You will be given a definition of a task first, then some input of the task.
Given a passage and a query based on the passage, generate an unambiguous, concise and simple answer to the query from information in the passage. Note that the answer may not be present in exact form.

summary: O2 could form a linear complex with the Fe atom, but then it would start to look more like a mineral Fe=O...O bond - a linear bond would be like a transition state to an iron oxide (rust). An Fe-O-O bent bond preserves more of the electronic character of the O2 molecule and promotes strong but reversible binding to Hemoglobin. The oxygen molecule forms a partial covalent bond with the iron in the heme (the ringlike molecule in the picture below ). The lone pair of electrons of the oxygen atom which forms the bond to the iron has a 120 degree angle with respect to the oxygen molecule itself. In chemistry we say it is sp2 hybridized. The heme iron serves as a source or sink of electrons during electron transfer or redox chemistry. In peroxidase reactions, the porphyrin molecule also serves as an electron source. In the transportation or detection of diatomic gases, the gas binds to the heme iron. During the detection of diatomic gases, the binding of the gas ligand to the heme iron induces conformational changes in the surrounding protein. Heme or haem is a coordination complex consisting of an iron ion coordinated to a porphyrin acting as a tetradentate ligand, and to one or two axial ligands. The definition is loose, and many depictions omit the axial ligands. The iron atom of heme is directly bonded to one of the histidines called the proximal histidine of the globin protein at the fifth coordination position. The oxygen binding site is present on the other side of the heme plane, at the sixth coordination position. Assuming that the deprotonated forms of HA and HZ bind selectively to the heme iron, the pH dependence of K D,app can be expressed by the following equation : K D, app =K D ·(1+[H +]/K a), where K D is a pH-independent dissociation constant and K a is the acid dissociation constant. Your question says “how is the heme molecule attached to globin protein”. Let’s first get the idea of what heme is. Heme is a non-protein group, which is the active site for haemoglobin, as well as myoglobin. Heme is held to the peptide portion (globin) of the protein by a combination of forces: 1  The coordination of iron by histidine nitrogen of the protein, 2  Hydrogen bonding, and. 3  Van der Waals forces between hydrophobic parts of the two molecules. Heme - Function. 1  Hemoproteins have diverse biological functions including the transportation of diatomic gases, chemical catalysis, diatomic gas detection, and electron transfer. The following genes are part of the chemical pathway for making heme: 1  ALAD: aminolevulinic acid, δ-, dehydratase (deficiency causes ala-dehydratase deficiency porphyria) 2  ALAS1: aminolevulinate, δ-, synthase 1. 3  ALAS2: aminolevulinate, δ-, synthase 2 (deficiency causes sideroblastic/hypochromic anemia) A2A: Quickly speaking, the heme group contains the famous iron ion that is in hemoglobin, that binds to and carries oxygen. The iron atom has a very strong covalent bond with the globular protein on the proximal histidine of the protein (the F8 histidine residue), on histidine's imidazole ring. Even with this strong covalent bond, the iron still has the ability to bind with oxygen via a dipolar or coordinate covalent bond. That is the electrostatic bond that hemoglobin is involved in, not the one that binds heme to the globin proteins in the first place. So the iron atom is not only involved in keeping the heme group attached to the globin proteins, but it is the functional oxygen-carrying component of the hemoglobin molecule. Query: how does iron bind to heme
Output:
The iron atom has a very strong covalent bond with the globular protein on the proximal histidine of the protein, on histidine's imidazole ring. Even with this strong covalent bond, the iron still has the ability to bind with oxygen via a dipolar or coordinate covalent bond. That is the electrostatic bond that hemoglobin is involved in, not the one that binds heme to the globin proteins in the first place. So the iron atom is not only involved in keeping the heme group attached to the globin proteins, but it is the functional oxygen-carrying component of the hemoglobin molecule.