Structure of hemoglobin. The protein subunits are in red and blue, and the iron-containing heme groups in green

The structure of hemoglobin.

The evolution of oxygen carrying proteins represents a major adaptation in the history of life. Overcoming the low solubility of oxygen in water, oxygen carriers make aerobic respiration possible in tissues. The oxygen carriers in vertebrates are myoglobin and hemoglobin. Myoglobin is located in muscle. Hemoglobin serves as the oxygen carrier in blood, increasing its oxygen carrying capacity fifty fold.

The properties of oxygen carriers are worthy of extensive, dedicated focus in MCAT preparation, not only because topics such as the Bohr effect are long-standing favorites. The former was true in the days of the old MCAT and is even more true with the new, biochemistry focused MCAT. Additionally, the properties of hemoglobin are a model for how proteins behave. Hemoglobin provides a model for understanding how a comformational change may alter the pKa of a residue side-chain, for example, and it is the place to learn how cooperativity works in allosteric, multisubunit proteins, both important concepts to bring to enzyme mechanics and regulation.

WikiPremed Resources

Learning Goals

Proficiency 

Be able to describe the structure of the heme prosthetic group.

Understand the role of hindrance in the heme environment for reversible oxygenation.

Be prepared to describe the tertiary structure of myoglobin.

Know how to compare and contrast the oxygen dissociation curves of myoglobin and hemoglobin.

Understand how the multi-subunit structure of hemoglobin functions in cooperative binding and how cooperative binding facilitates oxygen transport physiologically.

Beyond understanding the consequences of the Bohr and Haldane effects, be able to describe their mechanisms in terms of the effect of conformational changes on specific residues.

Comprehend the mechanism by which DPG decreases oxygen affinity in hemoglobin.

Know how to describe the variations in structure and behavior with the hemoglobin isoforms hemoglobin F and hemoglobin S.

Suggested Assignments

Conceptual Vocabulary for Oxygen Transporters

Carbohydrate
Carbohydrates or saccharides are a major class of biomolecules which are simple compounds, aldehydes or ketones with many hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone functional group.
Starch
Starch is a mixture of amylose and amylopectin. These are both complex carbohydrate polymers of glucose.
Glucose
Glucose, a monosaccharide, is an important carbohydrate in biology, used by the living cell as a source of energy and metabolic intermediates. It is one of the main products of photosynthesis and starts cellular respiration in both prokaryotes and eukaryotes.
Sucrose
Sucrose (table sugar) is a disaccharide formed by the condensation of glucose and fructose.
Disaccharide
A disaccharide is a sugar composed of two monosaccharides.
Glycogen
Glycogen is a polysaccharide of glucose which functions as the primary short term energy storage in animal cells.
Cellulose
Cellulose is a polysaccharide derived from beta-glucose. It is the primary structural component of green plants.
Amylose
Amylose is a linear polymer of glucose linked with mainly alpha (1 - 4) bonds which can consist of several thousand glucose units. It is one of the two components of starch, the other being amylopectin.
Monosaccharide
Monosaccharides are the simplest carbohydrates. They cannot be hydrolyzed into simpler sugars.
Hexose
A hexose is a monosaccharide with six carbon atoms, twelve hydrogens, and six oxygens.
Fructose
Fructose (or levulose) is a simple sugar found in many foods and is one of the three most important blood sugars along with glucose and galactose.
Lactose
Lactose is a disaccharide that consists of beta-D-galactose and beta-D-glucose monomers connected by a beta-1-4 glycosidic linkage.
Maltose
Maltose, or malt sugar, is a disaccharide formed from two units of glucose joined with an alpha(1 - 4) linkage.
Galactose
Also called brain sugar, galactose, is a type of simple sugar which is less sweet than glucose and not very water-soluble.
Deoxyribose
Deoxyribose is a deoxy sugar derived from the pentose sugar ribose by the replacement of the hydroxyl group at the 2 position with hydrogen
Ribose
Ribose is simple pentose sugar which is a component of RNA.
Chitin
Chitin is a long-chain polymer of beta-glucose that forms a hard, semitransparent material found throughout the natural world, for example, in fungi, the exoskeletons of arthropods, the radula of molluscs and the beaks of cephalopods.
Aldohexose
An aldohexose is a hexose with an aldehyde group on one end.
Amylopectin
Amylopectin is a highly branched polymer of glucose found in plants. It is one of the two components of starch, the other being amylose.
Glycolipid
Glycolipids are carbohydrate-attached lipids.
Glyceraldehyde
Glyceraldehyde is a monosaccharide three carbon atoms, six hydrogen atoms, and three oxygen atoms, the simplest of all common aldoses.
Dextrin
Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch. They are mixtures of linear alpha-(1,4)-linked D-glucose polymers.
Glycosylation
Glycosylation is the process or result of addition of saccharides to proteins and lipids.
Glycosidic bond
A glycosidic bond is a certain type of functional group that joins a sugar moeity to an alcohol, which may be another carbohydrate.
Oligosaccharide
An oligosaccharide is a saccharide polymer containing a small number of component sugars, typically three to ten.
Glycoside
Glycosides are molecules in which a sugar part is bound to some other part.
Glucoside
A glucoside is a glycoside that is derived from glucose.
Advanced terms that may appear in context in MCAT passages
Reducing sugar
A reducing sugar is any sugar that, in basic solution, forms some aldehyde or ketone. This allows the sugar to act as a reductant, for example, in the Maillard reaction or Benedict's reaction.
Anomer
An anomer is a stereoisomer of a saccharide in the cyclic form that differs only in its configuration at the hemiacetal, or hemiketal, carbon
Anomerization
Anomerization is the interconversion between the alpha and beta anomers of a carbohydrate.
Benedict's reagent
Benedict's reagent contains blue copper(II) ions which are reduced to copper(I) in the presence of reducing sugars such as glucose, fructose, galactose, lactose or maltose.
Anomeric effect
The anomeric effect or Edward-Lemieux effect is a stereoelectronic effect that describes the tendency of heteroatomic substituents adjacent to a heteroatom within a cyclohexane ring to prefer the axial orientation instead of the less hindered equatorial orientation that would be expected from steric considerations..
Mutarotation
Mutarotation is the term given to the change in the specific rotation of plane polarized light, when it is passed through an aldohexose, refering to the conversion of a pure anomer of a hemiacetal carbohydrate to an equilibrium mixture of two anomers.
Glycosylamine
Glycosylamine is a biochemical compound consisting of an amine with a beta-N-glycosidic bond to a carbohydrate.
Glucuronic acid
Glucuronic acid is a carboxylic acid with a structure similar to glucose, though with the sixth carbon oxidized to a carboxylic acid.