Biologically Important Molecules Flashcards
Where do disulfide bridges occur?
Disulfide bridges occur between two Cysteine residues bonded to one another. The amino acid is known as a Cystine disulfide bridge occurs. Disulfide bridges are found only in extracellular polypeptides where they will not be reduced. Examples of protein complexes held together by disulfide bridges include antibodies and the hormone insulin. Oxidation reactions takes place outside a cell in the extracellular environment not inside a cell where antioxidants reside (reducing environment).
Where does trypsin cleavage occur?
Where does chymotrypsin cleavage occur?
The protease trypsin cleaves on the carboxyl side of the positively charged (basic) residues arginine and lysine.
Chymotrypsin cleaves adjacent to hydrophobic residue such as phenylalanine
Denaturation
Denaturation is an important concept. It refers to the disruption of a protein’s shape without breaking peptide bonds. Proteins are denatured by urea (which disrupts hydrogen bonding), but extremes of pH, by extremes of temperature, and by changes in Salt concentration (tonicity).
Alpha helices
Alpha helices of proteins are always right handed, 5 Å in width, with each subsequent amino acid rising 1.5 Å. There are 3.6 amino acid residues per turn with the Alpha carbonyl oxygen of one amino acid residue hydrogen bonded to the alpha amino proton of an amino acid three residues away.
** The unique structure of proline forces it to kink the polypeptide chain: hence the proline residues never appear within the alpha helix
Alpha helical transmembrane regions
Proteins such as hormone receptors an ion channels are often found with alpha helical transmembrane regions integrated into hydrophobic membranes of cells. The alpha helix is a favorable structure for a hydrophobic transmembrane region because all polar NH & CO groups in the backbone are hydrogen bonded to each other on the inside of the helix, and thus don’t interact with the hydrophobic membrane interior. Alpha helical regions that span membranes also have hydrophobic our groups which radiate out from the helix interacting with the hydrophobic interior of the membrane
Beta-pleated sheets
They are also stabilized by hydrogen bonding between NH & CO groups in the polypeptide backbone. However hydrogen bonding occurs between residues distant from each other or even on separate polypeptide chains. Also the backbone of the beta she is extended rather than coiled with side groups directed above and below the plane of the beta sheet. Bsheets with adjacent peptide strands running in the same direction are called parallel and when polypeptide strands run in opposite directions they are anti-parallel
What effect what a molecule that disrupts hydrogen bonding, example urea, have on protein structure?
Urea would disrupt the secondary structure by unfolding alpha helices and beta sheets. It would not affect primary structure which depends on the much more stable peptide bond. Disruption of secondary, tertiary, and quaternary structure without breaking peptide bonds is called denaturation
Carbohydrates
Carbohydrates can be broken down to CO2 in a process called oxidation, which is also known as burning or combustion. A single carbohydrate molecule is a monosaccharide also known as a simple sugar. monosaccharides have the general chemical formula Cn H2n On.
Two monosaccharides bonded together form a disaccharide. The bond between two sugar molecules is called a glycosidic linkage. This is a covalent bond, formed in a dehydration reaction that requires enzymatic catalysis
Hydrolysis of Glycosidic linkages
The hydrolysis of polysaccharides into monosaccharides is favorite thermodynamically. Hydrolysis essential in order for the sugars to enter metabolic pathways and be used for energy by the cell. The enzymes are named for the sugar they hydrolyze, they are very specific. Ex: Maltase hydrolyzes maltose.
We cannot hydrolyze beta glycosidic linkages for example cellulose (b 1-4 linkage). The only exception is lactose -which we produce lactose as children so we can digest mothers milk but many adults cannot produce lactose and are lactose malabsorbers and intolerant
If the activation energy of polysaccharide hydrolysis were so low that no enzyme was required for the reaction to occur, would this make polysaccharides better for energy storage?
No, because that then polysaccharides would hydrolyze spontaneously and they’d be unstable. The high activation energy of polysaccharide hydrolysis allows us to use enzymes as gatekeepers – would we need energy from glucose we open the gate of glycogen hydrolysis.
(hydrolysis of polysaccharides is thermodynamically favored; energy input is required to drive the reaction to word polysaccharide synthesis)
Lipids are oily or fatty substances that play three physiological rolls. What are they?
- in adipose cells, triglycerides (fats) store energy
- In cellular membranes, phospholipids constitute a barrier between intracellular and extracellular environment.
- Cholesterol is a special lipid that serves as the building block for the hydrophobic steroid hormones
Micelles
Fatty acids interact in aqueous solution by forming a structure called a micelle. Water molecules form an orderly solvation shell around each hydrophobic substance. the problem is that forming a solvation shell is an increase in order and thus a decrease in entropy which is unfavorable according to the second law of thermodynamics.
Triacylglycerol
Fatty acids are stored as fat and the technical name for fat is triglyceride or Triacylglycerol. It’s composed of three fatty acids esterified to glycerol molecule. It’s necessary to store fatty acids in the relatively inert form of fat because free fatty acids are reactive chemicals.
Lipases
Lipases are enzymes that hydrolyze fats Triacylglycerols are more efficient energy storage than carbohydrates for two reasons: packing and energy content.
P: their hydrophobicity allows fats to pack together much more closely than carbohydrates. carbohydrates carry a greater amount of water of salvation. The amount of carbon per unit area or unit weight is much greater in a fat droplet than in a dissolved sugar.
E: fat molecule store much more energy than carbohydrates. Fats are much more reduced. Since carbohydrates are more oxidized to start with, oxidizing them releases less energy.
Things that increase membrane fluidity
Unsaturation, decreasing the length of fatty acid tails, and the steroid cholesterol at a low temperature
