BIOCHEM FINAL Questions

What are the characteristics of kinases?

1) add phosphoryl groups to proteins 2) may use ATP as a phosphoryl group donor 3) PKA (protein kinase A) is an example

What are the characteristics of phosphatases?

1) catalyze dephosphorylation reactions 2) turn off signaling pathways triggered by kinases

What 3 amino acid side chains absorb UV light at 280 nm?

1) phenylalanine 2) tyrosine 3) tryptophan

What characteristics of 2D electrophoresis describe isoelectric focusing?

1) proteins are separated by charge 2) carried out in a pH gradient 3) final net protein charge is zero

What characteristics of 2D electrophoresis describe SDS-PAGE?

1) proteins are separated by mass 2) carried out in a detergent solution 3) final net protein charge is negative

What characteristics describe the genomic library?

1) represents genomic contant 2) identical between different cell types of the same organism

What characteristics describe the cDNA library?

1) represents mRNA content 2) varies between different cell types of the same organism

Polymerase chain reaction (PCR) is a technique used to amplify (copy) DNA. Suppose a single, linear molecule of double-stranded DNA (dsDNA) is amplified by PCR. After one PCR cycle, how many molecules of dsDNA will there be?

2

Polymerase chain reaction (PCR) is a technique used to amplify (copy) DNA. Suppose a single, linear molecule of double-stranded DNA (dsDNA) is amplified by PCR. After three PCR cycles, how many molecules of dsDNA will there be?

8

One method for separating polypeptides makes use of their different solubilities. The solubility of large polypeptides in water depends upon the relative polarity of their R groups, particularly on the number of ionized groups: the more ionized groups there are, the more soluble the polypeptide. Which polypeptide (Glu)20 or (Phe-Met)3, is more soluble at a pH of 7.0? A. (Glu)20 B. (Phe-Met)3

A. (Glu)20

Select the true statements about SDS-PAGE, a method of separating proteins. Assume that SDS-PAGE is performed under reducing conditions. A. A protein binds roughly 1.4 times its mass of SDS, resulting in a large overall negative charge. B. Proteins are visualized using a dye that binds to the gel matrix, but not to proteins. C. Sodium dodecyl sulfate binds proteins, resulting in protein-SDS complexes that are similar in size. D. Proteins are separated in a polyacrylamide gel matrix. E. Smaller proteins migrate faster through the polyacrylamide gel. F. Protein-SDS complexes migrate towards the negative electrode.

A. A protein binds roughly 1.4 times its mass of SDS, resulting in a large overall negative charge. D. Proteins are separated in a polyacrylamide gel matrix. E. Smaller proteins migrate faster through the polyacrylamide gel.

A protein was purified to homogeneity. Determination of the mass by gel-filtration chromatography yields a value of 60 kDa. Chromatography in the presence of urea results in a 30 kDa species. Repeating the chromatography in the presence of both urea and β-mercaptoethanol, however, results in a single molecular species of 15 kDa. What do the data suggest about the structure of the protein? A. The 60 kDa protein is composed of two 30 kDa subunits linked by noncovalent interactions. Each 30 kDa subunit is composed of two 15 kDa subunits joined by disulfide bonds. B. The 60 kDa protein is composed of two 30 kDa subunits held together by disulfide bonds. Each 30 kDa subunit is composed of two 15 kDa subunits linked by noncovalent interactions. C. The 60 kDa protein is composed of multiple 15 kDa domains that hydrolyze upon treatment with an oxidizing agent. Some of the domains reassociate in the gel filtration column.

A. The 60 kDa protein is composed of two 30 kDa subunits linked by noncovalent interactions. Each 30 kDa subunit is composed of two 15 kDa subunits joined by disulfide bonds.

Signal transduction is part of a cell's response to an external signal. Although signal transduction pathways can differ in their details, there are some common elements. Select the six statements that accurately describe signal transduction pathways. A. a second messenger may carry a signal from the cell membrane to an organelle B. a receptor changes conformation upon binding, transmitting a signal across the cell membrane C. a receptor may pass on a signal by interacting with another protein or by acting as an enzyme D. a second messenger carries a signal from a tissue or organ to a target cell E. a ligand, such as a hormone, binds to a specific cell surface receptor on a target cell F. Signal transduction cascades, often involving protein kinases, amplify a signal intracellularly. G. phosphatases remove phosphoryl groups from polypeptides, regulating a cell's response

A. a second messenger may carry a signal from the cell membrane to an organelle B. a receptor changes conformation upon binding, transmitting a signal across the cell membrane C. a receptor may pass on a signal by interacting with another protein or by acting as an enzyme E. a ligand, such as a hormone, binds to a specific cell surface receptor on a target cell F. Signal transduction cascades, often involving protein kinases, amplify a signal intracellularly. G. phosphatases remove phosphoryl groups from polypeptides, regulating a cell's response

Polymerase chain reaction (PCR) is a technique used to amplify (copy) DNA. Suppose a single, linear molecule of double-stranded DNA (dsDNA) is amplified by PCR. After 30 PCR cycles (a typical number of cycles), how many molecules of dsDNA will there be? A. about 1 billion B. 625 C. 120 D. about 34 million E. 900 F. 60

A. about 1 billion

An IgG antibody that specifically recognizes a protein of interest, p50, was used to isolate p50 from a whole-cell lysate. The resulting protein mixture can undergo SDS-PAGE to separate p50 from IgG. The molecular weight of p50 is 50 kDa, and the protein does not contain any cysteine residues. The molecular weight of IgG is 160 kDa. IgG contains two heavy chains and two light chains that weigh 50 kDa and 25 kDa, respectively. Both types of chains contain cysteine residues. Select the components of the SDS-PAGE loading buffer. Select all that apply. A. bromophenol blue B. sodium chloride C. dithiothreitol D. sodium dodecyl sulfate E. glycerol

A. bromophenol blue D. sodium dodecyl sulfate E. glycerol

Which of the following molecules are used as second messengers in signal transduction pathways? A. cAMP B. inositol triphosphate C. epinephrine D. Ca2+ E. adenyl cyclase

A. cAMP B. inositol triphosphate D. Ca2+

What are the possible applications for restriction digestion? Select all that apply. A. genome editing B. gene cloning C. quantification of gene expression D. detection of mutations

A. genome editing B. gene cloning D. detection of mutations

DNA libraries are useful for identifying and isolating eukaryotic DNA fragments of interest in research. Select the ways in which genomic and cDNA libraries differ. A. genomic libraries are difficult to express in prokaryotic cells, which lack a splicing mechanism B. genomic libraries contain DNA fragments from an organism's entire genome C. cDNA libraries are the DNA fragment collections that are stored and propagated in host cells through cloning D. cDNA libraries contain more sequence information than genomic libraries

A. genomic libraries are difficult to express in prokaryotic cells, which lack a splicing mechanism B. genomic libraries contain DNA fragments from an organism's entire genome

Scientists have engineered bacteria to produce human proteins, such as insulin used to help treat diabetes. Suppose the unmodified eukaryotic gene coding for insulin is inserted directly into a bacterial chromosome. Select the reasons why no expression would be seen in the bacterial cell for the insulin gene. A. the bacteria will not recognize the eukaryotic promoter to transcribe the gene B. the bacterial nucleoid does not have the proper proteins to transcribe the eukaryotic gene C. the gene will induce attenuation in the bacterial cell, which will cause a delay in the transcription and translation of the insulin gene D. bacteria cannot remove intronic sequence from a gene, so if the gene for insulin were transcribed, it would translate to a nonfunctional protein

A. the bacteria will not recognize the eukaryotic promoter to transcribe the gene D. bacteria cannot remove intronic sequence from a gene, so if the gene for insulin were transcribed, it would translate to a nonfunctional protein

Select the statements that accurately describe receptor tyrosine kinases (RTKs). A. the structure includes a transmembrane helix B. RTKs are usually monomeric in the absence of a ligand C. the alpha subunit contains seven alpha helices that span the membrane D. a ligand binds to the extracellular domain E. the intracellular domain has phosphatase domains F. ligand binding is required for autophosphorylation (cross-phosphorylation)

A. the structure includes a transmembrane helix B. RTKs are usually monomeric in the absence of a ligand D. a ligand binds to the extracellular domain F. ligand binding is required for autophosphorylation (cross-phosphorylation)

What are the roles of sodium dodecyl sulfate (SDS) in two-dimensional electrophoresis? Select all that apply. A. to cause bound proteins to have a large negative charge B. to denature proteins C. to cause bound proteins to have a large positive charge D. to preserve the structure of the bound proteins E. to preserve zero net charge of the proteins

A. to cause bound proteins to have a large negative charge B. to denature proteins

Which of the following amino acids can be phosphorylated? A. tyrosine, serine, threonine B. tyrosine, serine, tryptophan C. serine, threonine, asparagine D. histidine, serine, phenylalanine E. tyrosine, methionine, tyrptophan

A. tyrosine, serine, threonine

In the course of purifying an enzyme, a researcher performs a purification step that results in increase in the total activity to a value greater than that present in the original crude extract. Explain.

An inhibitor of the enzyme being purified might have been present and subsequently removed by a purification step. This removal would lead to an apparent increase in the total amount of enzyme present.

Which protein would show up as the band at the top of an SDS-PAGE gel after electrophoresis? Why?

Apolipoprotein B

One method for separating polypeptides makes use of their different solubilities. The solubility of large polypeptides in water depends upon the relative polarity of their R groups, particularly on the number of ionized groups: the more ionized groups there are, the more soluble the polypeptide. Which polypeptide (Ala-Asp-Gly)5 or (Asn-Ser-His)5, is more soluble at a pH of 7.0? A. (Ala-Asp-Gly)5 B. (Asn-Ser-His)5

B. (Asn-Ser-His)5

One method for separating polypeptides makes use of their different solubilities. The solubility of large polypeptides in water depends upon the relative polarity of their R groups, particularly on the number of ionized groups: the more ionized groups there are, the more soluble the polypeptide. Which polypeptide (Ala-Ser-Gly)5 or (Asn-Ser-His)5, is more soluble at a pH of 7.0? A. (Ala-Ser-Gly)5 B. (Asn-Ser-His)5

B. (Asn-Ser-His)5

One method for separating polypeptides makes use of their different solubilities. The solubility of large polypeptides in water depends upon the relative polarity of their R groups, particularly on the number of ionized groups: the more ionized groups there are, the more soluble the polypeptide. Which polypeptide (Gly)20 or (Lys-Ala)3, is more soluble at a pH of 7.0? A. (Gly)20 B. (Lys-Ala)3

B. (Lys-Ala)3

Consider an experiment where you digest a DNA molecule with restriction enzyme Bc1I. You resolve the products of the digest by gel electrophoresis. Then, you cut out each of the resulting two bands from the gel and sequence the DNA. The sequencing results return two DNA segments. Segment 1 5' CCGCGGT 3' 3' GGCGCCACTAG 5' Segment 2 5' GATCAGGATCC 3' 3' TCCTAGG 5' What is the sequence of the Bc1I recognition site? A. 5' GATCAG 3' B. 5' TGATCA 3' C. 5' GATCAC 3' D. 5' GTGATC 3'

B. 5' TGATCA 3'

Consider an experiment where you digest a DNA molecule with restriction enzyme Bc1I. You resolve the products of the digest by gel electrophoresis. Then, you cut out each of the resulting two bands from the gel and sequence the DNA. The sequencing results return two DNA segments. Segment 1 5' CCGCGGT 3' 3' GGCGCCACTAG 5' Segment 2 5' GATCAGGATCC 3' 3' TCCTAGG 5' What type of ends do the byproducts have? A. blunt ends B. 5' overhang C. 3' overhang

B. 5' overhang

You prepare a cell line that overexpresses a mutant form of epidermal growth factor receptor, or EGFR, in which the entire intracellular region of the receptor has been deleted. Predict the effect of overexpression of this construct on epidermal growth factor (EGF) signaling in this cell line. What will be the effect of the overexpression of this truncated receptor? A. EGFR will be unable to bind to its ligand, blocking the signal and stopping cell growth B. EGFR will dimerize but will not cross-phosphorylate, inhibiting normal cell growth C. EGFR will activate other proteins without dimerization, causing uncontrolled cell growth D. EGFR will not form a dimer in the presence of EGF, stopping normal cell growth

B. EGFR will dimerize but will not cross-phosphorylate, inhibiting normal cell growth

Which restriction endonucleases would cleave a DNA molecule at the given sequence? The complementary DNA substrate strand is omitted for clarity. Select all that apply. 5' TCCAAGCTTGAATTC 3' A. EcoRV B. EcoRI C. HindIII D. HaeIII

B. EcoRI C. HindIII

Which restriction endonucleases would cleave a DNA molecule at the given sequence? The complementary DNA substrate strand is omitted for clarity. Select all that apply. 5' GCCAATTGAAGCTT 3' A. EcoRV B. HindIII C. HaeIII D. EcoRI

B. HindIII

Polymerase chain reaction (PCR) is a technique used to amplify (copy) DNA. Suppose a single, linear molecule of double-stranded DNA (dsDNA) is amplified by PCR. Consider that a typical PCR does not start with a single molecule of template DNA, but rather something in the range of 25 nmol of template DNA. What does this tell you about the potential of PCR to amplify DNA? A. PCR is an inefficient technique with the potential to produce a small amount of DNA B. PCR is an efficient technique with the potential to produce a large amount of DNA

B. PCR is an efficient technique with the potential to produce a large amount of DNA

If protein A has a pI of 3.1, protein B has a pI of 6.8, and protein C has a pI of 8.9, which protein would elute FIRST from a cation exchange column (negatively charged beads in the column) at pH 7? A. all 3 proteins would elute at the same time from the column B. Protein A C. Protein B D. Protein C

B. Protein A

Consider an experiment where your goal is to isolate Bruton's tyrosine kinase (BTK) enzyme from a whole‑cell lysate. You have an affinity chromatography column with a tyrosine kinase inhibitor molecule covalently attached to the beads. The tyrosine kinase inhibitor binds and inhibits BTK. As a result of the experiment, you are able to elute BTK from the column, but in a mixture of other tyrosine kinases. Why are tyrosine kinases other than BTK present in the eluate? A. BTK is rapidly degraded during cell lysis B. The kinase inhibitor has low specificity C. BTK is inactive in the cell D. The kinase inhibitor has low binding affinity

B. The kinase inhibitor has low specificity

An amino acid in a medium with a pH above its isoelectric point would have: A. a net positive charge B. a net negative charge C. a neutra charge D. either a net positive or net negative charge depending on the pKa value

B. a net negative charge

Odorant receptors and beta-adrenergic receptors are 7TM receptors that initiate a signal cascade through G proteins. Which of the following steps is common between the signal-transduction cascade mediated by the odorant receptor and the signal-transduction cascade mediated by the beta-adrenergic receptor? A. an increase in intracellular levels of IP3 B. an increase in intracellular levels of cAMP C. an influx of cations into the cell D. activation of protein kinase A

B. an increase in intracellular levels of cAMP

A protein with a high percentage of aspartate and glutamate residues would be BEST purified and concentrated with which type of column? A. cation (negatively charged beads in the column) exchange B. anion (positively charged beads in the column) exchange C. size-exclusion chromatography D. affinity chromatography

B. anion (positively charged beads in the column) exchange

Select the structural features common to all membrane-bound receptors. A. kinase domain B. intracellular domain undergoes structural changes upon extracellular ligand binding C. seven transmembrane helices D. signal molecule binding site on the extracellular domain

B. intracellular domain undergoes structural changes upon extracellular ligand binding D. signal molecule binding site on the extracellular domain

Do nucleic acids have a charge at physiological pH? A. nucleic acids are not charged molecules B. nucleic acids are negatively charged due to negatively charged phosphate groups in a backbone C. nucleic acids are positively charged due to phosphate groups in a backbone D. sugars in nucleic acids are positively charged, so nucleic acid molecules are positively charged

B. nucleic acids are negatively charged due to negatively charged phosphate groups in a backbone

Identify the applications for gel filtration chromatography. Select all that apply. A. separation of components in a mixture by hydrophobicity B. separation of components in a mixture by size C. purification of a specific protein from a mixture D. estimation of molecular weights

B. separation of components in a mixture by size D. estimation of molecular weights

Which tripeptide will be retained the longest on a cation-exchange (negatively charged beads in the column) chromatographic column in a pH 7.0 buffer? A. Asp-Glu-Ser B. Ser-Gly-Thr C. Gly-Lys-Arg

C. Gly-Lys-Arg

What is this molecule? A. cAMP B. UDP-glucose C. cholesterol D. sphingolipid E. oxaloacetate

C. cholesterol

Glucose is mobilized for ATP generation in muscle in response to epinephrine, which activates Gas. Cyclic AMP phosphodiesterase is an enzyme that converts cAMP into AMP. How would an inhibitor of cAMP phosphodiesterase affect glucose mobilization in muscle? A. it would increase cAMP levels, which would inhibit glucose mobilization B. it would reduce cAMP levels and inhibit glucose mobilization C. it would maintain high cAMP levels and elevate glucose mobilization D. it would increase AMP concentration, thereby increasing glucose mobilization

C. it would maintain high cAMP levels and elevate glucose mobilization

Which protein is to be retained the longest on a size-exclusion (gel-filtration) chromatographic column? Why?

Cytochrome C (human)

Which of the following reactions is catalyzed by reverse transcriptase? A. 3' -> 5' polymerization of DNA from an RNA template B. 3' -> 5' polymerization of RNA from a DNA template C. 3' -> 5' polymerization ofRNA from a polypeptide template D. 5' -> 3' polymerization of DNA from an RNA template E. 5' -> 3' polymerization of RNA from a polypeptide template

D. 5' -> 3' polymerization of DNA from an RNA template

According to the table, size-exclusion (gel-filtration) chromatography could not be used to separate which protein mixture? A. proteins A, D, and F B. proteins C, E, and F C. proteins B, E, and F D. proteins A, B, and C E. All of the answers are correct

D. proteins A, B, and C

What is pI (isoelectric point)? A. the pH at which a molecule does not have any electric charge B. the negative log of hydrogen ion concentration of an aqueous solution C. the pH of maximum electrolytic mobility D. the pH at which a solute has no net electric charge and does not move in an electric field E. -log(pK1+pK2)/2

D. the pH at which a solute has no net electric charge and does not move in an electric field

Which of the outcomes could potentially result if a mutation in the gene encoding the Ga subunit eliminates its GTPase activity? Choose three outcomes. A. the rate of GTP hydrolysis would increase B. the cell would be unresponsive to molecules that act via cAMP C. Ga would be ineffective as a signal molecule (that is, it could not activate other proteins) D. the signaling pathway could be activated for an extended period, possibly resulting in undesirable cell proliferation. E. the concentration of cAMP in the cell would be continuously elevated F. Ga would be activated for an extended period

D. the signaling pathway could be activated for an extended period, possibly resulting in undesirable cell proliferation. E. the concentration of cAMP in the cell would be continuously elevated F. Ga would be activated for an extended period

Genomic mammalian DNA cannot be expressed directly using bacterial expression vectors because mammalian genomic DNA contains introns, which the bacterial cell cannot excise. Complementary DNA, or cDNA, enables scientists to express mammalian DNA from a bacterial plasmid. Place the sentences in the correct order to describe the formation of double-stranded DNA. Not all steps will be placed. A. a primer site is created on the single-stranded DNA B. the DNA to be cloned is removed from the mammalian cell and incubated with reverse transcriptase C. the mRNA is hydrolyzed by raising the pH D. a complementary DNA strand is synthesized, forming a double-stranded cDNA molecule E. mature mammalian mRNA is isolated F. reverse transcriptase, provided with a primer, synthesizes a strand of cDNA without introns G. mRNA is hydrolyzed by lowering the pH

DNA to be cloned E. mature mammalian mRNA is isolated F. reverse transcriptase, provided with a primer, synthesizes a strand of cDNA without introns C. the mRNA is hydrolyzed by raising the pH A. a primer site is created on the single-stranded DNA D. a complementary DNA strand is synthesized, forming a double-stranded cDNA molecule Double-stranded DNA

Suppose a team of researchers sequences the genome and measures the proteome of a human skin cell and a human kidney cell. Will there be a greater difference between the genome or proteome between these two cells? A. The genome is more different between the cells than the proteome because gene expression varies between the two cells. B. The genome is more different between the two cells than the proteome due to the variety of molecules associated with DNA including histones, regulatory proteins, and enzymes. C. Each gene codes for a specific protein, so there is no difference between the genome and the proteome of the two cells. D. The proteome is the genetic material in a prokaryotic cell, and thus these cells do not have a proteome because they are eukaryotic E. The proteome is more different between the cells due to differences in gene expression and post-transcriptional processing of mRNA.

E. The proteome is more different between the cells due to differences in gene expression and post-transcriptional processing of mRNA.

Consider a mixture of four proteins with various molecular weights. A histone molecule weighs 15 kDa, a p53 molecule weighs 53 kDa, an actin molecule weighs 42 kDa, and an IgG molecule weighs 150 kDa. Arrange the molecules in order of their elution from a gel filtration column. -histone (15 kDa) -p53 (53 kDa) -IgG (150 kDa) -actin (42 kDa)

Elutes first IgG (150 kDa) p53 (53 kDa) actin (42 kDa) histone (15 kDa) Elutes last

Scientists have engineered bacteria to produce human proteins, such as insulin used to help treat diabetes. Suppose the unmodified eukaryotic gene coding for insulin is inserted directly into a bacterial chromosome. Arrange the steps that would be used in a laboratory to engineer a bacterium that could express the human gene coding for insulin. Not all steps will be placed. A. Insert the insulin cDNA into a bacterial vector near a promoter site B. human DNA is introduced into the bacterial cell using direct injection by a small needle C. generate cDNA of the insulin gene using reverse transcriptase D. transform the vector into an E. coli bacterium E. the insulin protein is isolated from human tissues F. isolate the mRNA of the insulin gene

Isolate the human gene that produces insulin F. isolate the mRNA of the insulin gene C. generate cDNA of the insulin gene using reverse transcriptase A. Insert the insulin cDNA into a bacterial vector near a promoter site D. transform the vector into an E. coli bacterium E. coli expresses insulin

A researchers resolves a mixture of peptides using isoelectric focusing. Order the peptides based on their relative positions in the immobilized pH gradient strip at the end of the experiment. A. Arg-Leu-Ala-Arg B. Arg-Ala-Lys-Lys C. Glu-Gly-Glu-Asp D. Asp-Ala-Leu-Asp E. Arg-Gly-Glu-Lys

Low pH C. Glu-Gly-Glu-Asp D. Asp-Ala-Leu-Asp E. Arg-Gly-Glu-Lys A. Arg-Leu-Ala-Arg B. Arg-Ala-Lys-Lys High pH

Proteins are separated on the basis of their ability to migrate in an electric field based on their size.

SDS-PAGE

Arrange these oligonucleotides according to the distance traveled in an electrophoretic separation. A. GTATGTTCACCCGTC B. TTTGCGTATTT C. CTACGTGCG D. AAACT E. AGGGAAG

Shortest distance traveled A. GTATGTTCACCCGTC B. TTTGCGTATTT C. CTACGTGCG E. AGGGAAG D. AAACT Farthest distance traveled

What wavelength does DNA denaturation require to absorb UV light?

UV spectrophotometry at 260 nm

What molecule is this?

cAMP

Place all eight steps of the insulin signaling pathway in the correct order. A. activation of Akt B. GLUT4 receptors are transported to cell membrane C. activation of PIP3 is dependent on protein kinase B (PDK1) D. phosphorylationof IRS proteins E. activation of insulin receptor tyrosine kinase F. phosphorylation of phosphoinositide 3-kinase (PI-3K) G. conversion of PIP2 to PIP3 H. binding of insulin to the alpha subunit of the insulin receptor

insulin is secreted H. binding of insulin to the alpha subunit of the insulin receptor E. activation of insulin receptor tyrosine kinase D. phosphorylationof IRS proteins F. phosphorylation of phosphoinositide 3-kinase (PI-3K) G. conversion of PIP2 to PIP3 C. activation of PIP3 is dependent on protein kinase B (PDK1) A. activation of Akt B. GLUT4 receptors are transported to cell membrane glucose enters cell

Lactate dehydrogenase (LDH) belongs to what class of enzymes?

oxidoreductase

What is the highest level of protein organization for insulin?

quaternary structure

What are the characteristics of both kinases and phosphatases?

regulate the activity of other proteins

Proteins are chromatographically separated solely on the basis of size.

size exclusion chromatography (gel-filtration)

A protein has a molecular mass of 400 kDa when measured by gel filtration. When subjected to gel electrophoresis in the presence of SDS, the protein gives 3 bands with molecular masses 180, 160, and 60 kDa. When electrophoresis is carried out in the presence of SDS and dithiothreitol, 3 bands are again formed, with molecular masses of 160, 90, and 60 kDa. Determine the subunits composition of the protein.

the protein has 4 subunits: 160, 90, 90, 60 subunits linked by disulfide bonds