Protein Modifications Flashcards
Glycosylation
The most common protein modifcation (with most common protein modification -50% of all proteins undergo some type of glycosylations Cyclic hemiacetals (alpha= oh group is below, beta= oh is above) oligosaccharide= polymer containing 2 or more monosaccharides joined together by O-glycosidic linkages (glycogen) 1. Carbohydrates are the building blocks to form oligosaccharides on proteins (mannose fructose galactose, GalNAc, GlcNAc) 2. Carbohydrates can be joined by different linkages Glucose (Glc, Sialic acid Neu5Ac, xylose, GlcA, Ido A)
Nucleotide sugars are the substrates. Nucleotide sugars are activated high energy compounds which are immediate precursors in glycoprotein/ glycolipid synthesis. Synthesis occurs in the cytoplasm (except CMP-Neu5Ac)
nucleoside triphosphate + sugar 1 phosphate-> nucleoside diphosphate sugar + PPi
Synthesis of nucleotide sugars and galactosemia
- Glucose is the (Glc) is the major in vivo source for the synthesis of nucleotide sugars
- ATP is required for early intermediates, then UTP, GTP and CTP is used in the late stages
- Glutamine is required for the synthesis of UDP- GlcNAc
Classic galactosemia: decreased synthesis of a nucleotide sugar UDP-galactose
caused bu deficient GALT-> failure to thrive, hepatomegaly, jaudice, and cataracts
accumulation of metabolites contribute to symptoms, treatment is elimination of lactose and galactose from the diet
Nucleotide sugars are involved in a variety of pathways
UDP-glucose is used by glycogen synthase to make glycogen
UDP-glucaronic acid: is the substrate for glucuronyltransferases involved in (glycoprotein/proteoglycan synthesis), drug detox of xenobiotics, excretion of steroids, heme metabolism and excretion of bilirubin
Glycosyltransferases are the enzymes and are resident integral membrane proteins of ER, they are very specific to (sugar that is transferred, acceptor substrate type of linkage)
Anomeric/C1 carbon of the sugar bonds to hydroxyl group of the acceptor glycosidically
Synthesis and trafficking of glycoproteins
Majority of glycosylation reactions occur in the lumen of the ER and golgi (secretory path)
Secreted and transmembrane proteins enter the ER as they are being synthesized by ribosomes, proteins exit the ER in transport vesicles, proteins travel through the cisternae of the golgi, proteins are sorted at the trans golgi network (TGN), soluble proteins are secreted from the cell, transmembrane is transported to PM with glycan on the outer membrane)
Major classes of glycoproteins
- Glycoproteins with N-linked glycans
- Proteo Glycans with o linkied
- Mucins
N-glycosidic linkage
N-glycosidic linkage between Asparagine and GlcNAc
An initial Glc3Man9GlcNAc2 structure can be changed or processed by the action of glycosyltransferases and glycosidases as the glycoprotein moves thru the ER and golgi so you can make thousands of different glycan structures
Glycosidases (hydrolytic enzymes that remove carbohydrates grouped based on the type of carbohydrate) via chaperones
o-glycosidic linkage between Xyl and ser
in proteoglycans; a specific oligosaccharide/glycan called glycosoaminoglycan
Proteoglycans often contain large numbers of glycosamino glycan chains
Proteoglycans (aka glycosaminoglycans) are negatively charged polysaccharides composed of repeating disaccharide units (amino sugar GlcN or Gal N+uronic acid GLCUA or IDUA) which are sulfated
Hyaluranic acid gives the brushed look
Mucins
heavily o-glycosylated glycoproteins
Muc genes: linked to ser and thr
found in mucous secretion or cell surface synthesized by many epithelial cells and by goblet cells of trachea GI and GU system
hydrate and protection
Cancer antigen 125 (marker for cancer)
what level is a protein concentration and activity regulated
protein concentration is regulated at the transcription, post transcriptional transcriptional processing, translational regulation and protein degredation
Protein activity of a protein can be regulated at all levels and covalent modifications to the protein itself (phosphate/ carbohydrate) and the subcellular location of a protein
Post translational modifications increase the number of lettters in the amino acid alphabet
Proteins contain more than just Amino acids
Writers- enzymes that add mark (kinases and acetylases)
Readers- proteins that bind to and interpret the mark (SH2 domains, lectins)
Erasers- enzymes that remove the mark (phosphatases and deacetylases)
Functions of carbohydrates
glycosylation is critical for normal development:
Numerous genetic defects of glycan synthesis involving defective synthesis of: N glycans: large family of diseases called congenital disorders of glycosylation CDGs, glycosaminoglycans( chondrodysplasia), olinked glycans (alpha dystroglycans, muscular dystrophies)
Protein folding and conformation is regulated by chaperones
Modification of protein function (erythropoietin)
Erythropoietin (EPO) is a growth factor secreted by the kidney stimulates the production of red blood cells (induces the proliferation and differentiation of erythroid progenitors)
Used in anemia and performance enhancing drugs
EPO is a protein that has an N-linked oligosaccharides(constitutes 40% of mass of glyco protein)
Glycans are crtical for EPO funtions
Modification of protein function (heparin)
Most widely used drug in the world as an anticoagulant
High affinity complex with antithrombin (which increases its activity 10000fold)
Antithrombin inhibits 2 principle procoagulants : factor Xa and thrombin
Modification of protein function (Structural support)
proteoglycans of cartilage: glycosaminoglycans bind large amounts of water, allows cartilage to resist compression in joints
disease: HYALGAN hyaluronic acid prep thats injected in joints
Functions of Carbohydrates
Carbs interact with carbohydrate binding proteins (lectins and receptors
1. Cell recgnition (for leukocyte adhesion selectins bind immune system cells to bind to sites of inflammation
2. Intracellular targeting (lysosomal enzymes, leysosmal storage diseases)
3. Binding sites for bacterial toxins
Antigenic- blood groups
antigens are oligosaccharides and determined by genetic glycosyltransferases
Glycosylation and disease
Cancer for diagnosis and tratment (CART cells)
Diabetes and glycation
Glycation: is a non enzymatic process that affect long lived proteins in the lens, plasma and red blood cells( glucose becomes covalently attached to lysine residues in hemoglobin in HbA1C
Advanced glycation end products -> bad things
The flu host ( glyco proteins and glycolipids contain sialic acid as cell surface receptors). Virus encodes sialidase that cleaves sialic acid from glycoproteins and that helps the virus through the mucosa (rich in sialic acid) of the respiratory tract and in the release of the virus from infected cells. Sialidase (neuramindase inhibitors)tamiflu and zanamivir relenz are analogs of sialic acid
Glycosylation and disease lysosomal storage diseases
incomplete degredation: (catabolism of glyco lipids, proteoglycans, and glycoproteins occurs in the lysosome via hydrolytic enzymes glycosidases solely from the non reducing end of oligosaccharide)
Hydroxylation and acetylation
Collagen and histones (etc)
Hydroxylation and 4 hydroxy proline: stabilize collagen structure (scurvy)
Acetylation of lysine: removes the positive charge and inhibits the ability of histones to interact with the negatively charged DNA
Phosphorylation
Can do 3 things:
- convert an inactive protein into an active on off switch
- serve as a docking/binding site for other proteins in signaling cascade
- serve as a rheostat to modulate the activity of enzymes
ADP ribosylation
Bacterial toxins such as diptheria, cholera, and pertussis toxins act by ADP ribosylation of crucial cell proteins using NAD as the substrate
Cholera toxin permanently activates G protein increasing cAMP and leads to diarrhea
Ubiquitinationand protein degredation
Ubiquitin tags a protein for degredation and is highly conserved
ATP dependent pathway involves 3 enzyme classes
E1 Ub activating
E2 UB conjugating
E3 Ub ligase (picks which protein to degrade via coarbosyl teminal glycine and linked to lysine residues)
BRCA is an E3 ub ligase(tumor suppressor gene)
Hydrophobic modification (Fatty acids
Palmitolation (adding palmitic acid to a protein)
Myritylation
Farnesylation: intermediate of cholesterol biosynthetic path and covalently attached to cys rediues
Fanesyltransferase inhibitor drugs: cancer (Ras has to be farnesylated to associate to PM and is essential for the transforming activity of oncogenic variants of Ras)
HG progeria syndrome (retaining a farnesyl group rather than making lamin A)
Glycsyl phosphatidylinositol (GPI)- attached to the c terminus of a target protein through the phosphoethanolamine portion of the molecule and fatty acidsare imbedded in the bilayer
