Food
Source of building material and energy for building up or renewing body structures
Digestion
Degradation of nutrient molecules into components simple enough to be absorbed in intestine
Absorption
Uptake of digested components
Protein received
diet and endogenous sources
Protein from diet
70-100 g
Protein from endogenous sources
35-200 g
Endogenous sources for protein
enzymes and epithelial cells
Amount of protein lost in feces daily
6-12 g
Endopeptidases
cleave internal peptide bonds
Exopeptidases
cleave one amino acid at a time
Exopeptidase subclassifications
carboxypeptidase and aminopeptidase
Carboxypeptidase
cleave the -COOH terminal
Aminopeptidase
cleave the -NH2 terminal
Endopeptidases cut large polypeptides to smaller ____________ which are then acted on by _________
oligopeptides; exopeptidases
Final products of protein digestion
dipeptides/tripeptides and amino acids
Stomach
- secretes HCl (pH 1-2)
- pepsinogen and pepsin
Pepsinogen
secreted by the chief cells in the stomach lining and is activated to pepsin in the acidic environment
Pepsin
digests protein into large peptide fragments and some free amino acids
Pancreas
- produces several proteases: typsin, chymotrypsin, elastase
- large amounts of sodium bicarbonate to neutralize the acid from the stomach to promote pancreatic protease activity
Trypsin
cleaves proteins at lysine and arginine residues
Chymotrypsin
cleaves at aromatic amino acids
Elastase
cleaves at smaller hydrophobic amino acids
Membrane-bound enzymes, endopeptidases, dipeptidases, and amino peptidases act on _____ and _____ in the GI tract
oligopeptides and dipeptides
Oligopeptides and dipeptides are broken down to _____ and _____
free amino acids and dipeptides and tripeptides
Free amino acids and di and tripeptides are absorbed across the membrane by ______
specific carrier-mediated transport
Di and tripeptides are absorbed with the help of _____
H+-dependent symporters
Amino acids are absorbed using _____
Na+-dependent transporters
Di and tripeptides are further hydrolyzed into ______ inside the _____
amino acids; enterocyte (intestinal cell)
Endoproducts are carried into the ______
portal venous system
Aliphatic amino acids
saturated hydrocarbon side chains
Aromatic amino acids
involved in hydrophobic reactions
Serin and threonine
hydroxyl side groups
Asparagine and glutamine
amide-bearing side chains and polar
Serine, threonine, and asparagine
primary sites of linkage of sugars to proteins forming glycoproteins
Acidic amino acids
carboxylic acids on their side chains
Lysine
additional amino group at side chain
Histidine
imidazole ring as the side chain and catalyst in many enzymes
Methionine
nonpolar methyl group in its side chain
Cysteine
stabilizes the protein structure and participates in the formation of disulfide bonds with other cysteine residues
Cyclic amino acid
pyrrolidine ring side chain
Amino acids
amphoteric molecules (zwitterions)
Isoelectric point
the point at which the acidic and basic side chains in a protein are balanced and the overall net charge of the protein is zero
Basic proteins
lysine and arginine
Acidic proteins
aspartate and glutamate
pI
pI=pK1+pK2/2
Buffers
solutions that minimize a change in H+
Buffer pair
weak acid and weak base
Angiotensin
peptide hormone that affects blood pressure
Proteins contain between _____ and _____ amino acid residues
50; 2000
Molecular weight can be determined by…
mass spectrophotometry
Primary Structure
linear; charge and polarity
Secondary Structure
local structure of the polypeptide chain; alpha helix or beta pleated sheet
Alpha helix
- rod like structure
- 3.6 amino acid residues per turn of the helix
- wind clockwise
- ribbon, stick, or space-filling models
Beta-pleated sheet
- several parallel alpha helixes
- extended structure
- carbon-carbon bonds are tetrahedral
- cannot be planar configuration
Tertiary Structure
- folding of the peptide chain
- three dimensional
- stabilized by interactions between the side-chain functional groups
Tertiary structure stabilization
- covalent disulfide bonds
- hydorgen bonds
- salt bridges
- hydrophobic interactions
Quaternary Structure
- interactions between peptide chains
- assembly of two or more separate peptide chains that are held together by non covalent or covalent interactions
- tetrameric protein