HLTH 230 Chapter Notes - Chapter 6: Essential Amino Acid, Alpha Helix, Amine
20 Nov 2018
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Chapter 6: Protein: Amino Acids
6.1 Chemist’s View of Proteins
Amino Acids
• structure
− amino group, carboxyl group, and side chain all surrounded by a carbon molecule
• unique side groups
− the side group is what determines the amino acid
− there are about 20 unique R groups – or 20 unique amino acids
• glycine
− simplest amino acid
− a hydrogen as the side group
• non-essential amino acids
− more than half of the AAs
− the body can synthesize them itself
• essential amino acids
− 9 AAs
− Body can not make them on its own must be supplied by diet
• Conditionally essential amino acids
− When a non-essential AA becomes essential under special conditions
− Ex. body normally uses Phe (essential) to make Tyr (non-essential) BUT some diseases don’t
allow this conversion and therefore Try becomes a conditionally essential amino acid
Proteins
• Amino acid chains
− A combination of the 20 amino acids makes up lots of different complex proteins
− Linked thru condensation reactions
− Dipeptide: 2 AAs bonded together
− Polypeptide: 3+ AAs bonded together
− Most proteins are a few dozen to several hundred AAs long
• Primary structure
− The actual amino acid sequence of the proteins
• Secondary structure
− The shape of the protein determined by hydrogen bonds
− Alpha helix
− Beta pleated sheet
• Tertiary structure
− The further folding of the protein determined by hydrogen, ionic, and disulfide bonds, and
hydrophobic interactions
• Quaternary structure
− Some proteins can only function in association with other polypeptides
− Ex. hemoglobin has a quaternary structure because it contains 4 associated polypeptide chains
• Protein denaturation
− Heat, acid or other conditions that disturb their stability
− Denaturation: the uncoiling of their shape which causes them to lose their ability to function
− Past a certain point, denaturation is irreversible
− Proteins are denatured when they are exposed to stomach acid, which then allows it to become
more digestible
6.2 Digestion and Absorption of Proteins
Protein Digestion
• Stomach
− Hydrolysis of proteins
− Hydrochloric acid denatures the protein (uncoils) so digestive enzymes can attack peptide bonds
− HCl also turns pepsinogen into active pepsin
→ Pepsin = digests proteins by cleaving them into smaller polypeptides/amino acids
• Small intestines
− pancreatic and intestinal proteases hydrolyze polypeptides further into short peptide chains (tri-
, di-peptides, and individual AAs)
− peptidase – enzyme that splits dipeptides into single AAs
Protein Absorption
• specific carriers transport AAs into intestinal cells
• AAs not used for energy are transported across cell membrane into surrounding fluid to make their
way to the liver
• enzymes in foods are digested just as all proteins are
− enzyme pepsin (low pH of stomach) becomes inactive and digested when in higher pH of small
intestine
• digestive system handles whole proteins better than predigested ones
6.3 Proteins in the Body
Protein Synthesis
• instructions
− transcription
→ DNA is used as template to make mRNA via RNA polymerase
− Translation
→ mRNA leaves the nucleus and enters the cell body
→ ribosomes (APE) read the mRNA and link together corresponding amino acids into a
polypeptide chain
• linking amino acids
− tRNA
→ specific to amino acids (i.e. 20 different tRNAs)
→ codon and anticodon
• sequencing errors
− if AAs are genetically mixed up, protein shape and function can change
− i.e. sickle cell anemia 2/4 chains in the hemoglobin are altered and can’t carry O2
• nutrient and gene expression
− cell protein = gene expressed
− cells can regulate gene expression (amount of protein being made)
Roles of Proteins
• proteins as building materials for growth and maintenance
− protein = building blocks for many muscles, blood and skin (i.e. protein collagen mainly makes
up a bone)
− proteins needed to replace dead or damaged cells
Document Summary
A hydrogen as the side group: non-essential amino acids. More than half of the aas the body can synthesize them itself: essential amino acids. Body can not make them on its own must be supplied by diet: conditionally essential amino acids. When a non-essential aa becomes essential under special conditions. Ex. body normally uses phe (essential) to make tyr (non-essential) but some diseases don"t allow this conversion and therefore try becomes a conditionally essential amino acid. A combination of the 20 amino acids makes up lots of different complex proteins. Most proteins are a few dozen to several hundred aas long: primary structure. The actual amino acid sequence of the proteins: secondary structure. The shape of the protein determined by hydrogen bonds. The further folding of the protein determined by hydrogen, ionic, and disulfide bonds, and hydrophobic interactions: quaternary structure. Some proteins can only function in association with other polypeptides.
