون الفكرية يقيم معرضًا وثائقيًا ضمن مهرجان (الشهيد) في جامعة الكوفة

قسم الشؤون الدينية يشهد إقبالًا واسعًا من المؤمنين لتنظيم عقود الزواج

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الآخبار الصحية

دواء جديد يحمي القلب ويعزز علاج السرطان في آن واحد

سر غذائي بسيط لحماية القلب مع التقدم في العمر

علامات تشير إلى اختلال عمل الغدة الدرقية

السمنة واللقاحات.. دراسة تكشف تأثيراً سلبياً

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مواضيع ذات صلة

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Protein-Ligand Interactions Can Be Described Quantitatively

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Myoglobin Has a Single Binding Site for Oxygen

Protein Denaturation and Folding:- Some Proteins Undergo Assisted Folding

Protein Denaturation and Folding:- Polypeptides Fold Rapidly by a Stepwise Process

Protein Denaturation and Folding: - Amino Acid Sequence Determines Tertiary Structure

Protein Tertiary and Quaternary Structures: -Protein Quaternary Structures Range from Simple Dimers to Large Complexes

Protein Tertiary and Quaternary Structures: -Protein Motifs Are the Basis for Protein Structural Classification

Protein Tertiary and Quaternary Structures: -Analysis of Many Globular Proteins Reveals Common Structural Patterns

Protein Tertiary and Quaternary Structures:- Globular Proteins Have a Variety of Tertiary Structures

Protein Tertiary and Quaternary Structures:- Myoglobin Provided Early Clues about the Complexity of Globular Protein Structure

Protein Tertiary and Quaternary Structures: -Structural Diversity Reflects Functional Diversity in Globular Proteins

Protein Tertiary and Quaternary Structures: -Fibrous Proteins Are Adapted for a Structural Function